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APPENDIX 25
EELS IN THE MUNSTER BLACKWATER RIVER, IRELAND1

by

C.I.D. Moriarty
Fisheries Division, Department of Agriculture and Fisheries
Dublin, Ireland

1. INTRODUCTION

Eels are plentiful in all of the lakes, estuaries and rivers in Ireland with the exception of a few small, land-locked lakes. Commercial capture of eels is affected by nets and traps for migrating silver eels and by long line for the feeding yellow eels. Wicker traps, baited with small fish, have been used on a small scale and until recently spear-fishing was carried out in the south-east of the country. Summer fyke nets have been used experimentally since 1961 in certain river estuaries.

Virtually nothing is known of the size and age composition of the stocks of eels in the Republic of Ireland. The present paper gives the results of the first of a series of investigations on the biology of the eel population being undertaken by the author. The Blackwater was chosen as a river estuary with a completely unexploited stock of eels.

2. EXPERIMENT

Samples of yellow eels were collected in the estuary of the River Blackwater, County Waterford, in 1965 and to a lesser extent in 1966. The tidal portion of this river is approximately 25 km in length and varies in width from 40 m to 800 m. The stretch selected for study was 5.6 km in length and extended from 4 km to 9.6 km downstream of the top of the estuary. Its width at high tide varied from 125 to 400 m. The bed of the estuary for the most part had a thick covering of mud. There were a few sandy areas but fishing on them gave poor results, and they were avoided as far as possible.

The gear used was a string of summer fyke nets, having an opening hoop of 54 cm diameter. In the first season (June to September 1965) each end of the string was attached to a stake and the nets were stretched across the current. In the second season (May 1966) the nets were attached to anchors and set parallel to the current. The nets used in 1965 had a mesh size (stretched) of 2.1 cm at the codend and those used in 1966, 1.7 cm. Each string consisted of from two to six pairs of nets with single leaders.

3. RESULTS

The catches per net are set out below:

 Mesh sizeNumber of eels/net/dayWeight of eels/net/day
1965 (June to September)2.1 cm2.14317 g
1966 (May)1.7 cm2.28283

The lengths and weights of the eels captured on a basis of 5 cm length groups are given in Tables I and II. Otoliths of 106 eels taken in 1966 were read. They were examined immersed in creosote but not ground. Considerable difficulty was encountered in making the readings, and the results given in Table III are set out on a two-yearly basis with a view to minimising the errors. The stomach contents of 168 specimens were examined, of which 44 (26 percent) were empty. The occurrence of the most important food items (those present in more than 10 percent of the stomachs) is set out in Table IV. Examination of the gonads of 38 of the specimens taken in 1965 revealed that 28 were female, 3 male and 7 could not be determined. The females ranged in length from 34 to 67 cm, the males 31 to 33 cm and the unknowns 33 to 37 cm.

Table I

Length Distribution of Blackwater Eels

Length
(cm)
19651966
NumberPercentNumberPercent
21–2530.1237.0
26–301115.89428.6
31–3543222.810632.3
36–4034518.1289.0
41–4538220.1289.0
46–5026413.9144.2
51–551517.9133.9
56–60924.882.4
61–65613.241.2
66–70502.630.9
71–7540.220.6
76–8040.210.3
81–85--10.3

Table II

Weight Distribution of Blackwater Eels

Length19651966
Weight (g)
Total
MeanTotalMean
21–251173969330
26–305 161463 84340
31–3528 300656 83864
36–4032 527942 76198
41–4551 0661334 149148
46–5050 7951923 098221
51–5540 3172673 492268
56–6032 8993572 986373
61–6528 0504592 016504
66–7028 7745752 200733
71–752 9056861 669834
76–803 704926892892
81–85--1 1101 110
Total304 615 35 747 

Table III

Length and Age of Blackwater Eels (1966)

Age GroupNumberMeanLengths (cm)Mode
MinimumMaximum
5 and 62131.0243926
7 and 84230.6243931
9 and 102741.2317845
11 and 12944.93860-
13 and 14440.25268-
15 and 16273.56582-
17171.0---

Table IV

Food of Blackwater Eels

 Present inPercentage
Amphipoda
(Gammarus sp.)
5947
Fish2117
Mysidacea
(Praunus sp.)
2016
Insects1411

The fishery is not commercially exploited, and in the virtual absence of large predatory fish which might eat the well grown eels it may be assumed that the fall in numbers of eels captured above the 31–35 cm groups results from the spawning migration. The proportion of eels smaller than these depended on the mesh size. In 1965, using the 2.1 cm mesh, 5.9 percent of the total catch were less than 31 cm; in 1966 with 1.7 cm mesh, these made up 35.6 percent. The mean weights of the small eels were substantially lower when the small mesh was used.

It appears from the examination of the gonads that male eels in the fishery do not exceed 37 cm in length. The length distribution figures show a decrease in the numbers of eels in the 36–40 cm group but similar numbers in that and the 41–45 cm group. This can be explained by the migration of a large proportion of male eels before they reach a length of 40 cm. The next abrupt drop in numbers comes after the 50 cm length by which size many of the females have left. Larger eels show a steady decline in numbers. The age determinations indicate that the majority of eels migrate at seven or eight years old. The rate of growth is very uneven and shows great variation amongst individuals.

Ten of the stomachs contained unidentifiable material. In some cases this may have been remains of offal from a bacon factory upstream, but otherwise the diet of the eels in the area consisted of living animals. Of the 21 stomachs containing fish flounder, Pleuronectes flesus occurred in five, eels in two, and the others were too much digested for identification. The insects in most cases were larvae of Trichoptera and Ephemeroptera.

1 Submitted at the Fifth Session of EIFAC as FI/EIFAC 68/SC I-3


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