Behaviour and Exposure to Bird Predation of Elvers Anguilla anguilla following Mass ReleaseAuthor(s): Christopher MoriartySource: The Irish Naturalists' Journal, Vol. 21, No. 5 (Jan., 1984), pp. 220-221Published by: Irish Naturalists' Journal Ltd.Stable URL: http://www.jstor.org/stable/25538790 .

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220 /r.Ator.7.VoL.21No.5 1984

-(1972) Interesting fishes taken in Irish waters in 1971. Ir. Nat. J. 17: 210-215.

-(1973) Some interesting fishes taken in Irish waters in 1972. Ir. Nat. J. 17: 375-379.

-(1974) Some interesting fishes taken in Irish waters in 1973. Ir. Nat. J. 18: 57-65.

-(1975) Interesting fishes taken in Irish waters in 1974. Ir. Nat. J. 18: 205-208.

Went, A. E. J. and Kennedy, M. (1969) List of Irish Fishes. 2nd Ed. National Museum of Ireland, Dublin.

-and-(1976) List of Irish Fishes. 3rd Ed. National Museum of Ireland, Dublin.

Wheeler, A. C. (1969) The fishes of the British Isles and North-West Europe. MacMillan, London.

-(1978) Key to the Fishes of Northern Europe. Warne, London.

BEHAVIOUR AND EXPOSURE TO BIRD PREDATION OF ELVERS ANGUILLA ANGUILLA FOLLOWING MASS RELEASE

Christopher Moriarty

Department of Fisheries and Forestry, Fisheries Research Centre,

Abbotstown, Castleknock, Co Dublin

Overland transfer of elvers from the tidal boundary to the lakes on the River Shannon has been in progress since 1959 (O'Leary 1971). The elvers are carried by lorry in a tank of aerated water and released at various points on the margins of lakes where access by road to

the water's edge is convenient. Although the practice of elver transport has taken place in

Europe at least since 1908 (Tesch 1977), their behaviour on release does not appear to have been described. The matter is of economic interest since elvers released in shallow water in

daylight are subject to predation by birds. This note describes the behaviour of elvers and birds on 25 May 1982 at Rinnaman point in Lough Derg (R6878). The lake is 1.3 km wide at this point. Observations from 1155 hrs onwards were made from a parked car and care was

taken to avoid disturbing the feeding birds.

The elvers were collected from traps at Ardnacrusha at 0930 hours. Nearly all

belonged to 0+ age group and the catch weighed 801b (36.3kg). These elvers are reckoned to weigh 1,000 to the pound and therefore the number collected was 80,000. Release began at 1015 hours and lasted for about a quarter of an hour. After release the elvers swam near the surface, within one metre of the shore in both upstream and downstream directions.

After five minutes the fastest swimmers had moved 22m. A small number, fewer than ten, of black-headed gull Larus ridibundus were visible but none were close to the point of release.

At 1050 hrs, 35 minutes after release had begun, the gulls were still some distance

away and had not begun to feed. Most of the elvers were massed at the point of release and it was easy to catch them in numbers in a pond net. The fastest swimmers had travelled a

distance of 50m and continued to keep close to the shore. The gulls began to gather soon after 1050 hrs and by 1115 hrs thirty gulls and a merganser Mergus senator had begun to feed on the elvers. The behaviour of the birds showed that some of the elvers had moved offshore into deeper water but were still swimming close to the surface.

By 1130 hrs active feeding had ceased and 21 gulls had left the bay; the remaining nine were resting on the shore or in the water. All had gone at 1137 hrs, though a number had by this time assembled at a headland about 500m distant. They did not appear to be hunting.

The merganser, however, continued to feed. All the gulls which had been present had

gathered at the headland at 1140 hrs but within the next ten minutes they and others moved back to the bay where the elvers had been released and resumed feeding. There had been no

sign of disturbance of the gulls and no apparent reason for their temporary departure from the feeding area. By the end of the second phase of feeding the number of gulls had increased to approximately fifty. At 1200 hrs only 14 remained and hunting activities had ceased. The merganser had also departed. The next inspection of the lake shore was made at 1230 hrs by which time a few elvers were visible on the concrete boatslip at the point of release but none could be seen in the shallows elsewhere.

The fact that the feeding gulls were closely gathered and very active made it difficult to observe the behaviour of individuals. However, the diving frequency of three gulls and of

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Ir. Nat. J. Vol. 21 No. 5 1984 221

the merganser was recorded for short intervals. The gulls hunted by rising to one or two metres above the surface and making shallow dives. One made two dives in 15 seconds, one three dives in 15 seconds and one eight dives in 75 seconds after which it flew ashore to rest.

The merganser made 15 dives in 163 seconds. No attempt was made to assess the degree of success in diving for elvers, but it

appeared that the birds did not secure an elver at every dive. When a successful dive was

made, the bird came to the surface with the elver held between the mandibles, wriggling vigorously and frequently wrapping itself around the bird's bill. Sometimes the bird subdued and swallowed the elver rapidly, otherwise the fight lasted for a number of

seconds, with the bird having to make several bites across the elver's body before it was immobilised.

The elvers were exposed to the risk of attack by birds for a period of 2Va hours.

However, the birds were present in the area for only l3/4 hours and actually feeding for less than 3/4 hours. There were two periods of feeding: one of half an hour with 30 gulls or fewer and one of J/4 hour with up to 50 gulls. At any one time many of the gulls were resting and it

appeared that frequent diving was not maintained for more than a minute or two. Making a

very rough assumption that an average often gulls were diving at intervals of 8 seconds for the full 45 minutes and secured an elver at each diwt, they would have consumed 3,375 elvers. This represents only 4% of the elvers released. It is very likely that even this is a considerable overestimate since there was evidence that success in a bird's dive led to a

delay while the elver was swallowed and thus reduced the diving frequency. Moreover, it seems certain that a large proportion of the dives was unsuccessful.

Two methods of preventing bird predation completely are available: either stationing one person at the point of release for two hours to scare the birds or using a boat to release the elvers in deeper water where they would be out of reach of the predators. It is concluded, however, that the predation rate is so low that the cost of preventive measures would not be

justified. references

O'Leary, D. P. (1971) An account of some experiments in fishing with eel nets in Ireland. EIFAC Technical Paper 14: 135-142.

Tesch, F.-W. (1977) The Eel. Chapman and Hall, London.

NOTES ON THE IRISH PARAONIDAE (POLYCHAETA) WITH RECORDS OF TWO SPECIES NEW TO IRELAND

B. O'Connor, P. Dinneen, M. Conneely and T. Bowmer

Zoology Department, University College, Galway

The Paraonidae is a family of small infaunal, direct-deposit feeding polychaetes which until recently was poorly recorded from the Irish intertidal and subtidal zones. This paper reviews previous records and includes comments on species density, substratum type, depth of water in which each species occurred, and where appropriate, gives information on the

reproductive state of specimens collected. Two species are added to the Irish fauna.

Records come from several sampling locations around the Irish coast and are treated under West, South and East coast headings.

West coast

North Bay, Inner Galway Bay This area was surveyed between June 1978 and June 1979 at 103 locations with a

0. lm2 van Veen grab, two faunal samples being taken per station. A 1mm square-aperture mesh was used during on-board sieving prior to preservation in 4% buffered formalin.

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