EELS (ANGUILLA ANGUILLA (L.)) OF THE
LOWER RIVER SHANNON, WITH
PARTICULAR REFERENCE TO
SEASONALITY IN THEIR ACTIVITY AND
FEEDING ECOLOGY
P. Cullen and T.K. McCarthy
ABSTRACT
The general biology and seasonal ecology of eels from a riffle section of the lower reaches of Ireland’s
largest river system, the River Shannon, were investigated. Eels at this location occurred in high
densities, had a slow growth rate and were predominantly male at maturation. Although they fed to
some degree all year round, foraging activity of the eels was highest in the months of May to
September, inclusive. Their diet, which was dominated by larval Ephemeroptera and Trichoptera,
reflected the typical composition of the macroinvertebrate species assemblage of the riffle habitat
sampled. Little evidence of piscivory or of the potential impact of the high density eel population on
the salmonid stocks of this river section was recorded.
P. Cullen
(corresponding
author e-mail:
paula@dspsrv.ch)
and T.K. McCarthy,
Zoology Department,
National University
of Ireland, Galway,
Galway.
Received 9 May
2006. Accepted
15 March 2007.
Published 9 July
2007.
BIOLOGY
AND
INTRODUCTION
The European eel Anguilla anguilla (L.) population
has declined strongly throughout its range since
the 1980s (Feunteun 2002). The causes of this
decline are not clear, though it seems likely that
several factors may be to blame, including loss
of habitat, water quality problems, obstacles to
migration, overfishing of all life history stages,
introduced pathogens and perhaps also changes in
oceanic conditions due to climate change. As a
result there is an urgent need for research on the
population biology of the species, at all stages in
its typically catadromous life cycle. Though
many studies have been undertaken on the
ecology of the eel in inland European waters, the
vast majority of these have concentrated on
lacustrine eel populations. Riverine population
studies have often concentrated on stream studies
or have been based on studies of the seasonal
spawning migration of the mature component of
the river system’s eel population.
The River Shannon, Ireland’s largest river
system, like most European inland waterways, has
been subject to various anthropogenic impacts
(McCarthy and Cullen 2002). Its eel population
has been affected by a steady decline in juvenile
recruitment, although the effects may not be as
extreme as experienced in many other European
systems. The lower reach of the river, although
ENVIRONMENT: PROCEEDINGS
OF THE
subject to flow regulation since the late 1920s, is
one of the remaining areas within the Shannon
system that still harbours a relatively dense eel
population.
In this paper we document the general
population characteristics of a large, lower reach
riffle section of the main river channel and describe
variation in eel feeding ecology with particular
reference to seasonality and effects of eel size.
MATERIALS AND METHODS
STUDY AREA
The study site was located on a riffle stretch of the
lower reaches of the River Shannon, approximately
8km upstream of the tidal limit (Fig. 1). Flow rates
on this stretch of river channel are generally low
and well regulated at a statutory minimum flow of
10m3 sec1 as a result of the construction of
the Ardnacrusha hydroelectric generating station
and the associated Parteen regulating weir in the
mid- to late 1920s. This regulating weir is located
approximately 10km upstream of the study site.
Although the bulk of the river’s waters are diverted
at the regulating weir, via a headrace canal, to the
power station, it is sometimes necessary in winter
months to ‘spill’ water at the regulating weir
down the main river channel. Although ‘spillage’
rates have been known on occasion to exceed
ROYAL IRISH ACADEMY, VOL. 107B, NO. 2, 87 94 (2007).
#
ROYAL IRISH ACADEMY
87
BIOLOGY
Fig. 1
*
AND
a) maps of the River Shannon, (b) the lower River Shannon and (c) and the study site.
600m3 sec1, this is the exception rather than the
norm, and during the period of this study water was
‘spilt’ over a three week period in late February and
early March at an average rate of 150.29SD
13.4m3 sec1.
Although some sections of the lower River
Shannon are important for coarse fishing, much of
this stretch of river channel has been successfully
developed, and is internationally renowned, for
salmon angling. The diversion of water from the
main river channel in 1929 had a profound impact
on the physical nature and ecology of the river.
Many areas of the river channel became dry,
resulting in a significant and immediate loss of
salmonid spawning habitat (Went 1970). Natural
encroachment of bankside vegetation also occurred, and islands emerged in many areas and
88
ENVIRONMENT
now permanently braid the channel. There is a
long history of construction of instream features on
salmon rivers in Europe, and prior to the impact of
the hydroelectric works numerous pools had been
developed on the lower River Shannon. These
pools have been maintained, and many instream
structures have been constructed to cater for the
reduced flows. This work has generally been carried
out on an ad hoc basis since the 1930s.
The survey site was typical of riffle sections of
this lower section of the River Shannon. The
channel was broken by a series of small islands, and
the site was bounded to the north and the south by
a series of small, artificially created falls (Fig. 1). The
water depth ranged between 15cm and 60cm, with
a substrate comprised mainly of pebbles and slightly
larger rocks.
EELS (ANGUILLA
ANGUILLA
(L.))
OF THE LOWER
RIVER SHANNON
METHODS
RESULTS
The site was electrofished on ten occasions between
April 1996 and April 1997. On each sampling
occasion a series of twenty-minute fishings
were carried out, with eel as the only target
species. Details of the number of fishings, water
temperatures, conductivity and the total area fished
were recorded. Although only single fishings were
carried out, electrofishing is known to be an
effective method of capturing eels at this location,
and so relative densities (no. eels m 2 fished) of
eels were calculated as an index of density. On
capture the eels were anaesthetised, weighed to the
nearest gram, individually bagged and numbered
and transported back to the laboratory on ice to
help preserve stomach contents. Eels were then
frozen and stored at 158C for several months
prior to further analysis. Following thawing the
length of each eel was measured. Subsamples of
eels representative of each sampling date were
selected for dietary analysis. Stomachs were
dissected out, percentage stomach fullness estimated by eye, and food items then identified to
group level.
Saggital otoliths were removed from all eels
and allowed to dry for at least a week before further
processing. The otoliths were aged using two
different methods. The first involved the burning
and cracking of otoliths and was used on larger
otoliths. Smaller otoliths were aged after they had
been cleared in creosote. These methods had
already been proven to be suitable for use on the
lower River Shannon eels (Cullen and McCarthy
2003).
A total of 1467 eels were captured over the
study period, and an overview of the results for
each of the ten occasions on which sampling took
place is presented in Table 1. There was an apparent seasonality to the capture rate of the eels,
with lower rates recorded during the winter
months. This was confirmed by means of a partial
correlation of capture rate with temperature,
controlling for conductivity (correlation coefficient: 0.696, p 0.037, n 13).
The sizes of eels captured ranged from 76mm
to 742mm, with an overall mean size of 200.5mm.
However, the largest individual captured was of
exceptional size in relation to the bulk of eels
captured, with approximately 99% of the overall
sample of eels captured less than 400mm length
(Fig. 2). The weights of eels captured ranged from
less than 1g to a maximum of 754g in the case of
the largest eel captured, with 58% of the individuals
caught less than 10g weight and 99% of the sample
less than 100g. The overall relationship between
length and weight was expressed by the regression
log wt 3.176 log lt 6.228, R2 0.96. There
were no apparent seasonal differences in the
condition factors of the eels captured. Eels in the
smallest length size category (B100mm), which
were generally accepted to be those spending their
first year in freshwater, were seen to be present in
all months, but were less numerous in winter
months. No apparent trend as to their time of
immigration upstream to this region from estuarine
waters was found.
A total of 1372 individuals were successfully
aged. Freshwater age was calculated in all cases, i.e.
Table 1
*
Details of the numbers of eels captured by electrofishing at the lower River Shannon
site on ten dates between April 1996 and April 1997, together with information on
water temperature, conductivity, areas fished and length of fishing times.
Water
Conductivity
temperature (mS cm 1 )
( 8C)
04 April 1996
10 May 1996
19 June 1996
25 July 1996
03 Sept 1996
08 Oct 1996
11Nov 1996
19 Dec 1996
27 Jan 1997
17 April 1997
8
11
19
19
16
11
9
6.5
6
11.5
152
164
230
470
460
423
384
404
549
460
Overall
No. of
eels
180
181
195
118
103
174
93
111
129
183
1467
Time fished Area fished
(min)
(m2 )
100
125
100
40
60
100
80
100
80
80
865
350
380
320
160
220
300
280
320
260
260
2850
Relative
Capture rate
density
(no. man-hr1 )
(no. m2 )
0.51
0.48
0.61
0.74
0.47
0.58
0.33
0.35
0.50
0.70
0.51
54.0
43.4
58.5
88.5
51.5
52.2
34.9
33.3
48.4
68.6
50.9
89
BIOLOGY
AND
ENVIRONMENT
450
400
15
350
10
Length (mm)
% frequency
20
5
0
0
Length (mm)
*
Fig. 2 Length frequency distribution of the 1467 eels
captured on the lower River Shannon during the current
study.
0 indicates an elver that has just arrived in
freshwater. In the overall sample eel ages ranged
from a minimum of 0, to a maximum of 30 in
the case of the largest individual sampled, although
the absolute age of this individual is somewhat
doubtful due to difficulties in reading the outermost
part of the otolith. The average age of the eels
sampled was 8.2 years. The relationship between
length and age was used in the calculation of a von
Bertalanffy growth curve (Fig. 3).
As was the case with the mean lengths and
weights of the eels, there was some variation in the
mean age of eel capture in each of the ten samples
analysed. Similarly, variation occurred in respect of
the mean size of eel in each of the age groups
throughout the sampling period. It had been
hoped that it would be possible to analyse the
seasonal changes in growth rates of the eels sampled.
However, due to a number factors* the number
of age groups present in each sample, the low
number of eels in each of the age groups in each
sample and the variable size range of eels at each
age* it was felt that this analysis would not be
possible.
Stomach contents from a subsample of 30 eels
from each of the ten sampling dates were analysed.
As can be seen in Fig. 4 there is a clear seasonal
pattern to the feeding behaviour of the eels in terms
of both mean percentage stomach fullness and the
number of eel stomachs containing food items.
With the exception of the sample from the month
of July 96, when eels larger than 300mm were
generally absent from the sample analysed, no
overall differences in the length frequencies of eels
whose stomach contents were examined were
noted (Mann Whitney U tests, p 0.05).
Similarly no overall differences were found
between the length frequencies of the samples of
eels whose stomach contents were examined and
the monthly sample from which they originated
(Mann Whitney U tests, p 0.05). Of the 120 eel
stomachs examined from the months of May, June,
July and September 1996, 94 contained prey
remains, and of these only eleven contained
remains that could not be positively identified.
90
y = 747.4(1-e-0.02(t+3.91))
250
200
150
0
75
70
65
0
0
0
60
55
0
0
0
0
50
45
40
35
0
30
25
0
0
15
20
10
0
0
300
100
50
0
0
5
10
15
20
25
30
35
Age
*
Fig. 3 Length at age relationship and the associated von
Bertalanffy growth curve, for the 1372 lower River
Shannon eels that were aged.
In total 1215 individual prey items were
identified from the diet analysis of these eels, and
Table 2 presents a summary of the results of
these analyses, both on a monthly basis and
overall. Overall, the diet of the eel appeared to be
dominated by five main groups: trichopteran larvae,
ephemeropteran nymphs, gastropods, dipteran
larvae and isopods. A summary of the overall
relationship between eel size and the type of prey
consumed is presented in Table 3. The average
number of prey items per eel increased with eel
size category. Salmonid remains were recorded on
three occasions, in the stomachs of eels in the largest
size category.
DISCUSSION
The relative densities of eels at the lower River
Shannon site during the current study were similar
to values recorded during previous electrofishing
surveys of the same region (McCarthy et al. 1999).
These high relative densities are typical of the lower
reaches of a river system and were shown to be
significantly higher than those recorded further
upstream in the Shannon system (Cullen 1999).
Similar results have been recorded for many
other European river systems (Aprahamian 1986;
Feunteun et al. 1998). Another feature that is
typical of high density, lower river eel populations
is small eel size, and results similar to those recorded
in the current study have also been recorded by
many authors (Legault 1986; Naismith and Knights
1993; Lobon-Cervia et al. 1995; Feunteun et al.
1998). Indeed, previous studies of eels on the
Shannon system have indicated that the lower
River Shannon region produces predominantly
male eels, which mature at a maximum of
approximately 430mm (McCarthy et al. 1999;
McCarthy and Cullen 2000a).
The results of the ageing studies in the present
investigation indicated that the eels had a relatively
slow growth rate. This is not atypical of a
population that is dominated by male eels, and
ANGUILLA
(L.))
OF THE LOWER
RIVER SHANNON
30
60
25
50
20
40
15
30
10
20
5
10
0
% stomach fullness
No. eel stomachs containing food items
EELS (ANGUILLA
0
Apr- May- Jun96
96
96
Jul96
Aug- Sep- Oct- Nov- Dec- Jan- Feb- Mar- Apr96
96
96
96
96
97
97
97
97
Sampling date
Ide nti fi abl e pr ey
Unidentifiable prey
% stomach fullness
*
Fig. 4 Results of the analysis of the stomach contents of eels captured on the lower River Shannon over an annual cycle.
The number of eels containing food items, either identifiable or unidentifiable, is presented along with the mean
percentage stomach fullness for each sampling date. On each occasion the stomach contents of 30 eels were analysed.
these results agree with those of other studies of
eels of temperate waters (Naismith and Knights
1993; McCarthy et al. 1999). The current study
involved the comprehensive sampling, where
possible, of all year classes present in the
population. This is important in the study of
growth rate of eels due to a number of reasons
(Sinha and Jones 1967b; Tesch 1998). Irregularities
occur in length at age data, with wide ranges on the
sizes of eels in each age class and considerable
overlap between age classes. The wide variation in
eel growth throughout the species range is also
Table 2
*
the reason why growth models such as the von
Bertalanffy growth curve, which is frequently used
for other fish species, sometimes do not fit, as eels
do not necessarily follow the conventional growth
patterns of other fish species.
As eels are temperature dependent, populations
in Northern European locations tend to become
noticeably less active as water temperatures drop in
the winter months, and they remain inconspicuous
and relatively inactive until temperatures again rise
in the spring (Tesch 1999). Water temperature is
known to affect the efficiency of capture of fish by
Summary of the dietary composition of the lower River Shannon electrofished eels
examined during the months of May, June, July and September 1996. Data is
presented both in terms of numbers of prey individuals identified (No.) and their
frequencies of occurrence, i.e. no. of eels in which they occurred (Freq.).
May
No.
Hirudinea
Cladocera
Isopoda
Amphipoda
Gastropoda
Ephemeroptera
Trichoptera
Odonata
Hemiptera
Diptera
Coleoptera
Fish
1
15
3
5
55
29
4
3
3
June
July
September
Freq.
No.
Freq.
No.
Freq.
No.
Freq.
1
3
3
4
14
7
3
2
3
1
69
2
37
99
231
3
9
1
1
8
2
8
20
15
2
5
1
1
4
52
53
19
7
1
1
2
8
14
8
6
1
6
1
26
25
371
76
3
4
1
5
12
6
7
3
91
BIOLOGY
AND
electrofishing. Because many fish are less active at
colder temperatures they can remain hidden in
burrows, crevices, etc., and thus are more difficult
to catch by electrofishing (Zalewski and Cowx
1990). If the rate of capture recorded during the
current study is taken as an index of eel activity,
then by correlating water temperature against rate
of capture while controlling for water conductivity,
which is also known to affect the efficiency of
electrofishing (Zalewski and Cowx 1990), it was
possible to demonstrate this temperature influenced
nature of the lower River Shannon eels’ activity.
Seasonality in the feeding behaviour of eels has
also been documented elsewhere (Thomas 1962;
Sinha and Jones 1975), and eels in temperate
regions generally tend to cease feeding or feed to
a lesser extent during the winter months (Tesch
1999). There was a definite seasonal pattern in the
feeding behaviour of the lower River Shannon eels,
and while some feeding did occur during the colder
months of the year, the main feeding season ran
from May to September. Similarly there was some
evidence of seasonal differences in the composition
of the diet. While some food items were only seen
in the diet of these eels in the earlier months (e.g.
fish), others (such as dipteran larvae) increased in
importance later in the season. Eels are known for
the diverse range of prey items that they ingest,
making use of much of the aquatic fauna in the
regions they inhabit, and it has also been noted that
the composition of the diet can change seasonally,
reflecting changes in the aquatic community (Tesch
1999). Coupled with the fact that eels are also
known to stay within limited home ranges,
especially during the feeding season (McGovern
and McCarthy 1992; Tesch 1999), it is therefore
Table 3
*
likely that the diet of the lower River Shannon
eels reflects composition of the available macroinvertebrate community at the study site and in
the area surrounding the site. In general, the
macroinvertebrates recorded from the food of the
eels are not unusual for a riffle area. Indeed, in a
review of the macroinvertebrate communities of
the River Shannon, Bowman (1998) found that in
the faster flowing reaches of the lower River
Shannon the macroinvertebrate community is
dominated by ephemeropterans and trichopterans.
In general, larger prey, such as fish, were absent
from the diets of most of the lower River Shannon
eels. As with many other fish species, the size of
prey generally increases with that of the eel (Cairns
1941; Costa et al . 1992). Although during this study
no overall trend was observed in respect of increases
in prey size with eel size, it was clear that some of
the larger prey items were only found in larger eels.
Leech remains were only found in eels of length
greater than 200mm, and larger prey items such as
Odonata and fish were only found in eels greater
than 300mm.
The impact of eels on other fish stocks, in
particular salmonids, has long been the concern of
salmon fishery managers. Although preliminary
analysis by Mann and Blackburn (1991) suggested
strong overlap in the diets of salmonids and eels,
further analysis indicated distinct differences in the
feeding behaviours of both groups that would
minimise competitive impacts. They also concluded that as the eel population in the stream
they surveyed consisted primarily of individuals less
than 400mm length, piscivory on salmonids was not
likely to be a problem. In a survey of a number of
studies carried out in England (Frost 1946; Jones
A summary of the numerical composition of the diets of the lower River Shannon eels
in each of four size categories.
0 100mm
No. eels
No. eels with prey
Hirudinea
Cladocera
Isopoda
Amphipoda
Gastropoda
Ephemeroptera
Trichoptera
Odonata
Hemiptera
Diptera
Coleoptera
Fish
Total
92
ENVIRONMENT
13
3
3
3
100 200mm
45
32
4
3
80
13
43
4
147
200 300mm
34
24
2
72
4
64
98
69
6
1
316
300mm
28
24
1
18
2
53
51
568
4
3
46
3
749
EELS (ANGUILLA
ANGUILLA
(L.))
and Evans 1960; Thomas 1962; Sinha and Jones
1967a) by Michel and Oberdorff (1995) the authors
found that salmonophagic eels represented only 1%
of the overall numbers examined. Bearing these
facts in mind, and given that the lower River
Shannon eel population is generally dominated by
smaller male eels (McCarthy et al. 1999; McCarthy
and Cullen 2000a), it is likely that piscivory among
lower River Shannon eels is minimal. The fact
that this area of the Shannon has historically been
important as a commercial silver eel fishery yet
remains an internationally renowned salmon fishery
suggests that it is unlikely that the local eel
population has had or will have a deleterious
impact on the salmonids of the area.
The widespread collapse of European eel
Anguilla anguilla (L.) stocks is a matter of great
concern (Moriarty and Dekker 1997; ICES
2005), and there is now increasing need for
greater knowledge of the population biology
and general ecology of all populations of
European eel throughout its range (W. Dekker,
pers. comm.), in particular if conservation strategies
are to be successfully implemented. As this study
has indicated, there is considerable knowledge
available on lake populations of eels, and to a
lesser degree on stream populations, in particular
the mature downstream migrating proportion of
stream populations. There are, however, few
studies on the resident populations of some of
Europe’s large rivers. This study has highlighted
the presence of a dense population of eels in the
lower reaches of one such large river, the River
Shannon. This population is of importance not
only for the numbers of mature migrating silver
eels it can potentially contribute to the spawning
stock but also because it has been recognised
as a good source of fingerlings and bootlace eels
suitable for restocking the lakes of the Shannon
system (McCarthy et al. 1999; McCarthy and
Cullen 2000b). The approach taken in this study,
of analysing many aspects of the eels’ biology
and ecology over an annual cycle, and other
long-term approaches to the study of eels should
be encouraged. As was shown there was considerable variation in many of the factors information
recorded in each of the individual samples (sizes and
ages of eel, dietary composition, rate of capture).
However when taken as a whole the information
gathered from all samples gives a clearer picture
of the biology and ecology of the population
studied. The longevity of European eels and the
seasonality of their behaviour, in particular in
populations inhabiting the cooler waters of
northern Europe, means that in order to truly
understand the biology of the species and maximise
the outcome of conservation strategies, long-term
and integrated studies are necessary.
OF THE LOWER
RIVER SHANNON
ACKNOWLEDGEMENTS
The assistance of the Zoology Department Staff, in
particular Mr Eoin MacLoughlin and Mr Terry
Callanan, is greatly acknowledged. This study was
undertaken as part of the Electricity Supply Board
River Shannon Eel Management Programme. We
are most grateful to ESB Fisheries Conservation for
their support and funding of this work.
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