22. Jellyman described growth rates of Anguilla australis and A. dieffenbachii in New Zealand fresh waters. Length at given age shows wide variation within and between populations, with growth rates averaging 2 to 3 cm per year in the wild, but more rapid in culture. Generation times are 25 and 40 years respectively for females of the two species, necessitating conservative management policies such as upper size limits. Ages of at least 90 years had been observed. With densities in the wild up to 1 000 kg per ha, growth may be suppressed by availability of food and interference competition.
23. Chan commented on differences in growth rates between wild and cultured eel and asked whether slow growth in culture is of behavioural or genetic origin. The problem of sex determination was aired by several speakers, including Boetius and Knights. Gelin commented on the potential of old eels for bioaccumulation.
24. Lobon-Cervia discussed longitudinal variations in population dynamics in a Cantabrian river. Eels were small, short-lived and about 99% were male and sex ratio appeared to be independent of density. Mean size increased with distance from the river mouth. Seasonal catch variations were related to temperature and activity.
25. Moriarty described a long-term study, begun in 1981, of the eel population in a small section of Lough Derg in Ireland. Numbers caught by constant effort showed expected variation between winter and summer and an unexpected low catch at midsummer. Variation between years correlated with total degree days from January to the end of May. The results confirmed the need for very long term observations in interpreting eel population material.
26. Cumaranatunga presented biological details for A. nebulosa and A. bicolor in sympatry and allopatry. The former was less tolerant of low oxygen than the latter, which was the dominant species in lagoons and reservoirs.
27. McCarthy summarized an external report on natural recruitment, fishery yields and current management in Ireland. Lack of adequate data has precluded accurate assessments but, for example, modelling the lough Neagh/River Bann system has allowed calculation of target elver restocking densities for the four major catchments.
28. Various management systems to enhance stocks (glass eel/elver translocation, facilitating upstream migration) were discussed and the need for a comprehensive large scale management plan noted.
29. Data from a study of two Swedish lakes stocked with elvers in 1980 were presented by Wickström. Recapture rates were about 11% (mainly migrating silver eel) in one lake, compared to only 1.7% (mainly fyke-netted yellow eel) in the other. Positive economic returns were achieved in the former within 10 to 11 years, profitability being strongly dependent on ecological productivity and growth rates and returns from male and female silver eel catches. Such long-term studies are essential in validating biological and economic bases for stocking and commercial exploitation.
30. Boëtius outlined studies over three seasons with different mesh sizes in fyke nets. The percentage of undersized eels ranged between 56 and 69, but did not differ significantly between nets; thus, despite an increase from 12 to 18 mm meshes, small eels were still retained. She proposed that small eels might be more effectively excluded by the use of a larger mesh size in the leader.
31. Donnelly presented past data analysis of records of silver eel catches and elver stockings in the River Shannon in Ireland dating back to 1938 and 1959 respectively. Using regression analyses a return after 15 years of 2.9% was found for the lower catchment and a return of 0.4% in the upper catchment after 11 years. The value of such analyses was discussed, with reference to the varying quality of the data sets.
POSTERS
32. Hegedis described a number of short (3–4 km) streams on the south Adriatic coast of Montenegro, all of which held populations of eel. Their size distribution depended mainly on the characteristics of the bottom and the macrophytic vegetation. All life stages were observed, glass eel in March and silver eel migration beginning in October and November.
