THE ECOLOGY AND CONSERVATION
OF EUROPEAN SMELT (OSMERUS
EPERLANUS L.) FROM WATERFORD
ESTUARY, IN SOUTHEASTERN
IRELAND
D. Doherty and T.K. McCarthy
ABSTRACT
The smelt (Osmerus eperlanus L.) is a little-known indigenous fish species in Ireland. All known Irish
populations are anadromous. A total of 173 smelt were captured using a variety of fishing methods in
Waterford Estuary over the period June 1997 /June 1998. Analyses of the stomach contents revealed
that they fed almost exclusively upon the marine mysid Praunus neglectus and other macroinvertebrates such as Hyperia galba and Crangon allmani . Piscivorous feeding, including cannibalism, was also
evident, and three fish species were recorded in the smelt stomach contents (Merlangus merlangus,
Sprattus sprattus and Osmerus eperlanus ). Seasonal variation in feeding intensity was also noted. Six
metazoan parasite species were recorded from the Waterford Estuary smelt. These included two
digeneans (Diplostomum spathaceum and D. gasterostei ), one cestode (Proteocephalus longicollis ), two
nematodes (Hysterthylacium aduncum and Anisakinae sp.) and one ectoparasitic crustacean species
(Caligus eperlanus ). Parasite prevalences and mean intensities of infection were recorded. The smelt
examined ranged from 1/to 5/years, with the majority belonging to the 2/and 3/cohorts.
They varied in size (125 /260mm fork length) and in weight (15 /129g). Their growth rates were
broadly similar to those of smelt populations elsewhere in Europe. Maturation of gonads and
increasing egg diameter in ovaries of female smelt in the Waterford Estuary were noted prior to a
spring spawning period. No clear correlations were found for either mean egg diameter or fecundity
when parental body size or age were taken into account. However, this may be due to the relatively
small sample size.
D. Doherty
(corresponding
author; e-mail:
denis.doherty@
mail.esb.ie) and T.K.
McCarthy, Zoology
Department,
National University of
Ireland, Galway.
Received 20 February
2003. Read 22 April
2004. Published 31
August 2004.
BIOLOGY
AND
INTRODUCTION
In Ireland, all known populations of smelt are
anadromous marine or coastal fish, although several
non-migratory freshwater populations occur in
Scandinavia and Eastern Europe. Smelt in Ireland
have previously been recorded from the Shannon,
Fergus, Maigue and Foyle estuaries as well as
inshore waters at Larne and in Belfast Lough
(Kennedy 1948; Bracken and Kennedy 1967;
Vickers 1974; Vickers and Watson 1974;
Moorehead and Service 1992). According to
Kennedy (1948) there were unconfirmed reports
from the Rivers Nore, Suir and Barrow (Fig. 1).
Quigley (1996) confirmed the presence of smelt
from the River Suir, although he concluded that no
evidence exists of a spawning population in any of
the three rivers. The only known spawning site in
Ireland is located at the upper tidal limit in the
Lower River Shannon. This newly discovered
population from Waterford Estuary may represent
a newly colonising population such as occurred in
ENVIRONMENT: PROCEEDINGS
OF THE
the River Thames (Hutchinson 1983) or may be an
unnoticed established stock.
Smelt from Waterford Estuary, although
locally abundant, have been classified as
‘vulnerable’ and in need of special conservation
measures in Ireland (Whilde 1993). Smelt have not
been extensively studied in Irish waters. In the
British Isles, smelt were known to have occurred in
at least 29 river systems but have disappeared from
some of these, only to reappear in others again. It
was once thought to be a common species, but
threats such as pollution, overfishing, obstruction of
rivers by physical barriers and habitat degradation of
spawning sites have reduced several populations and
made others extinct (Hutchinson and Mills 1983;
Maitland and Lyle 1990).
In this paper, new data is presented on the
growth, diet, parasitology and fecundity of a
population of European smelt sampled in
Waterford Estuary. Their future conservation is
also discussed.
ROYAL IRISH ACADEMY, VOL. 104B, NO. 2, 125 /130 (2004).
#
ROYAL IRISH ACADEMY
125
BIOLOGY
AND
ENVIRONMENT
STUDY AREA
Waterford Estuary (Fig. 1), which is situated in
south-eastern Ireland, is the exit point of the
Barrow, Suir and Nore rivers and has a catchment
area of 9,000km2 and a long-term discharge of
123m3 sec1. Despite the varied geology of the
south-eastern region of Ireland, the River Barrow is
for the most part floored with limestone and
consequently has relatively calcium-rich waters
(pH range /7.2/9.3; hardness range /20 /
428mg l 1 CaCO3) (Lucey 1998). Waterford
harbour has extensive commercial finfish and
shellfish fisheries and is Ireland’s third most
important commercial cargo port.
MATERIALS AND METHODS
Samples of smelt from Waterford Estuary were
captured using a variety of fishing methods: drift,
snap and stake nets, salmon/sprat weirs and eel pots.
Details of the monthly samples are listed in Table 1.
All fish (n /173) were labelled and deep-frozen
within four hours of landing for later examination.
Fish samples, stored at /208C, were dissected and
examined for parasites according to the method
adopted earlier by Conneely and McCarthy (1984).
Gonad and fecundity analyses followed standard
protocols (Bagenal 1978). Dietary analysis followed
standard protocols (Hyslop 1980), and the results
are presented as percentage numerical and
gravimetric taxonomic composition. Smelt were
aged using scale analysis (Belyania 1969).
RESULTS
In the present study, the estuarine smelt samples,
as shown in Table 1, had fork lengths of
125 /260mm, weights of 15 /129g and ages of
1/to 5/years. Seasonal fluctuation in the
percentage of females in the samples ranged from
44.4 to 82.6. The largest and oldest smelt were
present in the February sample before spawning
would have occurred. Male smelt were significantly
Table 1
*
Details of the monthly samples
from Waterford Estuary.
/
Sampling
month
June 1997
December 1997
January 1998
February 1998
June 1998
126
Method
of capture
Drift and snap nets
Drift net and sprat weir
Ground nets
Ground nets
Drift net and eel pots
No. of
fish
27
29
20
46
51
*
Fig. 1 /A map of Waterford estuary with the smelt
sampling sites highlighted by filled squares.
younger overall than females, with a mean age of
2.95 years compared to 5.6 years for females. Smelt
length at age for the Waterford Estuary samples is
shown in Fig. 2, together with data from samples of
other anadromous European populations (Belyania
1969), and smelt sampled in 1996 from the upper
tidal limits of the River Shannon (Geraghty 1996).
Smelt from Waterford Estuary showed a larger
initial size, which slowed as smelt reached 3/years
(Fig. 2).
Mature smelt ranged from 2/to 5/years in
age. Mean egg size and fecundity values differed
between the three age classes sampled, although
no clear pattern was evident. As expected,
significant differences were noted between the
sexes for gonad weights and gonad:somatic indices
(GSI) values (Table 2) (Mann /Whitney U test:
U /47, p /1785). Maturation of gonads and
increasing egg numbers and diameters in ovaries
of female smelt in the Waterford Estuary were
noted prior to the spring spawning period (Table
1). No clear correlations were found for either
mean egg diameter or fecundity when parental
body size or age were taken into account using
partial correlation analysis.
Six metazoan parasite species (five endoparasites) were recorded in smelt from Waterford
ECOLOGY
OF THE
EUROPEAN SMELT
IN
WATERFORD ESTUARY
sprat Sprattus sprattus (L.) and smelt Osmerus
eperlanus L. Of these fish species, sprat
predominated, occurring in the June 1997,
December 1997 and June 1998 samples. Only
single specimens of whiting and smelt were
recorded in the June 1998 sample. Seasonal
variation in feeding intensity was evident. The
percentage of empty stomachs varied throughout
the sampling period, being highest in the pre- and
post-spawning samples (32.6% of the February 1998
and 21.6% of the June 1998 samples had empty
stomachs). Similarly the mean stomach content
weights also decreased for those stomachs
containing prey items at those same times.
*
Fig. 2 /The length at age of some anadromous European
smelt populations (after Belyanina 1969), including smelt
from the River Shannon (Geraghty 1996), and Waterford
Estuary (present study).
Estuary. These included two digenean eyeflukes
(Diplostomum spathaceum and D. gasterostei ), one
intestinal cestode (Proteocephalus longicollis ), two
intestinal nematodes (Hysterthylacium aduncum and
encysted Anisakinae sp.) and one ectoparasitic
crustacean species (Caligus eperlanus ). No
acanthocephalan species were recorded. Parasite
prevalences and mean intensities of infections are
shown in Table 3 for each of the five smelt samples.
Parasite species compositions were similar for all of
the samples. The mean number of parasite species
harboured in individual fish in the monthly smelt
samples varied from 0 /3. The exceptions to this
were the December 1997 and June 1998 samples,
which harboured a maximum of four parasite
species per fish. Varying portions of four of the
five smelt samples were entirely free of parasites (0 /
19.6%). None of the parasite abundances were
significantly correlated with host length, weight or
age or with prey items in the stomach contents of
the individual specimens. In the external
examination of smelt, metacercarial cysts of either
Cryptocotyle lingua (Creplin, 1825) or C. concava
(Creplin, 1825) were noted encapsulated in the skin
and on fins, but these were not counted.
Analyses of the stomach contents revealed that
individuals sampled fed almost exclusively upon
the marine mysid Praunus neglectus (G.O. Sars),
which formed 56 /90% of their diet by number and
73 /90% of their diet by weight. Other
macroinvertebrates also occurred, such as Hyperia
galba (Montagu) (0.9 /32% by number; 6.8 /16%
by weight) and Crangon crangon (L.) (0 /7% by
number; 0/25.7% by weight). Piscivorous feeding,
including cannibalism, was also evident, particularly
in larger smelt during the summer months. Three
juvenile marine fish species were recorded in the
stomach contents: whiting Merlangus merlangus (L.),
DISCUSSION
Variability in the length of smelt within the same
year class is a characteristic feature of smelt
populations (Belyania 1969). However, the length
at age of the two populations of Irish smelt as
shown in Fig. 2 is broadly comparable with other
European populations. Ivanova et al. (1969)
reported that features of the smelt lifecycle
(growth rates and dietary preferences) were highly
variable, being primarily dependent upon the
prevailing environmental conditions. The length
at age of Irish smelt populations from Waterford
Estuary and the River Shannon were similar,
although the Waterford Estuary smelt were larger
compared to the River Shannon smelt.
It is likely that spawning smelt populations
occur within the Nore, Suir or Barrow, as mature
gravid specimens of smelt were captured. The
nearest known reproducing population is that
recorded by Geraghty (1996) in the River
Shannon, where smelt spawn at the upper tidal
limit. Quigley (1996) suggested that the most likely
spawning site of the Waterford Estuary smelt was in
the upper reaches of the River Suir (Fig. 1).
In the present study, mature smelt ranged from
two to five years in age. Belyania (1969) found that
the age of sexual maturity was highly variable, with
most European populations reaching sexual
maturity between 2 and 3 years. As expected,
differences were noted between the sexes for
gonad weights and GSI values (Table 2a).
Maturation of gonads and increasing egg numbers
and diameters in ovaries of female smelt in the
Waterford Estuary were noted prior to a spring
spawning period. In addition, fecundity values
and egg diameters increased prior to spawning
(Table 2b). Fecundity values for smelt sampled
from the River Crea in Scotland were 40,100 /
105,900 per female fish (Hutchinson and Mills
1987), whereas River Shannon smelt fecundity
were 6,000 /67,500 per fish (Geraghty 1996).
However, both fecundity and mean egg diameter
127
BIOLOGY
AND
ranges may vary considerably according to parental
body size (Belyania 1969; Hutchinson and Mills
1987). The River Crea mature smelt lengths were
185 /279mm (Hutchinson and Mills 1987),
Shannon lengths were 153 /173mm (Geraghty
1996), and Waterford Estuary smelt lengths were
125 /260mm. No clear correlations were
established for either mean egg diameter or
fecundity when parental body size or age was
accounted for using partial correlation analysis.
Mean egg size and fecundity values differed
between the three age classes, although no clear
pattern could be established. The apparent lack of
correlation of fecundity with parental size and age
may be due to the relatively small monthly female
sample sizes.
The seasonal variation in feeding intensity
recorded for smelt in the present study has also
been recorded by Belyania (1969) and Geraghty
(1996). Foltz and Norden (1977), who investigated
North American rainbow smelt Osmerus mordax ,
also recorded empty stomachs during the spawning
run in April, with maximum contents occurring in
June and minimum contents during the winter and
spring months. Similar results were observed from
the Shannon, where smelt (n /60) sampled during
the 1995 spawning season were not feeding
(Geraghty 1996). Smelt from Waterford Estuary
captured earlier and later in the season had fed
almost exclusively upon Praunus neglectus . Despite
the large number of invertebrates, piscivorous
feeding was also important, particularly for the
Table 2a
*
ENVIRONMENT
larger specimens. These larger smelt had a mean
length of 190.6mm, whereas non-piscivorous
feeding smelt had a mean length of 163.7mm.
Foltz and Norden (1977) also reported a marked
increase in piscivorous feeding for smelt longer than
180mm. Smelt from the Elbe Estuary had similar
diets, feeding upon amphipods, mysid shrimp and
small fish (smelt herring and gobids) (Thiel et al.
1996). Thiel et al. (1996) reported that smelt
formed an important part of the food web,
especially for other piscivorous fish species. A
single smelt specimen was reported in dietary
analyses of twelve allis shad (Alosa alosa (L.))
sampled from Waterford Estuary in 1996,
indicating that smelt are preyed upon by other
predatory fish species (Doherty and McCarthy
2001).
All of the parasite species recorded in the
present study (Table 3) were recorded from
Shannon smelt by Geraghty (1996), with the
exception of the external crustacean parasite
Caligus eperlanus . However Geraghty (1996) also
recorded six additional species: the external fish
leech Piscicola geometra , one species of
Acanthocephala (A. lucii ) and four species of
intestinal Digenea (Brachvphallus crenatus, Hemiurus
ocreatus, Lecithochiurm rufoviride and Lecithochirum
furolabiatum ). Differences in parasite species
infection levels between these two Irish smelt
samples may be due to the specificity of the
parasite or the characteristics of the host fish, such
as dietary preferences or movements at sea. As in
Details of male and female smelt length, weight, age and gonad weight for each of the monthly samples
from Waterford estuary.
/
Mean length (mm)
June 1997 (n /27)
December 1997 (n /29)
January 1998 (n /20)
February 1998 (n /46)
June 1998 (n /51)
Table 2b
*
/
Mean age (years)
Mean gonad weight (g)
Male
Female
Male
Female
Male
Female
Male
Female
181.79/27.0
194.19/18.5
179.99/6.3
204.49/21.7
159.19/16.1
200.39/17.63
186.09/10.4
193.79/14.7
198.89/17.98
191.59/28.2
52.99/22.3
64.79/19.0
44.49/3.7
72.19/26.8
32.19/8.2
64.69/18.9
49.49/5.8
61.99/10.3
66.09/19.2
55.09/23.7
2.79/1.0
3.09/0.8
2.39/0.6
3.69/0.7
2.39/0.5
3.19/0.6
2.09/0.01
3.39/0.5
3.49/0.6
3.29/0.7
3.59/0.01
0.29/0.01
2.69/0.2
3.69/0.2
0.19/0.01
0.29/0.01
2.39/0.2
6.09/0.4
8.19/0.6
0.49/0.01
Fecundity of gravid female fish (n/62) and details of their egg diameters.
Sample
June 1997
December 1997
January 1998
February 1998
June 1998
128
Mean weight (g)
No. of
female fish
Fecundity
(no. of eggs)
Mean egg
diameter (mm )
12
13
12
37
36
Spent fish
68089/
194909/
338189/
Spent fish
*/
0.49/0.05
0.79/0.06
0.89/0.07
*/
Maximum egg
diameter (mm)
Minimum egg
diameter (mm)
*/
0.5
0.8
0.9
*/
*/
0.3
0.5
0.6
*/
ECOLOGY
OF THE
EUROPEAN SMELT
the present study, Jarling (1982) also noted the
presence of metacercarial cysts of C. lingua or C.
concava , this time in Elbe Estuary smelt.
The adult nematode H. aduncum and the
encysted nematode Anisakinae sp., recorded in the
samples of smelt, may be acquired through various
crustaceans (such as P. neglectus ), or through
ingestion of paratenic hosts (Moravec 1989). The
single external parasite C. eperlanus recorded in the
present study has a direct lifecycle and thus is
acquired through the activities of the host fish. No
clear seasonal pattern was observed for any of the
parasite species recorded, as the infection
parameters (Table 3) were similar for all months.
Neither were parasite infection parameters related
to host sex, age or size, although both significant
positive and negative correlations were recorded for
some parasite species in some months.
Unlike other rare and endangered fish species,
smelt are not protected under any specific national
legislation in Ireland. The most relevant legislation
is aimed at the conservation of fish as an exploitable
resource rather than as an element of the native
fauna. Whilde (1993) designated smelt as
‘vulnerable’ in the Red Data Book of Irish
vertebrates, and they are also listed in the EU
Berne Convention. Cowx (2002) and Cowx and
Table 3
*
IN
WATERFORD ESTUARY
Collares-Pereira (2002), in an analysis of threats to
European populations of freshwater fish, concluded
that there was an immediate need to protect
threatened species from further loss through
sanctuaries and enforcement of legislation. Within
Waterford Estuary smelt are locally abundant at
certain periods of the year. However, smelt are
located within only a few Irish estuaries, and due to
this limited distribution they are vulnerable to a
number of potential threats. These threats include
deteriorating water quality, the destruction and/or
degradation of spawning sites, and inadequate or
difficult-to-enforce legislation. In addition to these
threats, the frequency of capture as bycatch is
unknown. This may be detrimental, particularly
for migrating shoals of mature smelt that are known
to be occasionally captured in large numbers as
bycatch (several thousand on some occasions)
within Waterford Estuary (M. Doherty, pers.
comm.). Lyle and Maitland (1997), in an
investigation of one of the few remaining
spawning populations of smelt in south-western
Scotland, stated that during the 1980s a
considerable portion of the population was being
netted, and this was the primary reason for the
decline. Since smelt are listed as being ‘vulnerable
to extinction’ in Ireland (Whilde 1993), further
research including the identification of spawning
Infection parameters of each parasite species in five samples of smelt from Waterford Estuary. The
intermediate host for each species is given, where appropriate.
/
Parasite species
recorded
Intermediate
host
June 1997
(n /27)
December 1997
(n/29)
January 1998
(n /20)
February 1998
(n/46)
June 1998
(n /51)
Prevalence Mean Prevalence Mean Prevalence Mean Prevalence Mean Prevalence Mean
(%)
intensity
(%)
intensity
(%)
intensity
(%)
intensity
(%)
intensity
Digenea
Diplostomum gasterostei
Williams
Bivalves
Mollusca
Gastropods
Bivalves
Mollusca
Gastropods
100.0
13.3
86.2
29.0
70.0
26.4
80.4
23.3
66.7
7.8
0.0
0.0
69.0
4.0
0.0
0.0
0.0
0.0
0.0
0.0
Crustaceans
Other fish
species
Hysterthylacium aduncum Crustaceans
Rudolphii
Other fish
species
14.8
0.5
31.0
2.7
10.0
2.0
17.4
1.6
0.0
0.0
14.8
1.0
10.3
1.0
50.0
1.6
37.0
2.1
27.5
4.8
59.3
42.0
37.9
7.9
50.0
12.0
56.5
8.0
45.1
7.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.0
1.0
Diplostomum spathaceum
Rudolphii
Nematoda
Encysted Anisakae spp
Cestoda
Proteocephalus longicollis
Rudolphii
Copepods
Crustacea
Caligus elongatus Kroyer None
(direct
lifecycle)
129
BIOLOGY
AND
sites and investigating aspects of their biology and
ecology is necessary, so that other localised
populations can be properly assessed and protected.
ACKNOWLEDGEMENTS
The assistance of the staff of the Zoology
Department of the National University of Ireland,
Galway, and the Cheekpoint fishermen is gratefully
acknowledged. Staff of the Natural History
Museum, London, verified parasite identifications.
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The subvention
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towards the cost of
publication of papers
by members of its
staff is gratefully
acknowledged by
the Royal Irish
Academy.