Transactions of the Royal Society of South Africa
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A preliminary survey of the estuaries on the
southeast coast of South Africa, Cape St Francis –
Cape Padrone, with particular reference to the fish
fauna
N.C. James & T.D. Harrison
To cite this article: N.C. James & T.D. Harrison (2010) A preliminary survey of the estuaries
on the southeast coast of South Africa, Cape St Francis – Cape Padrone, with particular
reference to the fish fauna, Transactions of the Royal Society of South Africa, 65:1, 69-84, DOI:
10.1080/00359191003652116
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Transactions of the Royal Society of South Africa
Vol. 65(1), February 2010, 69–84
A preliminary survey of the estuaries on the
southeast coast of South Africa,
Cape St Francis – Cape Padrone,
with particular reference to the fish fauna
1
N.C. James & T.D. Harrison
1
2
South African Environmental Observation Network, Elwandle Node, Private Bag 1015, Grahamstown, 6140 South Africa
e-mail: n.james@saiab.ac.za.
2
South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown, 6140 South Africa
e-mail: tdharrison@eircom.net
A basic ichthyofaunal and physico-chemical survey of the coastal outlets on the southeast coast of South
Africa (Cape St Francis – Cape Padrone) was conducted during September and October 1995. Eleven
systems have been identified along this stretch of coastline. Four systems (Kromme, Gamtoos, Swartkops
and Sundays) are permanently open and provide important habitats for a number of estuarine-resident and
marine fish species. Three systems (Seekoei, Kabeljous and Van Stadens) are moderately sized estuaries
characterised by intermittently open mouths. The Seekoei Estuary has undergone extreme human
disturbance, while the Kabeljous and Van Stadens are in a less disturbed state. All three systems provide
nursery habitats for both estuarine-resident and estuarine-associated marine species. The Maitland is a
small, shallow sandy system that is typically closed to the sea for extended periods; despite this it was still
found to support estuarine-associated species particularly mugilids. The remaining three systems (Bakens,
Papkuils and Coega) are all highly altered systems that provide little littoral habitat for either resident or
migrant marine species.
Keywords: ichthyofauna, estuarine survey, fish habitat, southeast coast, South Africa.
INTRODUCTION
The South African coastline extends approximately 3000 km
from the Orange (Gariep) River (28°38’S, 16°28’E) on the west
(Atlantic Ocean) coast to Kosi Bay (26°54’S, 32°53’E) on the east
(Indian Ocean) coast. Some 300 coastal outlets have been
identified along the coast of South Africa and these range from
relatively large, permanently open estuaries to inlets of coastal
streams and even dry river beds that only occasionally contain
water.
Estuaries are typically formed where freshwater from rivers
and saltwater from the sea meet and, as such, are unique environments characterised by variations in environmental conditions. Estuaries are amongst the most variable aquatic environments on earth and are known to be more productive than
adjacent freshwater and marine environments (Woodwell et al.,
1973; Haedrich & Hall, 1976). Estuaries are also sheltered
environments and often serve as important nursery areas for
various fish species (Dando, 1984; Wallace et al., 1984).
After reviewing the available scientific information on South
African estuarine systems Whitfield (2000) found that, of the
258 systems included in the review, the state of information on
68% was either ‘nil’ or ‘poor ’; the state of information on 22%
was classified as ‘moderate’, while only 10% were regarded as
‘good’ or ‘excellent’. This paper reports on ichthyofaunal surveys
of estuaries between Cape St Francis and Cape Padrone on the
southeast coast of South Africa conducted during 1995 and
describes physico-chemical variables, presents basic fish community data, and provides an appraisal of the nursery potential
ISSN 0035-919X print
© 2010 Royal Society of South Africa
DOI: 10.1080/00359191003652116
http://www.informaworld.com
Published online 23 Mar 2010
of these systems for fishes. This paper is a continuation of
the published series on the fish assemblages of South African
estuaries (Harrison 1997a,b; 1998; 1999a,b; James & Harrison,
2008, 2009).
STUDY AREA
The coastline in the vicinity of Port Elizabeth is dominated by
westerly and southwesterly winds throughout the year (Stone
et al., 1998). This coastline borders the southwestern Indian
Ocean and is influenced by the warm, south-flowing Agulhas
Current (Heydorn, et al., 1978). Eleven river outlets intersect
the coast in this geographical sector (Figure 1). Rainfall along
the coastline west of Port Elizabeth occurs throughout the year
with maximum falls usually experienced during spring and
autumn (Kopke, 1988). The coastline that extends from Port
Elizabeth to Port Alfred tends to receive most of its rain in
winter (Kopke, 1988).
MATERIALS AND METHODS
The estuaries between Cape St Francis and Cape Padrone
were sampled during September and October 1995. Each system
was sampled once and took 1–3 days to survey, depending on
the size of the system.
Physico-chemical
During each survey, selected physico-chemical parameters
were measured at various sites within each system. Water
depth and transparency were measured using a 20 cm diameter
70
Transactions of the Royal Society of South Africa
Vol. 65(1): 69–84, 2010
Figure 1. The locality of study estuaries between Cape St Francis and Cape Padrone on the southeast coast of South Africa.
Secchi disc attached to a weighted shot line graduated at
10 cm intervals. Temperature (°C), salinity (‰), conductivity
(mS cm–1), pH and dissolved oxygen (mg l–1) were measured
using a Horiba U-10 Water Quality Checker.
Where water depth permitted (usually >0.5 m), both surface
and bottom waters were measured. The mouth state of each
system at the time of sampling was also noted.
Ichthyofauna
The ichthyofauna of each estuary was sampled using a
30 m × 1.7 m × 15 mm bar mesh seine net with a 5 mm bar mesh
purse and a fleet of gill nets. The gill nets were either 10 m or
20 m in length and 1.7 m in depth and consisted of three equal
sections of 45 mm, 75 mm and 100 mm stretch meshes. Seine
netting was carried out during daylight hours in shallow
(<1.5 m deep), unobstructed areas with gently sloping banks.
Fish caught were identified and measured to the nearest
millimetre standard length (SL) before being released. Where
large catches of a species were made a sub-sample was kept
and returned to the laboratory where they were identified,
measured (mm SL) and weighed to the nearest 1.0 g. The
remaining specimens were counted and the batch weighed.
Specimens that could not be identified were also kept and identified in the laboratory by reference to Smith & Heemstra (1991)
and Skelton (1993). Taxonomic identities of certain species
were adjusted using information provided in Heemstra &
Heemstra (2004). The total species composition, by number
and mass, was calculated for each system. The relative biomass
contribution of each species was calculated using actual
recorded masses as well as masses derived from length–mass
relationships presented in Harrison (2001). Where appropriate,
the length frequency distribution (in 10 mm size classes) was
calculated for the most abundant species within each system.
Using information from Whitfield (1998) the species recorded
were divided into four estuarine association categories:
freshwater, estuarine-resident, estuary-associated marine and
marine species. The relative contribution made by each
category to the total ichthyofaunal assemblage of each system
was calculated in terms of number of species, relative abundance and relative mass.
RESULTS AND DISCUSSION
Kromme
The Kromme Estuary (34°09’S, 24°51’E) is situated approximately 55 km west of Port Elizabeth and is one of the largest
estuaries in the area (Strydom & Whitfield, 2000). The Kromme
River is about 95 km long and drains a catchment area of
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
71
Table 1. Physico-chemical parameters measured in southeast coast estuaries (Cape St Francis – Cape Padrone) during September/October 1995
(S, surface; B, bottom; 999, Secchi disc visible on the bed).
System
Site
Depth
(m)
Temperature
(°C)
Salinity
(‰)
Conductivity
(mS cm–1)
pH
Dissolved oxygen
–1
(mg l )
S
B
S
B
S
B
S
B
S
B
Secchi depth
(m)
Kromme
1
2
3
4
5
6
7
8
9
0.9
3.0
3.0
1.4
2.9
1.2
3.2
3.2
3.4
17.9
17.8
18.3
18.2
18.7
19.6
19.9
19.8
20.6
17.9
17.9
18.3
18.2
18.4
19.1
18.9
19.6
20.1
32.5
32.2
32.0
31.4
30.6
28.4
28.6
26.9
27.7
32.8
32.7
32.0
31.5
30.7
28.4
29.2
28.0
28.5
49.9
49.7
49.0
48.3
46.9
43.8
44.1
42.0
42.9
50.1
50.1
49.0
48.4
47.4
44.5
45.3
43.7
44.4
7.8
7.8
7.8
7.8
7.8
7.5
7.8
7.8
7.8
7.9
7.9
7.8
7.8
7.8
7.5
7.8
7.8
7.7
7.7
7.5
6.9
6.4
6.6
5.2
6.5
6.6
6.6
7.7
7.7
7.0
6.5
6.5
4.3
6.3
6.5
5.0
999
1.3
0.9
0.9
0.9
0.6
1.7
2.1
1.9
Seekoei
1
2
3
4
1.0
0.5
1.6
1.2
17.6
20.4
18.7
20.8
17.7
20.4
18.7
19.8
9.0
6.9
4.6
3.8
9.3
6.9
4.6
3.9
15.4
12.0
8.4
7.1
15.9
12.1
8.5
7.3
8.3
8.8
8.4
9.2
8.2
8.8
8.5
9.4
8.0
12.6
8.9
11.5
9.7
12.4
8.9
12.9
999
999
0.6
0.9
Kabeljous
1
2
3
1.0
0.9
1.1
17.2
17.5
17.9
17.3
17.6
18.4
17.1
16.5
15.2
16.9
16.5
16.5
27.5
26.7
24.7
27.4
26.9
26.8
8.0
7.9
8.1
8.0
7.9
8.0
6.2
4.6
7.1
5.0
4.1
5.7
999
999
999
Gamtoos
1
2
3
4
5
6
7
8
1.3
1.1
2.6
2.6
1.9
1.1
1.1
1.0
18.5
18.6
18.6
20.0
20.2
19.4
19.7
19.6
18.5
18.6
18.0
19.2
19.4
19.4
19.4
19.4
31.4
29.6
18.8
22.8
15.0
10.7
7.4
3.1
31.4
29.8
23.8
28.6
28.0
24.5
8.8
3.1
48.2
45.7
30.3
35.9
22.2
16.3
12.9
5.86
48.2
45.9
37.5
44.5
43.8
39.0
15.2
5.92
7.8
8.0
8.1
8.1
8.3
8.4
8.3
8.2
7.9
8.0
8.0
7.9
7.9
7.6
8.0
8.2
7.8
7.6
8.4
8.4
9.5
10.3
11.6
9.8
7.8
7.7
8.1
7.2
6.3
3.3
5.8
9.7
0.3
1.0
0.9
1.0
0.7
0.6
0.5
0.6
Van Stadens 1
2
3
4
2.6
3.3
1.4
1.5
19.5
20.5
21.4
22.0
18.5
20.6
21.4
21.6
15.0
15.0
15.2
10.5
15.1
15.3
15.1
15.2
24.5
24.6
24.7
16.2
24.8
25.1
24.9
25.1
7.8
7.8
7.6
7.3
7.8
7.7
7.6
7.3
7.5
6.9
6.0
5.3
7.2
7.0
5.7
3.1
999
2.5
999
999
Maitland
1
0.3
22.7
Bakens
1
2
3
1.0
0.2
0.2
19.2
19.0
20.5
19.2
29.8
9.6
0.7
31.7
46.2
20.1
1.6
48.6
7.8
7.5
8.1
6.8
999
999
999
18.2
18.4
18.5
18.5
18.7
19.1
32.7
32.9
31.5
30.2
27.0
25.2
32.7
32.9
31.6
30.2
28.6
27.0
50.0
50.3
48.3
46.5
41.9
39.5
50.0
50.3
48.4
46.5
44.3
42.0
7.9
7.9
7.7
7.7
7.5
7.4
Papkuils
1
0.1
24.0
Swartkops
1
2
3
4
5
6
2.3
0.6
2.4
2.5
2.0
3.5
18.2
18.4
18.5
18.5
18.8
19.2
1.3
2.6
9.7
8.4
15.7
12.9
7.9
6.2
9.1
15.9
7.9
7.9
7.8
7.7
7.6
7.5
6.8
7.7
6.3
6.1
5.6
5.0
7.4
999
3.4
999
6.9
7.8
6.5
6.2
5.3
5.0
1.1
999
1.0
1.2
1.3
1.6
Coega
1
0.7
13.4
13.3
47.2
47.2
61.6
61.7
8.2
8.2
7.2
7.1
999
Sundays
1
2
3
4
5
6
7
1.2
3.5
2.1
3.5
1.9
2.5
2.2
19.1
20.0
21.2
21.9
21.5
22.1
21.9
18.9
19.1
19.6
19.8
20.2
20.6
20.4
32.8
22.6
15.4
13.9
10.4
6.5
3.4
32.8
24.3
26.6
24.8
21.7
15.1
3.2
50.2
35.6
25.0
22.7
17.6
11.5
6.4
50.1
38.0
42.1
39.0
34.7
24.8
6.0
8.1
8.0
8.1
8.1
8.2
8.4
8.5
8.1
8.0
7.8
7.9
7.9
8.1
8.5
8.7
7.9
8.3
9.0
8.8
8.9
9.9
8.9
6.6
4.9
5.5
5.4
5.9
8.1
0.5
0.7
0.6
0.5
0.4
0.4
0.4
approximately 936 km2 (Reddering & Esterhuysen, 1983).
Numerous tributaries enter the Kromme, with the main tributary being the Geelhoutboom that flows into the estuary about
9 km upstream of the mouth (Bickerton & Pierce, 1988). There
are two large dams in the catchment and these severely restrict
freshwater input into the estuary. The Kromrivier Dam, which
was completed in 1943, is situated approximately 35 km from
the head of the estuary, while the Mpofu Dam, completed in
1982, is situated 4 km from the tidal head. Together these dams
have the capacity to store about 133% of the mean annual
runoff of the Kromme River catchment (Scharler & Baird,
2000). In addition, there are also numerous small farm dams
72
Transactions of the Royal Society of South Africa
within the catchment that further restrict the flow of freshwater
into the estuary (Bickerton & Pierce, 1988). The upper part of
the estuary is narrow and is bounded on either side by rocky
cliffs but nearer the sea the banks become less steep and the
system widens before discharging into St Francis Bay (Marais,
1983a). A well-developed flood-tidal delta is present in the
lower reaches, and these sand banks are exposed at low tide
(Bickerton & Pierce, 1988). The Kromme Estuary is a popular
recreational area; a marina development is situated near the
mouth and numerous holiday houses have been built further
upstream. Two road bridges cross the estuary approximately
3 km and 15 km from the mouth, respectively. According to
Whitfield (2000) the state of information on the Kromme is
regarded as excellent.
Physico-chemical
Physico-chemical parameters were measured at eight sites
from the mouth to the head of the estuary; one site (Site 6) was
located in the inlet of the Geelhoutboom tributary. The mouth
of the Kromme Estuary is permanently open; the presence of a
flood-tidal delta in the lower reaches of the system, together
with the low river inflow, indicates that tidal currents serve to
maintain a connection with the sea. The channel depth during
this survey ranged from 0.9 m near the mouth to 3.4 m recorded
at the uppermost site (Table 1). Bickerton & Pierce (1988) reported
similar water depths of between 1.5 and 3.5 m in the system.
Surface water temperatures during this survey ranged
between 17.8°C recorded in the lower reaches and 20.6°C at the
uppermost site; bottom water temperatures showed a similar
pattern and ranged between 17.9 and 20.1°C (Table 1). Overall,
surface values were slightly higher than those at the bottom
and this is probably due to solar radiation warming the surface
waters. The influence of cooler marine waters also probably
accounted for the horizontal thermal gradient recorded. Day
(1981) reports a seasonal temperature range of between 14.0
and 24.0°C for the Kromme Estuary. According to Bickerton &
Pierce (1988) surface water temperatures in the system range
from 12.0°C in winter to 28.0°C in summer. Average summer
temperatures recorded by Scharler et al. (1997) measured
between 20.0 and 24.0°C, while winter temperatures averaged
17.0–18.0°C.
Both surface and bottom salinities recorded during this survey
exhibited a horizontal salinity gradient and decreased from the
mouth upstream (Table 1). Surface salinities ranged between
32.5 and 26.9‰; bottom salinities were slightly higher and
measured between 32.8 and 28.0‰. According to Hanekon &
Baird (1984) salinities in the Kromme Estuary are normally
about 33.0‰ but can decline to as low as 1.0‰ during winter
floods. Extensive water abstraction in the catchment, however,
has resulted in high salinities in the estuary with occasional
incidences of hypersaline conditions developing in the upper
reaches (Bickerton & Pierce, 1988; Scharler & Baird, 2000).
Average salinities in the Kromme vary between 26.0 and 33.0‰
(Emmerson & Erasmus, 1987; Scharler et al., 1997). Salinties
above 28.0‰ have been recorded throughout the estuary, even
following heavy rain and the release of freshwater from the
Mpofu Dam (Bickerton & Pierce, 1988; Heymans & Baird, 1995;
Scharler & Baird, 2000; Strydom & Whitfield, 2000).
The pH of the water in the Kromme Estuary ranged from 7.5
to 7.8 (Table 1). Heymans & Baird (1995) recorded an annual
mean pH of 8.0 reflecting the strong marine influence. Hecht
(1973) found that the pH of Kromme Estuary water ranged
between 7.9 and 8.4. The dissolved oxygen of both the surface
and bottom waters decreased from the mouth upstream
(Table 1). Surface water dissolved oxygen concentrations
Vol. 65(1): 69–84, 2010
ranged from 6.4 to 7.7 mg l–1 while those at the bottom ranged
from 5.0 to 7.7 mg l–1. Surface water dissolved oxygen concentrations, particularly those above the Geelhoutboom tributary
were also slightly higher than those at the bottom and this
is probably a reflection of salinity stratification and lack of
mixing. Hecht (1973) found little variation between surface and
bottom waters, with surface values ranging from 5.7 to 7.9 mg
l–1. Overall, the waters of the Kromme Estuary are well oxygenated with dissolved oxygen values generally exceeding 6.0 mg
l–1 (Emmerson & Erasmus, 1987; Bickerton & Pierce, 1988;
Heymans & Baird, 1995; Scharler et al., 1997). The waters in the
estuary were relatively clear with Secchi disc values generally
exceeding 0.9 m (Table 1). This is probably a result of the strong
marine influence in the system. Other workers also found the
waters in Kromme Estuary to be fairly clear with mean Secchi
disc values measuring between 0.8 and 1.8 m (Marais, 1984;
Bickerton & Pierce, 1988; Scharler et al., 1997).
Ichthyofauna
Eighteen seine net hauls and nine gill nets captured a total of
4101 individuals representing 30 species from 15 families.
Mugilidae (6 species), Gobiidae (5 species) and Sparidae
(4 species) dominated the taxa. Atherina breviceps was the most
abundant species caught comprising 41.3% of the total catch
followed by Rhabdosargus holubi (14.7%), Gilchristella aestuaria
(13.9%), Liza dumerili (6.2%), Glossogobius callidus (4.8%), Liza
richardsonii (4.2%), juvenile mugilids (3.1%), Psammogobius
knysnaensis (2.7%), Lithognathus lithognathus (2.1%) and Liza
tricuspidens (1.1%). Together these species comprised 94% of
the total catch numerically (Table 2). A total species mass of over
95 kg was caught. Liza richardsonii was the dominant species
comprising 28.7% of the overall biomass, followed by Lichia
amia (23.1%), Pomadasys commersonnii (11.2%), Liza tricuspidens
(9.8%), Galeichthys feliceps (6.1%), Mugil cephalus (4.8%), L.
dumerili (3.9%), R. holubi (3.8%), A. breviceps (1.9%), Argyrosomus
japonicus (1.8%) and Platycephalus indicus (1.1%) (Table 3).
A total of 36 species has been recorded in the Kromme Estuary
from various surveys using either gill nets and/or seine netting
(Baird et al. 1981; Marais, 1983a; Bickerton & Pierce, 1988;
Hanekom & Baird, 1988). Twenty-four species (67%) were
common to those recorded during this survey. Many of the
important species reported were also similar to those that
dominated during this survey. Dominant taxa either numerically and/or in terms of biomass included A. japonicus, A. breviceps, Caffrogobius gilchristi, G. feliceps, G. aestuaria, G. callidus,
L. amia, L. dumerili, L. richardsonii, L. tricuspidens, Monodactylus
falciformis, P. commersonnii, P. knysnaensis and R. holubi (Marais,
1983a; Hanekom & Baird, 1984; Bickerton & Pierce, 1988).
Of the species collected in the Kromme during this survey,
nine were species that are able to live and breed in estuaries;
these included A. breviceps, Caffrogobius nudiceps, C. gilchristi,
Caffrogobius natalensis, Clinus superciliosus, G. aestuaria, G.
callidus, P. knysnaensis and Syngnathus temminckii. Atherina
breviceps captured during this survey measured between 29
and 58 mm; the majority of these were mature individuals,
which is estimated to occur at a size of 40 mm SL (Ratte, 1989).
Psammogobius knysnaensis ranged in size from 17 to 48 mm and
most of these were also mature individuals in the 30–40 mm
size class; P. knysnaensis matures at a length of approximately
30 mm SL (Bennett, 1989).
The remaining species captured during this survey were
estuarine-associated marine species whose juveniles are
dependent on estuaries to varying degrees. These included
A. japonicus, Diplodus hottentotus, Diplodus capensis, G. feliceps,
Heteromycteris capensis, L. amia, L. lithognathus, L. dumerili, Liza
Table 2. Relative abundance (%n) of fishes captured in all net types (seine (S) and gill (G) nets) in southeast coast estuaries (Cape St Francis – Sundays) during September/October 1995.
Species
Kromme
S
G
Seekoei
%n
S
G
Kabeljous
%n
S
G
%n
Gamtoos
S
G
Van Stadens
%n
S
G
%n
Maitland
S
G
Bakens
%n
S
G
Swartkops
%n
S
G
%n
8
0
0.2
0
3
0.1
51
0
1.1
S
G
Sundays
%n
0.7
57
0
1.6
2.3
95
0
2.7
0.4
1
0
0.0
0
1
0.0
0.0
Atherina breviceps
1695
0
41.3
Caffrogobius gilchristi
12
0
0.3
110
0
Caffrogobius natalensis
12
0
0.3
19
0
Caffrogobius nudiceps
13
0
0.3
6
0
0.1
Clinus superciliosus
25
0
0.6
4
0
0.1
Diplodus sargus capensis
10
0
0.2
29
0
0.6
Diplodus cervinus hottentotus
1
0
0.0
21.8
1529
0
68.0
0.6
219
0
5.1
31
0
0.7
141
0
8.2
4
0
4.8
1895
0
96.8
Carcharias taurus
23
Elops machnata
Galeichthys feliceps
0
12
0.3
Gilchristella aestuaria
570
0
13.9
345
0
28.9
Glossogobius callidus
198
0
4.8
31
0
2.6
1
0
0.0
0
0.5
1
0
0.1
1
0
1.2
0.1
0
23
0
109
2.6
0
16
0.3
0
8
0.2
1136
0
26.6
1
0
0.1
1389
0
28.7
30
0
1.5
1160
0
32.7
8
0
0.2
17
0
1.0
69
0
1.4
1
0
0.1
6
0
0.2
2
0
0.0
29
0
0.6
5
0
0.3
20
0
0.6
0
5
0.1
0
1.0
2
0
0.2
29
0
0.7
25
0.6
0
3
0.3
0
3
0.1
0
3
0.1
0
4
0.2
Lithognathus lithognathus
85
1
2.1
190
7
16.5
118
2
5.3
106
1
2.5
193
4
11.5
Liza dumerilii
245
9
6.2
10
10
1.7
676
14
16.2
15
0
0.9
Liza macrolepis
1
0
0.0
Liza richardsonii
99
75
4.2
1
10
0.9
Liza tricuspidens
26
18
1.1
43
Mugil cephalus
17
7
0.6
5
32
Juvenile mugilids
128
0
3.1
259
0
Myxus capensis
0
3
0.1
Monodactylus falciformis
9
4
0.3
184
70
11.3
483
36
12.1
1
47
2.8
4
0
0.2
1
14
0.4
0
3
0.2
3.1
21
4
1.1
137
3
3.3
38
0
2.2
21.7
261
0
11.6
340
0
8.0
922
0
1
5
0.3
25
6
0.7
79
0
1
4
0.2
1
2
0.1
5
3
0.5
5
0.4
0.6
5
0
0
0.0
0
40
Oreochromis mossambicus
0.1
0.1
Lichia amia
0.2
0
0
4
Heteromycteris capensis
2
4
1
0
Hemiramphus far
0
0.0
0
14
2
0
25
%n
1
0
260
0
G
12
Amblyrhynchotes honckenii
Argyrosomus japonicus
S
40
2
0
0
6.8
0.3
0
51.2
8
0
9.5
83
0
1.7
92
1
2.6
3
0
3.6
232
3
4.9
194
4
5.6
3
0
3.6
284
7
6.0
211
18
6.5
3
13
0.3
2
35
1.0
139
19
4.5
618
0
17.4
0
1
0.0
15
1
0.5
516
0
88.2
4
0
4.8
10
2
0.2
53.7
2
0
0.3
4
0
4.8
663
0
13.7
4.6
16
0
2.7
4
0
12
5
0
0
0.6
0.3
0.1
Platycephalus indicus
0
1
0.0
0
1
0.0
3
0
0.1
1
0
0.0
Pomadasys commersonnii
17
15
0.8
38
4
1.0
750
6
15.6
131
11
4.0
Pomatomus saltatrix
0
4
0.1
Psammogobius knysnaensis
112
0
2.7
Rhabdosargus globiceps
5
0
0.1
Rhabdosargus holubi
599
2
14.7
Sarpa salpa
2
0
0.0
Solea turbynei
1
0
0.0
Syngnathus temminckii
1
0
0.0
Total individuals
3923
178
1119
73
2159
89
4047
225
1657
61
585
0
84
0
4781
60
1957
Total nets
18
9
11
5
13
5
16
15
11
5
5
0
3
0
19
14
5
9
7
0
4
0.8
0.9
3
36
0
1
0.1
1.6
1
1
0.0
52
0
1.2
76
0
1.8
650
2
15.3
1
0
0.1
242
0
14.1
1
0
0.1
1
8
0
0
0.2
1.4
11
3
0
0
Sillago sihama
15
0
0.4
13.1
3.6
2
0
0.0
53
0
1.1
127
0
2.6
824
0
17.0
18
0
0.4
1
0
0.0
8
0
0.2
5
4
0
0
0.3
0.2
31
0
0.9
50
0
1.4
505
1
14.3
66
0
1.9
1
0
0.0
0
3413
137
0
16
14
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
Ambassis dussumieri
Coega
73
74
Table 3. Relative biomass ( %g) of fishes captured in all net types (seine (S) and gill (G) nets) in southeast coast estuaries (Cape St Francis – Sundays) during September/October 1995.
Species
Kromme
S
G
Seekoei
%g
S
G
Kabeljous
%g
S
G
Gamtoos
%g
S
G
Van Stadens
%g
S
G
Maitland
%g
S
G
Bakens
%g
S
G
Swartkops
%g
Ambassis dussumieri
Coega
S
G
%g
2.1
0
0.0
0
2215
4.3
S
G
Sundays
%g
Amblyrhynchotes honckenii
0
1766
1.8
Atherina breviceps
1848
0
1.9
Caffrogobius gilchristi
17.5
0
0.0
Argyrosomus japonicus
0
261.5
0
1.0
1395
0
3.0
30545 17.9
189.3
0
0.1
28.9
0
0.0
298.7
0
0.9
5.2
0
2.4
S
G
%g
9.9
0
0.0
428.5 9364.4 7.3
79.3
0
0.2
130.0
0
0.1
125.5
0
0.2
1597
0
91.2
126.2
0
0.1
0.3
0
0.0
Caffrogobius natalensis
9
0
0.0
16.2
0
0.0
Caffrogobius nudiceps
23
0
0.0
16.0
0
0.0
Clinus superciliosus
39.4
0
0.0
8.8
0
0.0
Diplodus sargus capensis
1.6
0
0.0
11.7
0
0.0
Diplodus cervinus hottentotus
0.4
0
0.0
Galeichthys feliceps
0
5884
6.1
Gilchristella aestuaria
256.6
0
0.3
144.7
0
0.5
747.5
0
0.4
1.2
0
0.0
1285.6
0
2.5
20
0
1.1
1054
0
0.8
Glossogobius callidus
208.8
0
0.2
45.5
0
0.2
19.9
0
0.0
25.4
0
0.1
79.2
0
0.2
5.1
0
0.3
8.26
0
0.0
0.0
Heteromycteris capensis
19.4
0
0.0
1.8
0
0.0
17.3
0
0.0
2.2
0
0.1
11.8
0
0.0
Lichia amia
0
22206 23.1
0
1375
5.1
0
4999 10.8
0
863
0.5
0
4047
12.2
85.0
1390 3.2
1857 1237
1.8
55.6
219
0.8
7520 2056
5.6
52.0
0
0.2
0
Carcharias taurus
6.1
13733
0
0.9
0
0.0
0.0
0.2
0
0.0
0.4
0
8.1
0.2
0
62760 36.9
Hemiramphus far
Lithognathus lithognathus
207.0
49
0.3
56.1
3614
13.5
Liza dumerilii
2604 1115
3.9
1187 1291
9.1
Liza macrolepis
612.9
0.6
3.4
10.2
0
2.6
0
1.2
115.6
0
0.2
157.9
50
0.2
0
13.1
3300
417
7.2
2718
641
2.5
119
0
56.0
2717
2701
10.5
20.7
3849
7.5
4.8
12.5 13886 51.1
16.3
0.3
28.4
453.9
Myxus capensis
0
952
1.0
Monodactylus falciformis
6.1
198
0.2
0
1076
1.1
0
979
0.6
26.0
0
0.1
9.6
526.8 10215 11.2
82.4
2459
1.5
721.8
4754
10.6
360.3
Pomadasys commersonnii
0
438
0.5
Psammogobius knysnaensis
92.2
0
0.1
Rhabdosargus globiceps
18.8
0
0.0
Rhabdosargus holubi
3491.1 166
3.8
1.0
0
0.0
Solea turbynei
2.3
0
0.0
Syngnathus temminckii
1.5
0
0.0
Total mass
13452 82484
Sarpa salpa
7.3
13.7
165.8
0
170
0.1
1.2
1643 2359
2.4
13.6
0
0.0
357.3
0
95.3
18.8
0
8.8
44.7
961
1.9
1.0
590.1
0
0.3 1492.4
0
4.5
0.2
0
0.0
0.8
0
0.4
1516
0
2.9
0.2
2344 5.0
5.5
2476
1.5
91.3
0
0.3
3.8
0
1.0
0.1
233
0.8
144
0.1
0.9
79
0.2
10.3
6.7
0
0
43
0.5
0.0
0.1
1.69
98
0.1
62.1
0
0.0
125.2
0
0.1
6067
74
3.6
1.2
0
0.0
116.4
0
0.4
0.5
0
0.0
11.4
1.3
0.9
0
0
0.4
0.2
9.2
16.4
0
0
Sillago sihama
Total nets
18
9
24.73
1879 25149
6466 39964
11
13
5
5
0
25169 145024
16
15
0.0
2528 30737
375
0
212
0
11
5
0
3
0
5
4.3
7.7
0
5.8
0
0.0
0
0.1
267.8
0
0.5
2614
0
5.0
14.1
0
0.0
2.3
0
0.0
19.7
0
0.0
19
14
3.8
0
0
6.1
0.2
0.0
52.5
13098 38725
106
15.4
2.4
0
0
1751
0
5
0
0.9
0.1
1991
6088 6.0
14.9
33304 24.8
145.7
9300 7.0
1005
0
0
329
0.7
0.2
34.0
60
0.1
0
0.0
5085 4.0
34.4
0
0.0
195.8
0
0.1
2595
64.7
2.0
134.1
0
0.1
0.9
0
0.0
11170 123393
16
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Vol. 65(1): 69–84, 2010
Pomatomus saltatrix
1994
5204 11.2
0.8
34.9
4090 3.0
27.8
0
0.0
0
4.6
645.3 3999
0
3.0
2369
303.9
0
10.3
0.0
0
Juvenile mugilids
Platycephalus indicus
3425
0
Mugil cephalus
0.2
0
0
0
9.8
59
7.0
6.7
17.6
47017
2013 25533 28.7
0
99.0 11880
2.3
0
18.1
502.8 8887
0.1
6081 13361 11.4 378.4 22967 70.2
16.2
0
5.8
9370
0.0
0.0
Liza tricuspidens
0
4481 25751 65.1
8.5
0
23.3
0
0
0
Liza richardsonii
Oreochromis mossambicus
2760
12.3
12089
4.3
0.4
Transactions of the Royal Society of South Africa
0
Elops machnata
0
8000 5.9
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
macrolepis, L. richardsonii, L. tricuspidens, M. falciformis,
M. cephalus, Myxus capensis, P. indicus, P. commersonnii,
Pomatomus saltatrix, Rhabdosargus globiceps, R. holubi, Sarpa salpa
and Solea turbynei.
Lithognathus lithognathus ranged in size from 18 to 130 mm SL.
Most of the individuals were newly recruited and fell in the
20–30 mm size class; a few larger individuals (>80 mm) were
also present. Lithognathus lithognathus enters estuaries along
the southeastern, southern and southwestern Cape coasts at
sizes below 50 mm TL (Bennett, 1993). The predominance of
small juveniles indicates recent recruitment into the system.
Liza dumerili ranged in size from 45 to 234 mm with two size
cohorts (60–70 mm and 170–180 mm) predominating; this also
suggests regular recruitment and utilisation of the system.
Recruitment of L. dumerili into estuaries on the southern Cape
coast occurs at <30 mm TL (Whitfield & Kok, 1992). Liza
richardsonii ranged in size between 44 and 340 mm and showed
a polymodal length frequency distribution; the first size class
comprised individuals of 60–70 mm, the second size class was
120–130 mm and the third size class was 280–290 mm. The
range of size cohorts captured indicates regular recruitment
and utilisation of the system. Juvenile L. richardsonii recruit into
estuaries on the southwestern and southern Cape coasts at
about 20 to 50 mm TL (Bennett, 1989; Whitfield & Kok, 1992)
and a length of about 75 mm FL is attained after a year (Ratte,
1989). Rhabdosargus holubi individuals ranged in size from 12 to
160 mm with most individuals less than one year old in the
50–60 mm size classes; R. holubi attains a length of approximately 100 mm SL by the end of their first year (Beckley, 1984).
The predominance of small individuals indicates that the
Kromme serves as an important nursery area for this species.
During this survey estuarine-resident species comprised
66.4% of the total catch numerically and 2.6% of the biomass,
while estuarine-associated marine species comprised 33.6% of
the catch numerically and 97.4% of the biomass. The high
proportion of estuarine and marine species in the Kromme
Estuary suggests that it serves an important function as an
estuarine nursery area for both resident and marine species.
This is also supported by the high numbers of mature
estuarine-resident species as well as the occurrence of a range
of size classes of estuarine-associated marine species.
Seekoei
The Seekoei (34°05’S, 24°54’E) is situated near the coastal
town of Jeffrey’s Bay. The river has a total length of approximately 35 km and a catchment area of 250–502 km2 (Bickerton
& Pierce, 1988). The main tributary, the Swartrivier, joins the
Seekoei approximately 1 km upstream of the mouth so that the
estuary consists of two arms; with the main arm formed by the
Seekoei River and the secondary arm by the Swartrivier
(Bickerton & Pierce, 1988). Numerous farm dams have been
built in the catchment, resulting in reduced river flow to the
estuary. The lower reaches of the estuary have been severely
modified. The mouth dynamics of the system were changed
with the construction of a protective embayment and parking
area at the coast, followed by a swimming pool complex
(Bickerton & Pierce, 1988). This caused a southerly deflection of
the position of the mouth and as a consequence the estuary
drained completely when open (Dundas, 1994). In an attempt
to retain water in the estuary a causeway, which also carried
a road, was built about 700 m upstream of the mouth. This
causeway only had a narrow opening in the centre and was
extremely restrictive to river flow (Dundas, 1994). Since this
survey was undertaken, however, the opening in the causeway
has been increased (DWAF, 2006). In addition to the causeway,
75
a concrete canal was constructed in an attempt to fix the position of the mouth at its historical location. This canal was artificially maintained and permitted limited tidal exchange to take
place; this practice has stopped, however, and the canal has
subsequently become blocked with sand and is no longer kept
open (DWAF, 2006). The state of information on the Seekoei
Estuary is considered good (Whitfield, 2000).
Physico-chemical
Physico-chemical parameters were measured at four locations
in the Seekoei Estuary from the mouth to the upper reaches of
the system. The mouth of the estuary was closed at the time of
sampling. According to anecdotal information the mouth of
the Seekoei was historically predominately open before major
anthropogenic disturbance. However, the deflection of the
estuary mouth and the construction of the causeway and
concrete canal caused the mouth to close for extended periods
and to stay open for only a few days (Bickerton & Pierce, 1988).
More recently, since this survey was undertaken, the increase
in the opening in the causeway and the cessation of artificially
maintaining the mouth via the concrete canal has allowed the
estuary to remain open for much longer periods of up to 18
months even when river flow is very low (DWAF, 2006).
The channel water depth during this survey ranged between
0.5 and 1.6 m (Table 1). Bickerton & Pierce (1988) also reported
depths of between 0.8 and 1.6 m in November 1984. Both
surface and bottom temperatures measured between 17.6 and
20.8°C and generally increased from the mouth upstream
(Table 1). Surface temperatures were only slightly warmer
than those at the bottom at the uppermost site (Site 4). Similar
conditions were also reported in spring (November 1984)
where water temperatures increased in an upstream direction
and ranged between 17.5 and 18.6°C (Bickerton & Pierce, 1988);
similarly, there was little difference recorded between surface
and bottom waters. Both surface and bottom salinities were
fairly uniform and decreased from 9.3‰ recorded at the
mouth to 3.9‰ at the uppermost site (Table 1). Salinities reported by Bickerton & Pierce (1988) were relatively constant
throughout the estuary, ranging between 26.0 and 27.0‰.
Salinity can be extremely variable in the Seekoei Estuary, with
severe hypersaline conditions sometimes developing in the
upper reaches (DWAF, 2006). Dundas (1994) measured
salinities of over 45.0‰ in summer (October–March 1992) and
this was attributed to high evaporation rates.
The pH of the water ranged between 8.2 and 9.4 and did not
vary much between surface and bottom waters (Table 1).
Bickerton & Pierce (1988) reported most sites having a pH typical of seawater (8.0) with the exception of the uppermost site
(6.8) and this was attributed to a more fluvial influence. Dissolved oxygen concentrations ranged between 8.0 and 12.8 mg
l–1 (Table 1). Dissolved oxygen concentrations of between 7.4
and 7.9 mg l–1 were recorded in November 1984, with bottom
and surface measurements being similar and this was attributed to wind-induced mixing (Bickerton & Pierce, 1988). The
waters of the Seekoei were relatively clear with the bed of the
system visible at the lower sites (Sites 1 & 2); Secchi disc measurements for the remaining sites measured between 0.6 and
0.9 m (Table 1). Bickerton & Pierce (1988) also found that the
Secchi disc readings in the system were mostly above 0.7 m.
Ichthyofauna
Eleven seine net hauls and five gill nets caught a total of 1192
individuals representing 13 species from nine families. Mugilidae
(3 species), Sparidae (2 species) and Gobiidae (2 species) dominated the taxa. Catches were numerically dominated by
76
Transactions of the Royal Society of South Africa
G. aestuaria, which comprised 28.9% of the catch, followed by
A. breviceps (21.8%), juvenile mugilids (21.7%), L. lithognathus
(16.5%), M. cephalus (3.1%), G. callidus (2.6%) and L. dumerili
(1.7%) (Table 2). A total species mass of over 27 kg was caught
and this was dominated by M. cephalus (51.1%), L. lithognathus
(13.6%), L. richardsonii (10.2%), L. dumerili (9.2%), Oreochromis
mossambicus (7.4%), L. amia (5.1%) and R. holubi (1.2%) (Table 3).
Based on the results of a gill net survey (Bickerton & Pierce,
1988) and a study using seine and gill nets (Dundas, 1994), a
total of 24 species have been recorded in the Seekoei Estuary.
Thirteen of these species (54%) were captured during this
survey. Important species, in terms of abundance and/or biomass included A. breviceps, G. feliceps, G. aestuaria, L. amia, L.
lithognathus, L. dumerili, L. richardsonii, M. falciformis, M. cephalus
and R. holubi (Bickerton & Pierce, 1988; Dundas, 1994).
Four estuarine-resident species (A. breviceps, G. aestuaria, G.
callidus and P. knysnaensis) were captured during this survey.
Atherina breviceps ranged from 27 to 57 mm in size; the predominant size classes included both immature (30–40 mm) and mature (40–50 mm) individuals. Gilchristella aestuaria measured
between 26 and 40 mm; most specimens were mature individuals in the 30–50 mm size classes; G. aestuaria are estimated to
mature at a size of 28 mm SL (Talbot, 1982). Glossogobius callidus
were between 20 and 62 mm; this species matures at approximately 35 mm (Whitfield, 1998) and most specimens during
this survey were mature individuals in the 30–50 mm size
classes. The presence of both mature and immature individuals
of a range of estuarine-resident species suggests that the
Seekoei serves as a viable habitat for this group of fishes.
Eight estuarine-associated marine species were captured; these
were represented by H. capensis, L. amia, L. lithognathus, L.
dumerili, L. richardsonii, M. falciformis, M. cephalus and R. holubi.
Lithognathus lithognathus ranged in size from 19 to 289 mm SL;
most specimens were newly recruited individuals (<40 mm)
although some larger specimens (>240 mm) were also present.
This indicates regular recruitment and utilisation of the system.
Liza dumerili were all large (>180 mm) individuals and
although no juvenile specimens were identified, the presence
of high numbers of juvenile mugilids (mostly <50 mm) suggests that the Seekoei serves a viable nursery function for this
group of fishes. Mugil cephalus ranged in size from 22 to
360 mm. Most specimens were large (>200 mm), but a few
small (<40 mm), newly recruited individuals were also recorded. Recruitment of M. cephalus into estuaries occurs at
<30 mm TL (Wallace & van der Elst, 1975).
Overall, both estuarine-resident species and estuarineassociated marine species dominated the ichthyofauna of
the Seekoei during this survey. Estuarine-resident species
accounted for 31% of the taxa captured while estuarineassociated marine species comprised 62% of the taxa; estuarine
resident species comprised 54.1% of the abundance and 1.7%
of the biomass, while estuarine-associated marine species comprised 45.5% of the abundance and 91.0% of the mass. These
results indicate that despite major anthropogenic disturbance,
the Seekoei Estuary still serves an important nursery function
for estuarine-associated fishes.
Kabeljous
The Kabeljous (34°00’S, 24°56’E) is situated just north of
Jeffrey’s Bay. The river is 30 km long and has a catchment area
of 238 km2 (Reddering & Esterhuysen, 1984). Several tributaries
flow into the Kabeljous and there are a number of farm dams in
the catchment (Bickerton & Pierce, 1988). The upper reaches of
the system are narrow but at the coast the system forms a wide,
shallow expanse of water. The system is relatively undisturbed;
Vol. 65(1): 69–84, 2010
the holiday township of Kabeljous-on-Sea is situated at the
coast on the lower southwestern side of the estuary but there is
little to no development in the middle and upper reaches of
the estuary (Dundas, 1994). Two road bridges cross the Kabeljous between 2 and 3 km upstream of the mouth (Bickerton &
Pierce, 1988). According to Whitfield (2000) the state of information on the Kabeljous is regarded as moderate.
Physico-chemical
Physico-chemical parameters were measured at three locations in the lower, middle and upper reaches of the estuary. The
mouth of the system was closed at the time of sampling. The
Kabeljous is typically separated from the sea by a large,
100–200 m wide sandbar. According to Bickerton & Pierce
(1988) the estuary remains closed for most of the year and normally only opens during floods. The water depth during this
survey ranged between 0.9 and 1.1 m (Table 1). The average
depth of the system has been reported to be 0.5 m, while depths
of between 1.6 and 2.3 m were recorded in the upstream
channel (Bickerton & Pierce, 1988). Surface water temperatures
increased from 17.2°C near the mouth to 17.9°C at the uppermost site; bottom water temperatures were slightly higher and
increased from 17.3°C to 18.4°C in the same direction (Table 1).
Surface and bottom water temperatures measured in November 1984 ranged between 18.5°C and 23.0°C and also generally
increased upstream from the mouth (Bickerton & Pierce, 1988).
Salinities measured during this survey decreased upstream
from the mouth; surface salinities ranged from 17.1 to 15.2‰
and bottom salinities from 16.9 to 16.5‰ (Table 1). Bickerton &
Pierce (1988) reported no differences between surface and
bottom salinities, which ranged between 30.0 and 32.0‰ in
November 1984.
The pH of the water during this survey was fairly uniform
throughout the estuary and measured between 7.9 and 8.1
(Table 1). The pH values recorded by Bickerton & Pierce (1988)
in November 1984 were slightly higher and ranged between 8.4
and 8.6; this is probably a reflection of the higher salinities
recorded at the time. The dissolved oxygen concentrations of
the surface water during this survey ranged between 4.6 and
7.0 mg l–1 and were slightly higher than those at the bottom,
which measured between 4.1 and 5.7 mg l–1 (Table 1). This is
probably a result of a lack of mixing and thermal stratification
of the water column. Dissolved oxygen values reported by
Bickerton & Pierce (1988) in November 1984 were higher than
those recorded during this study and measured between 8.3
and 10.5 mg l–1. The waters in the Kabeljous were clear and the
bed of the system was visible at all sites (Table 1). Bickerton &
Pierce (1988) also recorded high Secchi disc measurements
(>1.6 m) during their survey of the Kabeljous.
Ichthyofauna
A total of 13 seine net hauls and five gill nets caught 2248 individuals representing 11 species from seven families. Mugilidae
(5 species) and Sparidae (2 species) dominated the taxa.
Catches were numerically dominated by A. breviceps, which
comprised 68.0% of the total abundance; this was followed by
juvenile mugilids (11.6%), L. richardsonii (11.3%), L. lithognathus
(5.3%), R. holubi (1.6%) and M. cephalus (1.1%) (Table 2). A total
species mass of over 46 kg was caught and this was dominated
by L. richardsonii (65.1%), M. cephalus (11.2%), L. amia (10.8%),
M. capensis (5.0%), L. lithognathus (3.2%) and A. breviceps (3.0%)
(Table 3).
Based on information provided from a gill net and beam
trawl survey (Bickerton & Pierce, 1988) and the results of a
seine net and gill net study (Dundas, 1994), a total of 23 species
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
have been recorded in the Seekoei Estuary. Eleven of these
species (48%) were also recorded during this survey. The
dominant species either numerically and/or in terms of
biomass included A. japonicus, A. breviceps, G. aestuaria, L. amia,
L. lithognathus, L. richardsonii, L. tricuspidens, M. falciformis,
M. cephalus, O. mossambicus, P. commersonnii, Pomadasys
olivaceum and R. holubi (Bickerton & Pierce, 1988; Dundas,
1994).
Of the taxa collected during this survey, three species (27%)
were estuarine-resident species and these included A. breviceps,
G. aestuaria and P. knysnaensis. Atherina breviceps comprised both
immature and mature individuals between 26 and 68 mm, indicating that the Kabeljous serves an important habitat for
estuarine-resident species. Eight species (73%) were estuarineassociated marine species and these included L. amia, L. lithognathus, L. richardsonii, L. tricuspidens, M. falciformis, M. cephalus,
M. capensis and R. holubi. Lithognathus lithognathus comprised
mostly small individuals in the 20–40 mm size classes, with a
few large individuals (>250 mm) also present. Liza richardsonii
comprised individuals between 40 and 325 mm. Small individuals were mostly in the 40–50 mm size class and large individuals in the 230–240 mm size class, indicating regular use of the
system. Rhabdosargus holubi were mostly newly recruited individuals in the 15–25 mm size classes. Similarly, M. cephalus comprised small individuals in the 20–40 mm size classes, with a
few individuals larger than 250 mm. The high proportion of
newly recruited fish, together with the occurrence of a few
larger specimens indicates that the Kabeljous provides important nursery habitat for estuarine-associated marine species.
Overall, estuarine species comprised 68.2% of the catch
numerically and 3.0% by mass during this survey; estuarineassociated marine species comprised 31.8% of the catch numerically and 97.0% by mass. The high proportion of both marine
and estuarine-resident species indicates that the Kabeljous
serves an important nursery function for these fishes.
Gamtoos
The Gamtoos (33°58’S, 25°04’E) is located approximately
60 km west of Port Elizabeth. The system has a river length of
75 km and a catchment area of 34 438 km2 (Heinecken, 1981).
Most of the catchment area is situated in a region of low precipitation and there are three major dams within the catchment
that absorb and buffer a large proportion of the runoff. Despite
this, occasional floods completely flush the system with freshwater (Schlacher & Wooldridge, 1996). The upper reaches of
the estuary are narrow and flow through steep-sided banks;
the system gradually broadens from the middle reaches towards
the mouth and an extensive flood-tidal delta is situated in the
lower reaches (Heinecken, 1981). Three major structures cross
the system, the old National Road Bridge is located about 7 km
from the mouth, a water pipeline and the N2 National Road
cross the system approximately 5 km from the mouth
(Heinecken, 1981). The state of information on the Gamtoos is
regarded as good (Whitfield, 2000).
Physico-chemical
Physico-chemical parameters were measured at eight
locations in the Gamtoos. Site 1 was located in a tidal side arm
that represents the remains of the former main channel, Site 2
in the estuary mouth and Sites 3–8 in the lower, middle and
upper reaches. The mouth of the Gamtoos is permanently
open and is maintained by tidal currents rather than river flow;
this is supported by the presence of a flood-tidal delta in the
lower reaches (Heinecken, 1981). The channel depth during
this survey ranged between 1.0 and 2.6 m (Table 1). Heinecken
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(1981) reported depths of between 2.0 and 4.0 m in the estuary
channel in August 1980. According to Schlacher & Wooldridge
(1996) the tidal inlet is shallow (<1.5 m); beyond the flood tidal
delta the channel deepens to about 4.0 m in the middle reaches
and then becomes progressively shallower toward the tidal
head (approximately 20 km from the mouth) where the water
depth is less than 1.0 m.
Surface water temperatures during this survey ranged
between 18.5 and 20.2°C, and were generally higher than those
at the bottom, which measured between 18.0 and 19.4°C
(Table 1). Water temperatures tended to increase upstream
from the mouth. In August 1980, surface water temperatures
measured between 17.3 and 17.9°C and also increased in an
upstream direction (Heinecken, 1981). Schlacher & Wooldridge
(1996) found that winter temperatures decreased from 16.6 to
12.2°C in an upstream direction, whereas in summer this trend
was reversed and temperatures increased from 19.1 to 24.8°C
in an upstream direction. Both surface and bottom salinities
during this survey declined from the mouth upstream and
ranged from 31.4 to 3.1‰ (Table 1); surface values were also
generally lower than those recorded near the bottom. Schlacher
& Wooldridge (1996) found that axial salinity gradients in the
Gamtoos can vary dramatically with the magnitude of river
inflow. Marais (1983b) recorded bottom salinities of only 2.0‰
at the mouth when the river was in flood in October 1981; by
contrast, bottom salinity at the mouth averaged 30.0‰ when
the river was not in flood.
The pH of the surface waters during this survey measured
between 7.8 and 8.4, while the pH of the bottom waters ranged
between 7.6 and 8.2 (Table 1). Surface dissolved oxygen concentrations ranged from 7.6 to 11.6 mg l–1 and were higher on the
surface than at the bottom; bottom dissolved oxygen concentrations ranged between 3.3 and 9.7 mg l–1 (Table 1). Secchi disc
measurements during this study were less than 1.0 m (Table 1).
Heinecken (1981) also reported low Secchi disc readings of
0.8 m throughout the estuary.
Ichthyofauna
A total of 24 species from 14 families were caught in 16 seine
hauls and 15 gill nets; 4272 individuals were recorded with
Mugilidae (5 species), Sparidae (4 species) and Gobiidae
(3 species) dominating the taxa. Numerically, catches were
dominated by G. aestuaria (26.6%) followed by L. dumerili
(16.2%), R. holubi (15.3%), L. richardsonii (12.1%), juvenile
mugilids (8.0%), A. breviceps (5.1%), M. cephalus (3.3%),
G. feliceps (2.6%), L. lithognathus (2.5%), R. globiceps (1.8%),
P. knysnaensis (1.2%) and P. commersonii (1.0%) (Table 1). A total
species mass of over 170 kg was recorded with G. feliceps dominating; this species comprised 36.9% of the total biomass
followed by A. japonicus (17.9%), L. richardsonii (11.4%), Elops
machnata (8.1%), L. tricuspidens (7.0%), L. dumerili (5.6%),
R. holubi (3.6%), M. cephalus (2.4%), L. lithognathus (1.8%),
P. commersonnii (1.5%) and M. capensis (1.5%) (Table 3).
Marais (1983b) recorded 22 species from a gill net survey of
the estuary, 14 of which (64%) were recorded during this
survey. Similar species were also among the dominant taxa,
both numerically and in terms of biomass, and these included
A. japonicus, G. feliceps, L. amia and Mugilidae (L. dumerili,
L. richardsonii, L. tricuspidens, M. cephalus and M. capensis)
(Marais, 1983b).
From this survey, five species (20%) live and breed in estuaries;
these are represented by A. breviceps, C. gilchristi, G. aestuaria,
G. callidus and P. knysnaensis. Atherina breviceps ranged between
28 and 59 mm and included both mature and immature
individuals; most specimens fell within the 30–40 mm size class.
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Transactions of the Royal Society of South Africa
Gilchristella aestuaria measured 19–60 mm and the majority of
individuals were mature (>30 mm). All P. knysnaensis individuals
were mature and measured between 30 and 52 mm. The
presence of both juvenile and adult estuarine-resident species
indicates that the Gamtoos Estuary is an important habitat for
these fishes.
Nineteen estuarine-associated marine species were captured
during this survey. These included A. japonicus, D. capensis,
E. machnata, G. feliceps, H. capensis, L. amia, L. lithognathus,
L. dumerili, L. richardsonii, L. tricuspidens, M. falciformis,
M. cephalus, M. capensis, P. indicus, P. commersonnii, P. saltatrix,
R. globiceps, R. holubi and S. turbynei. The majority of G. feliceps
individuals during this survey were between 300 and 310 mm.
This species has a unique reproductive cycle in that the male
mouthbroods the eggs; mature males (>260 mm SL) with eggs
frequent estuaries during spring and summer (Whitfield,
1998). All G. feliceps specimens captured were mouthbrooding
males, which indicates that the Gamtoos is an important nursery for this species. Two size classes of L. lithognathus were
caught, the first was between 20 and 30 mm and the second
between 130 and 140 mm; this indicates regular recruitment
and utilisation of the system. Liza dumerili ranged in size from
31–245 mm; most specimens were small individuals in the
60–70 mm size class, which suggests recent recruitment. Two
size classes of L. richardsonii were caught; the first comprised
individuals of 80–90 mm and the second individuals >200 mm.
This also indicates regular utilisation of the estuary. Most
R. holubi captured were small, recently recruited individuals
between 40 and 50 mm. The presence of both small, recently
recruited individuals and larger specimens of estuarineassociated marine species indicates that the Gamtoos is an
important nursery for this group of fishes.
Both estuarine-resident and estuarine-associated marine
species dominated the fish fauna of the Gamtoos Estuary.
Estuarine-associated marine species comprised 66.2% of the
total catch numerically and 99.4% by mass, while estuarineresident species comprised 33.9% of the catch numerically and
0.6% by mass. These results indicate that the Gamtoos Estuary
serves an important habitat for both resident and estuarineassociated marine species.
Van Stadens
The Van Stadens (33°58’S, 25°13’E) is situated about 32 km
west of Port Elizabeth. The river is 25.8 km long with a catchment area of 271 km2 (Dundas, 1994). The estuary flows between
relatively steep banks for most of its length and shallow littoral
areas are limited. The Van Stadens forms part of a holiday resort
and a caravan park is situated near the coast on the eastern
shore of the system. All development, however, is set away
from the banks of the system, which are relatively undisturbed
(Dundas, 1994). A water pipeline crosses the system about
500 m from the mouth and the N2 National Road crosses the
river approximately 11 km from the mouth. The state of information on the Van Stadens is regarded as poor with limited
biological information (Whitfield, 2000).
Physico-chemical
Physico-chemical parameters were measured at four sites in
the Van Stadens from the mouth to the upper reaches. The
mouth of the system was closed at the time of sampling. The
estuary mouth is separated from the sea by a 50 m wide beach
with a low berm. Freshwater inflow can result in the sand
barrier being breached although high seas overwashing and
lowering the sand bar can also cause the mouth to open
(Dundas, 1994). The water depth during this survey ranged
Vol. 65(1): 69–84, 2010
between 1.5 and 3.3 m, with the deepest section in the lower
reaches; water depths at Sites 1 and 2 exceeded 2.5 m (Table 1).
Surface water temperatures increased from 19.5°C at the
mouth to 22.0°C in the upper reaches. Bottom water temperatures were similar, increasing from 18.5°C to 21.6°C in the same
direction (Table 1). Dundas (1994) recorded a mean surface
temperature of 24.0°C in summer (1992), with temperatures
also increasing from the lower to the upper reaches. Surface
salinities ranged between 10.5 and 15.2‰ with the lowest
value recorded at the uppermost site (Site 4); the salinity of the
bottom waters was fairly uniform and ranged between 15.1
and 15.2‰ (Table 1). Dundas (1994) recorded a mean surface
salinity of 21.1‰ in summer, with salinity decreasing from the
lower to the upper reaches.
The pH of the waters during this survey decreased from the
mouth to the upper reaches and ranged from 7.8 to 7.2 (Table 1).
Dundas (1994) recorded a mean pH of 8.5 in summer and found
that values were fairly uniform throughout the system. Both
surface and bottom dissolved oxygen values decreased upstream
from the mouth. The dissolved oxygen values of the surface
waters ranged from 7.5 to 5.3 mg l–1 and were higher than those
at the bottom, which ranged from 7.2 to 3.1 mg l–1 (Table 1).
The steep banks that characterise the Van Stadens Estuary
probably limit wind-induced mixing of the waters resulting
in lower bottom dissolved oxygen values. Dundas (1994)
recorded a mean dissolved oxygen concentration of 8.5 mg l–1 in
summer. The waters in the estuary were clear and the bed of the
system was visible at most sites; a Secchi disc value of 2.5 m was
recorded at Site 2 (Table 1). Dundas (1994) also reported relatively high water transparencies in the Van Stadens, with mean
Secchi disc values varying between 0.9 and 2.5 m.
Ichthyofauna
A total of 1718 individuals representing 15 species and seven
families were caught in 11 seine net hauls and five gill nets.
Mugilidae (5 species), Sparidae (4 species) and Gobiidae (2
species) dominated the taxa. Juvenile mugilids (53.7%)
dominated catches by number followed by R. holubi (14.1%),
L. lithognathus (11.5%), A. breviceps (8.2%), M. capensis (4.6%),
L. richardsonii (2.8%), M. cephalus (2.2%) and G. callidus (1.0%). A
total species mass of over 33 kilograms was recorded and this
was dominated by L. richardsonii, which comprised over 70.2%
of the catch, followed by L. amia (12.2%), L. tricuspidens (10.3%)
and juvenile mugilids (4.5%) (Table 3).
Dundas (1994) recorded fifteen fish species from the estuary
in a seine and gill net study. Ten species (67%) were common to
those captured during this survey. The dominant species
captured by Dundas (1994) included A. breviceps, L. amia,
L. lithognathus, L. dumerili, L. richardsonii, M. falciformis, M. cephalus,
Myliobatis aquila, P. commersonnii and R. holubi.
Of the fishes recorded during this survey, four (A. breviceps,
G. aestuaria, G. callidus and P. knysnaensis) are estuarine-resident
species. Atherina breviceps comprised mostly mature individuals
between 39 and 62 mm. Eleven species (D. capensis, L. amia,
L. lithognathus, L. dumerili, L. richardsonii, L. tricuspidens, M. falciformis, M. cephalus, M. capensis, R. holubi and S. salpa) are
estuarine-associated marine species. Lithognathus lithognathus
comprised mostly small individuals less than 30 mm; a few
larger individuals (>120 mm) were also present, which suggests
recent recruitment and regular utilisation of the system. Liza
richardsonii ranged in size between 202 and 339 mm. Although
most of these were large specimens in the 290–300 mm size
class the high numbers of juvenile mugilids (<50 mm) indicates that the Van Stadens serves as a nursery for this group of
fishes. This is also supported by catches of M. capensis being
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
dominated by newly recruited juveniles between 20 and
56 mm and M. cephalus comprising juveniles mostly <30 mm.
Rhabdosargus holubi were also all juveniles between 15 and
72 mm in length. The presence of large numbers of juvenile
estuarine-associated marine species indicates that the Van
Stadens is an important nursery habitat for these fishes.
Estuarine-associated marine species also dominated catches
comprising 91% of the catch numerically and 99% of the
biomass. Estuarine-resident species comprised 9% of the catch
numerically and only 1% of the biomass. The Van Stadens
Estuary appears to serve an important nursery function for
estuarine-associated marine species as well as estuarineresident species.
Maitland
The Maitland Estuary (33°59’S, 25°17’E) is situated about
26 km west of Port Elizabeth. The system consists of a small,
very shallow sandy system located behind a broad dissipative
beach and little published information exists on the estuary
(Whitfield, 2000).
Physico-chemical
Physico-chemical measurements were made at one site in the
Maitland Estuary. This was situated in the outflow channel of
the mouth which was open at the time of sampling. The
Maitland appears to be closed to the sea for extended periods
and, when the system opens, the mouth comprises a shallow
drainage channel for outflowing estuarine water. The water
depth measured 0.3 m; the water temperature was 22.7°C and
the salinity was 1.3‰, indicating very little seawater input. The
water had a pH of 8.4 and the dissolved oxygen measured
12.9 mg l–1; the bed of the system was also visible (Table 1).
Ichthyofauna
A total of five seine hauls caught 585 individuals representing
six species and three families, with Mugilidae dominating the
taxa (3 species). In terms of numbers, M. cephalus comprised
88.2% of the catch followed by L. lithognathus (6.8%), M. capensis
(2.7%) and R. holubi (1.4%) (Table 2). A total species mass of 0.4 kg
was caught; M. cephalus (95.3%) dominated the biomass followed by L. lithognathus (2.3%) and M. capensis (1.0%) (Table 3).
From this survey, five species (L. lithognathus, L. richardsonii,
M. cephalus, M. capensis and R. holubi) are estuarine-associated
marine species and only one estuarine-resident species
(P. knysnaensis) was recorded. All estuarine-associated species
caught comprised newly recruited individuals. L. lithognathus
ranged in size from 14–28 mm, M. cephalus measured
21–46 mm, Myxus capensis ranged between 20 and 27 mm, and
R. holubi were between 14 and 17 mm. Although the predominance of small individuals indicates recent recruitment, the
lack of larger specimens suggests that the system is not regularly
utilised and that the nursery function of this system may be
limited. It may also be possible; however, that larger specimens
had migrated back to sea immediately after the mouth had
opened and thus were not captured at the time of this survey.
Estuarine-associated marine species dominated catches
comprising 99.8% of the catch numerically and 99.6% of the
biomass. Estuarine-resident species comprised only 0.2% of the
catch numerically and 0.4% of the biomass. Although the
Maitland is a small system that is intermittently open to the sea,
it still appears to support estuarine-associated species.
Bakens
The Bakens Estuary (33°57’S, 25°37’E) is a heavily modified
system that discharges into the Port Elizabeth Harbour. Harbour
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and urban developments have resulted in the system being
canalised along much of its length and the bed is composed
mainly of rock and concrete. Nine bridges cross the system. The
state of information on the Bakens is regarded as poor
(Whitfield, 2000).
Physico-chemical
Physico-chemical parameters were measured at three sites in
the Bakens. Only the lower site (Site 1) was deep enough to
permit both surface and bottom measurements. Water depths
ranged from 0.2 to 1.0 m (Table 1). Water temperatures measured
between 19.0 and 20.5°C and were highest at the uppermost
site (Site 3). Salinities ranged from 31.7‰ at the mouth to 0.7‰
at the uppermost site. The pH values measured between 7.8
and 8.1. Dissolved oxygen concentrations ranged from 6.2 to
15.9 mg l–1 and were highest at the uppermost site. The bed of
the system was visible at all sites (Table 1).
Ichthyofauna
Two seine net hauls caught a total of 83 specimens representing nine species and five families. Mugilidae (4 species) and
Sparidae (3 species) dominated the taxa. The most abundant
species recorded was H. capensis and comprised 51.2% of the
catch followed by P. knysnaensis (13.1%), L. lithognathus (9.5%),
A. breviceps (4.8%), M. cephalus (4.8%), juvenile mugilids (4.8%),
L. dumerili (3.6%), R. holubi (3.6%), L. richardsonii (3.6%) and
D. capensis (1.2%) (Table 2). A total species mass of over 159 g
was caught; this was dominated by L. richardsonii (56.0%),
L. dumerili (13.1%), M. cephalus (8.8%), R. holubi (7.7%),
H. capensis (5.8%), P. knysnaensis (4.3%), A. breviceps (2.4%) and
L. lithognathus (1.2%) (Table 3).
Of the taxa captured during this survey, seven were
estuarine-associated marine species; these included D. capensis,
H. capensis, L. lithognathus, L. dumerili, L. richardsonii, M. cephalus
and R. holubi. Heteromycteris capensis were all immature individuals and were mostly in the 30–40 mm size class; this species
attains sexual maturity at approximately 80 mm SL (Cyrus &
Martin, 1991). Lithognathus lithognathus comprised newly
recruited individuals in the 20–30 mm size class, which
suggests that the Bakens appears to provide some nursery
function for marine species. This is supported by the occurrence
of small individuals of M. cephalus (20–30 mm) and R. holubi
(10–20 mm). Two estuarine-resident species (A. breviceps and
P. knysnaensis) were captured during this survey. Psammogobius
knysnaensis were all mature individuals (>30 mm) and were
mostly in the 30–40 mm size class.
Overall, estuarine-associated marine species were the dominant group, comprising 81.9% of the catch numerically and
91.0% by mass. Estuarine-resident species comprised 18.1% of
the catch numerically and 9.0% by mass. Although this survey
indicates that the Bakens supports estuarine-associated species,
the canalisation of the system and lack of habitat suggests that
this is limited.
Papkuils
The Papkuils (33°55’S, 25°36’E) is another severely modified
system situated in Port Elizabeth. The system is canalised along
its lower reaches and the bed is concreted; it also appears to
receive industrial and domestic effluent. Numerous road
bridges cross the system. The Papkuils has a permanent outlet
to the sea, which is maintained by concrete walls and rock
armouring. Physico-chemical parameters were measured at a
single site in the lower reaches of the system. A depth of 0.1 m
was recorded and the water temperature was 24.0°C. The
salinity recorded was 9.7‰ indicating limited seawater input.
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Vol. 65(1): 69–84, 2010
The pH of the water was 7.4 and dissolved oxygen was low,
measuring 3.4 mg l–1. Owing to the shallow nature of the system, the bed was visible at the sampling site (Table 1). No fish
sampling was conducted during this survey. Owing to the
modified state of this system it is unlikely to provide suitable
habitat for estuarine-associated species.
mean value of 7.2 mg l–1 with the waters being slightly less
oxygenated during summer. The waters of the Swartkops were
relatively clear with Secchi disc values exceeding 1.0 m
(Table 1). Baird et al. (1986) also found that the waters of the
Swartkops Estuary were clear with mean Secchi disc measurements of between 1.0 and 1.5 m.
Swartkops
The Swartkops (33°51’S, 25°38’E) is situated just north of Port
Elizabeth. The river is 155 km long and the catchment area is
estimated at 1438 km2 (Binning & Baird, 2001). The main tributaries of the system are the Elands River and the Chatty River
(Baird et al., 1986). The only major impoundment on the
Swartkops River is the Groendal Dam that supplies the industrial
area of Uitenhage (Binning & Baird, 2001). The storage capacity
of the dam is small relative to mean annual rainfall and it has
little effect on overall run-off (Baird et al., 1986).
The Swartkops Estuary is 16 km long and a concrete causeway marks the upper limit of the system (Dye, 1978). The upper
reaches of the system winds through steep banks of muddy
sand; the estuary widens considerably in the middle reaches,
traversing a broad flood plain and broadening from 80 to 200 m.
Large intertidal mudflats, islands and saltmarshes increase in
extent along the lower reaches. Several bridges cross the estuary,
one of which carries a dual carriageway and is located 500 m
from the mouth. The state of information on the Swartkops is
regarded as excellent (Whitfield, 2000).
Ichthyofauna
A total of 4841 fish, represented by 18 families and 30 species
were caught in 19 seine net hauls and 14 gill nets in the
Swartkops Estuary. In terms of the number of species recorded
Sparidae (5 species) and Mugilidae (4 species) were the dominant
families. Gilchristella aestuaria was the most abundant species
caught comprising 28.7% of the total catch, followed by
R. holubi (17.0%), P. commersonnii (15.6%), juvenile mugilids
(13.7%), L. richardsonii (6.0%), L. dumerili (4.9%), R. globiceps
(2.6%), C. gilchristi (2.3%), L. lithognathus (1.7%), G. callidus
(1.4%), P. knysnaensis (1.1%) and A. breviceps (1.1%) (Table 2). A
total species mass of over 51 kg was recorded and this was
dominated by E. machnata (23.3%), G. feliceps (18.1%),
P. commersonnii (10.6%), L. richardsonii (10.5%), L. tricuspidens
(7.5%), L. dumerili (7.2%), R. holubi (5.0%), L. amia (4.6%),
A. japonicus (4.3%), juvenile mugilids (2.9%), G. aestuaria (2.5%)
and M. cephalus (1.9%) (Table 3).
A total of 86 species of fish has been recorded in the
Swartkops Estuary (Baird et al., 1986). The high number of
species recorded is probably a reflection of a combination of the
number of studies undertaken and the strong marine influence
within the system. Many of the dominant species, however,
were also among the important species captured during this
survey. Abundant fishes reported in the Swartkops estuary
included A. japonicus, A. breviceps, C. gilchristi, D. capensis,
E. machnata, Engraulis japonicus, G. feliceps, G. aestuaria, L. amia,
L. lithognathus, L. dumerili, L. richardsonii, L. tricuspidens,
M. cephalus, P. commersonnii, P. saltatrix, P. knysnaensis and
R. holubi (Marais & Baird, 1980; Beckley, 1983; Baird et al., 1988).
Estuarine-resident species during this survey were represented
by nine species (Ambassis dussumieri, A. breviceps, C. gilchristi,
C. natalensis, C. nudiceps, C. superciliosus, G. aestuaria, G. callidus
and P. knysnaensis). Caffrogobius gilchristi specimens measured
between 25 and 55 mm; most specimens were immature and
were in the 30–40 mm size class. Caffrogobius gilchristi matures
at a length of approximately 50 mm SL (Bennett, 1989). The
majority of G. aestuaria, G. callidus and P. knysnaensis were
mature individuals with a modal size class of 30–40 mm. These
results indicate that the Swartkops is an important habitat for
estuarine-resident taxa.
Estuarine-associated marine species predominated during
this survey represented by 21 species (A. japonicus, D. capensis,
E. machnata, G. feliceps, Hemiramphus far, H. capensis, L. amia,
L. lithognathus, L. dumerili, L. richardsonii, L. tricuspidens,
M. falciformis, M. cephalus, P. indicus, P. commersonnii, P. saltatrix,
R. globiceps, R. holubi, S. salpa, Silago sihama and S. turbynei).
Lithognathus lithognathus were between 19 and 106 mm and
catches were dominated by small individuals (<40 mm), which
indicates recent recruitment. Liza dumerili individuals ranged
from 50 to 221 mm with most also represented by recently
recruited individuals in the 60–80 mm size classes. Liza
richardsonii ranged between 38 and 303 mm, with most individuals in the 60–70 mm size class. Pomadasys commersonnii ranged
in size from 19 to 546 mm, with most specimens between 20
and 40 mm. Recruitment of P. commersonnii individuals into
estuaries occurs at 20–30 mm TL (Wallace & van der Elst, 1975).
Rhabdosargus holubi individuals measured between 14 and
124 mm, with most individuals in the 20–30 mm size classes.
Physico-chemical
Physico-chemical parameters were measured at six sites in
the Swartkops Estuary extending from the mouth area to the
head of the system. The mouth of the estuary is kept permanently open by the action of strong tidal currents, which exceed
the average river flow by 60 times (Baird et al., 1986). Water
depth recorded during this survey ranged from 0.6 to 3.5 m
(Table 1). Baird et al. (1986) recorded depths of 2.0 m in the lower
reaches while the deepest areas (>3.7 m) were recorded in the
upper reaches and the mouth area. Both surface and bottom
water temperatures during this survey increased slightly from
the mouth upstream and ranged between 18.2 and 19.2°C
(Table 1). In general the head of the system is warmer than the
mouth region during summer and cooler during winter
(Marais & Baird 1980; Emmerson, 1985; Baird et al., 1986). Mean
summer temperatures varied between 21 and 26°C while mean
winter temperatures were 11–18°C (Marais & Baird, 1980;
Emmerson, 1985; Scharler et al., 1997). A horizontal salinity
gradient was recorded during this survey. Surface salinities
decreased from 32.7‰ in the mouth region to 25.2‰ in the
upper reaches. Bottom salinities decreased from 32.7‰ to 27.0‰
in the same direction (Table 1). Emmerson (1985) recorded a
permanent salinity gradient from the mouth to the head of the
estuary and this was attributed to a continual input of freshwater. There was no difference observed between surface and bottom salinities in the lower and middle reaches of the estuary.
Some stratification, however, was recorded in the upper reaches
(Sites 5 & 6). Emmerson (1985) attributed this lack of vertical
stratification in the lower reaches to wind and tidal mixing.
The pH values recorded during this survey ranged from 7.9
near the mouth to 7.5 in the upper reaches (Table 1). Emmerson
(1985) reported a mean pH of 8.1 but found that values were
generally higher at the head of the system. Dissolved oxygen
concentrations during this survey ranged from 6.9 to 5.0 mg l–1
(Table 1). Baird et al. (1981) reported a relatively constant
oxygen value of about 4.5 mg l–1 within the system. Both
Emmerson (1985) and Scharler et al. (1997) recorded an overall
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
The majority of the individuals captured during this survey
were less than one year old; again indicating recent recruitment into the system. These results show that the Swartkops
Estuary provides important nursery habitat for estuarineassociated marine species.
Estuarine-associated marine species also dominated the catch
both numerically (64.7%) and by mass (96.8%); estuarineresident species comprised 35.3% of the catch numerically and
3.2% by mass. The predominance of both estuarine-dependent
marine species and estuarine-resident species indicates that
the Swartkops Estuary is an important habitat for fishes with
different life-history styles.
Coega
The Coega system (33°47’S, 25°41’E) is situated approximately
18 km northeast of Port Elizabeth. The system is highly altered
from its original state; the entire lower reaches of the system
(situated below a road bridge which crosses the system about
3 km from the mouth) is utilised for salt evaporation ponds.
More recently, since the time of this survey, a harbour has been
developed on the coast adjacent to the estuary and the system
now opens into Ngqura (Coega) Harbour. In its present state
seawater is pumped into a canal that runs parallel to the former
estuarine channel and then into the evaporation ponds, which
occupy the estuary. The state of information on the Coega
system is regarded as poor (Whitfield, 2000).
Physico-chemical
Physico-chemical parameters were measured at one site in
the mouth area below the salt evaporation pans. The depth
recorded at this site was 0.7 m and the water temperature
measured 13.4°C (Table 1). The waters were hypersaline
(47.2‰) and the pH measured 8.2. Dissolved oxygen values
were between 7.1 and 7.2 mg l–1; the waters were clear and the
estuary bottom was visible (Table 1).
Ichthyofauna
Five seine net hauls caught a total of 1957 specimens representing eight species and six families. Mugilidae and Gobiidae
dominated the taxa with two species each. The most abundant
species included A. breviceps (96.8%) and G. aestuaria (1.5%),
which together comprised over 98% of the catch (Table 1). A
total species mass of over 1.7 kg was recorded.
Atherina breviceps (91.2%) was the dominant species by mass
followed by L. richardsonii (6.1%) and G. aestuaria (1.1%) (Table 2).
Of the taxa collected in the Coega four (A. breviceps, G. aestuaria,
G. callidus and P. knysnaensis) were estuarine species and four
(H. capensis, L. richardsonii, M. cephalus and R. holubi) were
estuarine-associated marine fishes. Atherina breviceps captured
during this survey measured between 12 and 62 mm; although
most individuals fell within the 30–50 mm size classes, a cohort
of small specimens 10–20 mm were also present. Most G. aestuaria
specimens were mature individuals in the 30–50 mm size
classes. The presence of both immature and mature specimens
suggests that the Coega is able to support estuarine-resident
fishes.
Estuarine-resident species were also the dominant group of
fishes both numerically and by mass, comprising 98.7% and
93.5% of the catch, respectively. Estuarine-associated marine
species comprised 1.3% of the catch numerically and 6.5% of
the mass. This survey indicates that although the Coega is
severely modified, it still supports estuarine-resident taxa.
However its suitability as a nursery habitat for estuarineassociated marine taxa is limited.
81
Sundays
The Sundays (33°43’S, 25°51’E) flows into Algoa Bay approximately 35 km northeast of Port Elizabeth. The river, which is
310 km long, has a catchment area of 20 729 km2 (Harrison &
Whitfield, 1990). One major impoundment, the Mentz Dam, is
situated on the river but this has not significantly affected the
flow of freshwater into the system (Marais, 1981). The estuary is
about 21 km long and is channel-like along its entire length
with steep banks and an absence of extensive mudflats or salt
marshes (Wooldridge & Melville-Smith, 1979; Beckley, 1984). A
sandy flood-tidal delta is situated in the lower reaches and
these sand banks are exposed at low tide. The estuary is widest
just inside the mouth (maximum width of 800 m) from where it
becomes progressively narrower to about 20 m at the head of
the estuary (Whitfield & Harrison, 1996). The state of information on the Sundays Estuary is considered excellent (Whitfield,
2000).
Physico-chemical
Physico-chemical parameters were measured at seven sites
in the Sundays Estuary ranging from the mouth area (Site 1) to
the upper reaches of the system (Site 7). The mouth of the
Sundays Estuary is permanently open and this is maintained
by tidal currents. Water depths recorded during this survey
ranged from 1.2 to 3.5 m (Table 1). According to Marais (1981)
the estuary is 2.5 m deep near the mouth and gradually
increases to a maximum depth of 5.0 m in the lower reaches
from where it gets gradually shallower to approximately 2.5 m
in the upper reaches. Both surface and bottom water temperatures during this survey increased from the mouth upstream;
surface water temperatures were also slightly higher than
those at the bottom. Surface water temperatures ranged from
19.1 to 21.9°C, while bottom water temperatures measured
18.9–20.4°C (Table 1). Jerling & Wooldridge (1991) recorded
water temperatures ranging from 13.0°C in winter (July) to
26.0°C in summer (January), with differences between surface
and bottom water temperatures rarely exceeding 1.0°C. Both
horizontal and vertical salinity gradients were recorded during
this survey. Surface salinities were generally lower than
those at the bottom and decreased from 32.8‰ in the mouth
region to 3.4‰ in the upper reaches. Bottom salinities decreased from 32.8‰ to 3.2‰ in the same direction (Table 1).
Jerling & Wooldridge (1991) also noted that horizontal and
vertical salinity gradients were evident throughout the
year, with salinity in the mouth region consistently above
30.0‰ and seldom above 5.0‰ at the head of the estuary;
differences between surface and bottom salinities increased
up the estuary.
The pH of both surface and bottom waters generally increased upstream from the mouth; surface values (8.0–8.5)
were also slightly higher than those recorded at the bottom
(7.8–8.5) (Table 1). Surface dissolved oxygen concentrations
were typically higher than those recorded at the bottom and
are probably a reflection of stratification and a lack of mixing.
Surface values ranged from 7.8 to 9.9 mg l–1 while those at the
bottom measured between 4.9 and 8.9 mg l–1 (Table 1). The
waters of the Sundays were found to be well oxygenated with
mean dissolved oxygen values exceeding 7.0 mg l–1 (Emmerson,
1989; Scharler et al., 1997). The water clarity during this survey
generally decreasing upstream from the mouth with Secchi
disc values measuring between 0.4 and 0.7 m (Table 1). Similar
values were reported by other workers who reported mean
Secchi disc values in the Sundays Estuary of between 0.3 and
1.0 m (Marais, 1984; Scharler et al., 1997).
82
Transactions of the Royal Society of South Africa
Ichthyofauna
Sixteen seine net hauls and 14 gill nets caught 27 species
representing 15 families. Mugilidae and Sparidae were each
represented by five species. A total of 3550 individuals was
recorded and these were dominated by G. aestuaria (32.7%),
juvenile mugilids (17.4%), R. holubi (14.3%), L. richardsonii
(6.5%), L. dumerili (5.6%), M. cephalus (4.5%), P. commersonnii
(4.0%), C. gilchristi (2.7%), L. lithognathus (2.6%), S. turbynei
(1.9%), A. breviceps (1.6%), R. globiceps (1.4%) and L. tricuspidens
(1.0%) (Table 2). A species mass of over 134 kg was recorded
with E. machnata (34.9%), L. tricuspidens (24.8%), A. japonicus
(7.3%), M. cephalus (7.0%), L. richardsonii (6.0%), Carcharias
taurus (5.9%), P. commersonnii (4.0%), G. feliceps (3.0%),
L. dumerili (2.5%) and R. holubi (2.0%) dominating the catch
(Table 3).
Based of the results of a gill net study (Marais, 1981) and a
seine net study (Beckley, 1984), a total of 55 species have been
recorded in the Sundays Estuary. Altogether 24 of these species
(44%) were common to this survey, many of which were dominant during all studies. Important species, either numerically
and/or in terms of biomass included A. japonicus, C. gilchristi,
G. feliceps, G. aestuaria, H. capensis, L. dumerili, L. richardsonii,
M. falciformis, M. cephalus and P. commersonnii, P. knysnaensis,
R. holubi and S. bleekeri (Marais, 1981; Beckley, 1984).
Of the taxa collected during this survey seven were estuarineresident species (A. breviceps, C. gilchristi, C. natalensis, G. aestuaria,
G. callidus, P. knysnaensis and S. temminckii). Atherina breviceps
were all mature individuals between 48 and 65 mm, with a
modal size class of 50–60 mm. Caffrogobius gilchristi specimens
were between 17 and 62 mm; most were immature and were in
the 30–40 mm size class. The majority of G. aestuaria caught
were mature individuals with a modal size class of 40–50 mm.
The presence of mature and immature individuals indicates
that the Sundays is an important habitat for resident taxa.
Seventeen species during this survey were estuarine-associated
marine species (A. japonicus, D. capensis, E. machnata, G. feliceps,
H. capensis, L. lithognathus, L. dumerili, L. richardsonii, L. tricuspidens,
M. falciformis, M. cephalus, M. capensis, P. indicus, P. commersonnii,
R. globiceps, R. holubi and S. turbynei). Liza dumerili ranged in size
from 30 to 240 mm; most of these were small individuals in the
50–60 mm size class. The majority of L. richardsonii were also
small individuals between 80 and 90 mm; a few individuals
>200 mm were also captured indicating regular usage of the
system. Mugil cephalus were predominantly small, newly
recruited individuals less than 30 mm; larger specimens
>290 mm were also represented. Pomadasys commersonnii
showed a bimodal length frequency distribution, the majority
of fish were recently recruited individuals in the 20–30 mm size
class with larger specimens represented by the 270–280 mm
size class. Rhabdosargus holubi individuals caught in this survey
were between 15 and 130 mm; most specimens caught were 0+
juveniles between 40 and 60 mm. The predominance of small
juvenile estuarine-associated marine species together with
larger individuals suggests regular recruitment and utilisation
of the estuary as a nursery area.
Estuarine-associated marine species comprised 61.9% of the
catch numerically and 93.0% by mass, estuarine-resident
species comprised 38.0% numerically and 1.0% by mass. The
predominance of both estuarine-associated marine and
estuarine-resident taxa highlights the importance of this
system as a habitat area for estuarine-associated fishes.
GENERAL DISCUSSION
Eleven rivers intersect the coastline between Cape St Francis
and Cape Padrone (Figure 1). Four systems (Kromme,
Vol. 65(1): 69–84, 2010
Gamtoos, Swartkops and Sundays) are large, permanently
open estuaries. A connection with the sea is maintained in
these estuaries by strong tidal currents. The flow regime of
the Kromme Estuary is highly altered with two large dams
severely restricting freshwater input into the estuary (Scharler
& Baird, 2000). The Gamtoos, although dammed, is subject to
occasional floods that completely flush the system with freshwater (Schlacher & Wooldridge, 1996). The Swartkops is
situated in the immediate confines of Port Elizabeth and flows
through a highly urbanized and industrial area. The Sundays
Estuary receives a steady flow of freshwater and is channel-like
along its entire length (Wooldridge & Melville-Smith, 1979).
Between 24 (Gamtoos) and 30 (Kromme and Swartkops)
species were recorded in these four estuaries, with Mugilidae,
Sparidae and Gobiidae dominating the taxa. Studies have
shown that permanently open estuaries often have a higher
species richness than estuaries that have an intermittent
connection with the sea and this can be attributed to an
increase in the number of marine-spawning species in permanently open estuaries (Bennett, 1989; Whitfield & Kok, 1992).
Dominant estuarine-dependent marine species during this
study included A. japonicus, E. machnata, G. feliceps, L. amia,
L. dumerili, L. richardsonii, P. commersonnii and R. holubi. Studies
in permanently open warm-temperate estuaries have also
reported comparable fish assemblages (e.g. Marais, 1981, 1983b;
Beckley, 1984; Baird et al., 1986).
Differences in fish assemblages between permanently open
and closed estuaries in the southeastern Cape have also been
attributed to a greater numerical proportion of estuarineresident species in closed estuaries (Vorwerk et al., 2003). In the
Gamtoos, Swartkops and Sundays estuaries, resident species
comprised <40% of the catch numerically; the exception was
the Kromme Estuary where resident species comprised over
66% of the catch. Strydom & Whitfield (2000) found that the
Geelhoutboom tributary serves as an important source of
larvae of estuarine-resident species to the Kromme Estuary.
Dominant estuarine-resident taxa included A. breviceps,
G. aestuaria, G. callidus and P. knysnaensis.
The diversity and abundance of fish species found in estuaries
is also affected by latitude. Maree et al. (2000) recorded a
decrease in the number of fish species in a southwesterly
direction, with a substantial decline occurring west of the
permanently open Swartkops Estuary. This was attributed to
the Agulhas Current moving further offshore in the Algoa Bay
region and consequently tropical coastal species reaching the
southern limit of their distribution. The increased occurrence
of tropical species may account for the greater number of
species recorded in the Sundays (27 species) than the Gamtoos
Estuary (24 species).
Three systems (Seekoei, Kabeljous and Van Stadens) are
moderately sized estuaries characterised by intermittently
open mouths separated from the sea by a broad dissipative
beach. The Seekoei Estuary has undergone severe human
disturbance in the form of reduced river flow and completely
transformed mouth dynamics. In comparison the Kabeljous
and Van Stadens estuaries are relatively undisturbed. The
Maitland is a small, shallow sandy system that is typically
closed to the sea for extended periods.
Between six (Maitland) and 15 (Van Stadens) species were
recorded in these four estuaries. Estuarine-resident taxa dominated the catches numerically in the Seekoei and Kabeljous
estuaries, comprising >50% of the catch, while estuarineassociated marine species dominated catches numerically and
by mass in the Van Stadens and Maitland Estuaries. Dominant
estuarine-associated marine species included mugilids
James & Harrison: Survey of the estuaries on southeast coast of South Africa, Cape St Francis – Cape Padrone
(M. cephalus, L. richardsonii and M. capensis) and sparids
(R. holubi and L. lithognathus). Dominant estuarine-resident
species included A. breviceps, G. callidus, G. aestuaria and
P. knysnaensis. Only one freshwater species, Oreochromis
mossambicus was recorded in the Seekoei Estuary. The remaining three systems (Bakens, Papkuils and Coega) are all highly
altered systems that provide little habitat for either resident or
migrant marine species.
The coastal sector between Cape St Francis and the Sundays
Estuary contains eleven coastal outlets of which four are large
permanently open estuaries. The greatest diversity of species
was found in the permanently open estuaries, while four
temporarily open/closed estuaries, although supporting lower
species diversity also provided important habitat for a number
of estuarine-associated marine species and resident taxa. Three
of the outlets are highly altered and provide little habitat for
fishes.
Of the systems assessed by Whitfield (2000) within this
coastal sector, the state of information on 45% of the estuaries
was regarded as poor and the remaining estuaries were
regarded as having moderate (9%), good (18%) to excellent
(27%) information. As a result of this survey basic physicochemical and biological (fishes) information now exists on all of
these systems, with much of this information having been
lacking for the smaller altered systems.
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