THE EEL (ANGUILLA ANGUILLA) POPULATIONS OF THE RIVER LEE SYSTEM
T. K. McCarthy1*, J. Grennan1, M. Murray1 and D. Doherty1,2
1Fish
Conservation Unit, Department of Zoology, National University of Ireland, Galway. 2ESB Fishery Conservation, Ballyshannon, Co. Donegal
Email: tk.mccarthy@nuigalway.ie
Introduction
The European eel Anguilla anguilla L, an important indigenous fish in Ireland, is a catadromous migratory
species. Its long-distance trans-Atlantic migrations, as Leptocehaus larval stages coming to Europe and North
Africa and as adults returning swimming to the Sargasso Sea spawning area have fascinated oceanographers
and fishery scientists over the past century. However, the panmictic stock is declining to critical levels and serious
concerns have been expressed at an international level about the need for immediate implementation of
conservation measures throughout the continental rage of the species.
A recent political response is a new EU Regulation for the recovery of the stock of European eel, requiring all
Member States to formulate National Eel Management Plans. The EU eel recovery plan is built on the principle
that each countries’ river systems should permit at least 40 % of the biomass of spawner escapement that
migrated to sea during former pristine conditions to do so in future. Thus, there is an urgent need for reliable
information on Ireland’s eel populations, so that they can be managed as required by the EU regulation and so
that the ultimate aim of restoring the species to sustainable levels can be achieved.
In anticipation of EU plan, to examine effects of hydropower dams on local eel population dynamics, we
undertook some surveys of some river and reservoir populations in the River Lee system Co. Cork. We also
wished to determine if introduced eel parasites were adversely affecting eels in the River Lee system
Results & Discussion
The introduced nematode A. crassus was not present in eels examined from the River Lee system. This probably
reflects the lack of eel fishing in the area, as commercial trafficking of eels seems to be the major factor affecting
the spread of this pathogenic parasite in Ireland. Initially recorded in the Waterford area, and soon after in the
major Irish eel fisheries of the Rivers Shannon, Erne, Corrib and Bann, this parasite is now being recorded in
various smaller exploited systems in the western and northwestern parts of the country. Protection of the Lee eel
stocks from this and other pathogens, that can affect the quality of spawners leaving the river as silver eels, is very
important.
Electrical fishing surveys revealed that high densities of small eels occurred in the River Lee downstream of the
reservoirs, where eel densities were approximately one eel per 2 m2 mostly less than 20cm in length. This shows
that successful recruitment to the river has occurred in recent years. Ensuring that such recruits are effectively
managed will be increasingly important to the EU recovery plan and to local fishery management. Electrical fishing
surveys of tributary sites upstream of both dams (Doherty, D., unpublished) and results of the present River Laney
survey suggests that eels reaching the reservoirs do not enter their minor tributaries to any great extent. However,
it seems likely that additional surveys in downstream tributaries such as the Rivers Bride and Martin would reveal
that they have significant numbers of eels and that such habitats may contain a higher proportion of male eels.
Results of 2006 fyke net and longline surveys are summarized below in Tables 1 and 2.
Study Area
The River Lee (catchment 1253 km2; mean annual discharge 27.1 m3sec-1), is has been exploited as a salmonid
fishery for at least 2000 yrs and more recently as a recreational coarse fishery. Its eel stocks, however, have
rarely been exploited. Scientific fish stock assessments have also generally neglected the eel populations of the
catchment area. The river was harnessed for hydroelectricity generation between 1953 and 1957 when the
Carrigadrohid and Iniscarra generating stations were commissioned. Large reservoirs were created upstream of
the two dams. The Iniscarra reservoir (9.3 km2) and the Carrigadrohid reservoir (5.3km2), as well as the natural
lakes of the upper parts of the river system (e.g. Lough Allua) are known to be inhabited by eel populations (Fig.
1). Eels are also widespread in the lower parts of the river basin, with 27% of the catchment area being
downstream of Iniscarra dam, and in parts of Cork Harbour to which the River Lee discharges 14.5 km below
Iniscarra.
Table 1. Summary of 2006 Iniscarra reservoir fyke net and longline eel survey results
No. of Fykes
25/07/06
26/07/06
19/09/06
38 (76 traps)
38 (76 traps)
28 (56 traps)
No. of Eels caught
CPUE-1
Average eel wt (g)
CPUE-2
Lines Set
87
166
24
1.14
2.18
0.429
207
186.7
250
0.237
0.408
0.107
No
No
Yes
No. of eel caught
70
CPUE-1
28.7
Average eel wt (g)
210.5
CPUE-2
4.91
Cork
Carrigadrohid
Reservoir
Inniscarra
Reservoir
Table 2. Summary of 2006 Carrigadohid reservoir fyke net and longline eel survey results
Electrofishing sites
No. of Fykes
No. of Eels caught
CPUE-1
Fig. 1 Location of reservoirs sampled on the river Lee, Co. Cork
Average eel wt (g)
Methods
CPUE-2
The 2006 River lee surveys involved three sampling methods, routinely used in River Shannon eel stock surveys
(1992-2006 ). These were: (1) electrical fishing for river surveys; (2) longline fishing and (3) fyke net fishing (Fig.
2).
In July 2006, three sites were electrofished: (1) the River Lee 1km downstream from Iniscarra dam, (2) the
River Lee 0.5 km downstream of Carrigadrohid dam, and (3) the River Laney about 2km upstream of the
Carrigadrohid reservoir.
(a)
Longlines set
27/07/06
28/07/06
20/09/06
38 (76 traps)
38 (76 traps)
28 (56 traps)
20
41
24
0.263
0.539
0.429
625
561
625
0.165
0.303
0.268
No
No
Yes
No. of Eel caught
51
CPUE-1
16.1
Average eel wt (g)
490.2
CPUE-2
8.33
The CPUE -1 values (Table 1 & 2) indicate a greater density of eels in Iniscarra Reservoir than in the Carrigadrohid
Reservoir. This was observed on all sampling dates and for both fishing methods.. However, from the CPUE- 2
values and the size frequency distributions ( Fig. 4) it is clear that the eels in Carrigdrohid Resevoir were
significantly larger than those in Iniscarra Reservoir. The CPUE values for these waterbodies were higher than
anticipated, when compared with results from the exploited River Shannon lakes. All eels caught at Carrigadrohid
were female, as reflected in their size, while <5% of eels caught at Iniscarra were indeterminate sex and possibly
be male. This low percentage of males in the Lee reservoirs will have implications for management of the eel
stocks in the river system.
(b)
% Length Frequency- Inniscarra v's Carrigadrohid July 2006
Fig. 2 (a) Standard longline set up (b) Standard fyke nets
35
Each site was fished three times in succession. Fishing time was approximately 20 minutes on each occasion
and the area fished was noted. Fyke nets were used in the two reservoirs in both July and September,
whereas due to administrative delays it was only possible to conduct the longline stock assessments in
September The fyke nets used were either new or had been steam cleaned to prevent movement of alien
species or pathogens between catchments. The nets were set at dusk and lifted at dawn in order to minimize
by-catch. A total of six fyke net surveys were undertaken, three per reservoir. Samples of eels were taken
from the fyke net catches and during electrofishing in July to check for the presence of the Asian parasite
Anguillicola crasssus. Two longlines surveys were undertaken, one per reservoir, in which bottom set lines (c.
300 earthworm baited hooks per line) were also deployed at dusk and lifted at dawn. The relative abundances
of eels, as recorded in fyke-net and longline surveys, are expressed CPUE-1, number of eels per sampling
unit (ie per trap or per 100hooks), or CPUE -2, weight (kg) of eels per sampling unit ( i.e. per trap or per100
hooks, hooks).
% Frequency
30
Inniscarra Fyke Net
Carrigadrohid Fyke Net
25
20
15
10
5
0
300-349
350-399
400-449
450-499
500-549
550-599
600-649
650-699
700-749
750-799
800-849
850-899
Length (mm)
Fig. 4. Length frequency histograms from fyke net surveys of Iniscarra and
Carrigadrohid reservoirs in July 2006
Conclusions
Fig. 3. Invasive parasitic Asian
nematode Anguillacola crassus from
the swimbladder of a River Shannon
eel
The large size of the eels, especially in the Carrigadrohid reservoir, and their potential
contribution to the declining European eel spawning stock, suggest that further study on the
escapement of migratory silver stage eels though the River Lee generating station dams will be
needed. Likewise, upstream migrations of juvenile eels will need to be investigated. Clearly,
periodic ascent of eels to the reservoirs has occurred in the past and this must have been via
Borland lift facilities. These were installed at the dams to facilitate salmon spawning migrations
and are used for capture of the salmon used for fry production at the ESB hatchery near
Carrigadrohid. Major technical problems still remain to be addressed if obstacles to natural eel
movements such as the hydropower dams.
Acknowledgments: This project was funded by ESB Fisheries Conservation.