EARLY STAGES
(Convener: Desaunay; Rapporteur: Aprahamian)
14. Growth of eel is known to be affected by both environmental and biological factors. Belpaire, Van Driessche, Gao and Ollevier studied the relationship between food availability and stocking density (7 kg elvers/ha and 26 kg elvers/ha). Density dependent growth was evident, with elvers stocked at the lower density reaching approximately 4 g by October compared with 1.6 g for the high-density eels. Growth rate was associated with a reduction of certain food organisms (molluscs, nematodes, copepods, Daphnia) in the high density pond suggesting prey was limiting. Peak feeding activity in the high density pond occurred between 4–6 a.m. and their daily consumption was estimated to be 4.38 mg dry weight/day. A digestion model was presented on the degree of fullness of the different parts of the digestive tract of eels sampled at 3 hourly intervals during 24 hours.
15. Klein Breteler (2 papers) studied the growth of eels at densities ranging from 160–1 600 glasseels/ha (approximately 0.05 to 0.5 kg/ha) associated with cyprinid (carp and bream) densities of between 320–420 kg/ha. No density dependent effects were apparent on length or weight attained by the beginning of October. Survival was, however, found to be inversely related to density. In another experiment, glasseels from the Gironde, Severn and Ijsselmeer were stocked at a density of 1 600/ha in association with cyprinids around 350 kg/ha. The results showed that there may be latitudinal differentiation in quality of eels with elvers from the Ijsselmer growing slower than those from the Gironde and from the Severn.
POPULATION AND FISHERIES
(Convener: Desaunay; Rapporteur: Aprahamian)
16. The eel stocks in the Dunkellin System, Western Ireland were studied by Callaghan and McCarthy. Eel densities and biomass inversely correlated with increasing distance from the sea and decreasing pH and conductivity. Growth appeared to be slow at between 1.1 to 4.9 cm/year. Dunkellin eels therefore grow slower than those in Scotland, Wales and Windermere. Reasons speculated could be the lower temperature and higher densities of eels.
17. Knights presented a paper on distribution, density and growth of eel in the Thames catchment by Naismith and Knights. Historic evidence indicates that the Thames once supported a healthy eel population, declining during the 19th century due to pollution. The data presented illustrated the present state of the population and was determined from sampling at 235 sites throughout the catchment. The findings indicate that the greater part of the population is confined to within 50 km of the tidal limit. With distance upstream there is a tendency towards slower growing, larger and mainly female eels. There are a number of possible reasons for the decline in fishing: barriers, perhaps pollution and the fall in elver supply. It was pointed out that while fisheries management approaches are limited, more can be done in enhancing catchment recruitment and utilisation, e.g. eel passes and stocking.
18. The value of eel stocking as a fisheries management tool was presented by Bninska and Leopold. Their study on Polish lakes indicated that bream and to a lesser extent roach stock composition could be controlled by the density of eels. In lakes where eel made up >5% of the catch the proportion of small bream was lower (17.2%) compared with lakes where eels constituted <5% of the catch (31.6%). The converse situation was true for medium and large bream. The situation for roach was less pronounced, though in lakes where eels made up >10% of the catch the proportion of small roach decreased. Since elvers for stocking are imported to Poland, the fishery managers now tend to allocate the stocking material to lakes where - apart from production aspects - eels could be used as a factor alleviating the symptoms of the eutrophication process. The reasons for the observed effect of eel on bream and roach stocks may be either competition for food or predation; these problems are now being studied.
19. A list of the names used for the different growth stages of Anguilla anguilla throughout Europe is being compiled by G. Williamson who requested the participants to assist with its preparation.
20. Dekker reported on the ICES Working Group which met from 16 to 18 May in Dublin and dealt with the assessment of eel stocks. A surplus production model has been proposed and is being evaluated using data from the Ijsselmeer and from Lough Neagh. Problems exist with variation in catchability. Temperature was shown to have an effect but other factors need to be considered.
21. Poole examined the age and growth of eels in two tributaries on the Burrishoole system, Western Ireland. This provided basic data on eels in extreme habitats with low pH, food availability and space. Growth was slow with a modal value of 0.5 to 1.5 cm/year. Otoliths were prepared using the “burning and cracking” method as recommended by EIFAC 1989. Large variation in both age for length and incremental growth was observed but it was noted that the population tended to grow towards the theoretical calculated value. The discussion highlighted differences both in techniques used and in the resulting growth estimated between different research bodies and it was recommended that the established burning and cracking method should be applied to the faster growing continental eels in order to assess their comparability.
22. A review of the development of the Corrib Eel Fishery was presented by Moriarty. Experimental fyke netting was carried out in 1967–69 and led to the development of a fyke net fishery commencing in 1971. The size of the fishery increased gradually up to 1983 with 200 authorized nets. The number of nets has fluctuated widely in recent years, the present level is approximately 200–250 nets. Silver eel catches reached a peak in 1982 after which there was a decline. The CPUE in the yellow eel fishery was approximately half that of the late 1960's; the decline was thought to reflect the decline in elver recruitment since the early 1980's. Comparison of the size structure of the eel population in 1969 with 1990 indicated little change over the time period.
REVIEW
ECOLOGICAL MATTERS
23. A review was presented by Fontenelle on age and growth of eels on a European scale. From 80 papers examined, 17 produced useful data sets. This produced 78 individual statistical data sets with more than 11 000 eels which were analysed using factorial analysis. 20% of the samples consisted of less than 30 individuals. The analysis, although preliminary, shows considerable potential for further development. The results indicated that shallow water and waters with a salinity of 20 ppm were associated with younger age groups. Some latitudinal differences in growth were apparent where the southern populations were greater than the mean, the converse being true for those from northern latitudes. Fontenelle also expressed caution when back-calculated values were used in the analysis. The participants were requested to cooperate in collecting all available data on growth for individual catchments, to assist with further evaluations.
The review and computer analysis of age and growth of eels for Europe as a continent showed the diversity and heterogeneity of data. This is to be expected due to different objectives of research, different methodologies and some other factors applied by different laboratories. The question arises whether extensive reviews are worthwhile and, if yes, what selection criteria should be applied for future analyses. The need for identifiation of clear objectives is evident, as objectives will determine the selection of key factors. Once trends are established supportive factors can be selected to assist in understanding local situations. This was the subject of further discussion.
