(Convener: Boetius; Rapporteur: Callaghan)
8. Moriarty reported that reports from 9 countries showed that the low level of elver recruitment seen throughout the 1980s had continued. Catches equal to the lowest on record were reported from 6 countries for 1992, with some slight improvements over recent years in the others. Great concern about these data was expressed by the Session.
9. Antunes & Weber compared the experimental glass eel fishery and by-catch data for the Rio Minho between 1981–2 and 1991–2. Official statistics from Spain and Portugal indicate an average catch of 14.2 t, falling by 18% in the course of the decade, with an increase to 13.4 t in 1991–2. Figures are probably underestimates because of under- and non-declaration of catches. The experimental fishery indicated a general decline and possibly changes in length and weight. By-catch yields were more important in summer months and included juveniles of some commercial fish species. It is not yet possible to estimate the effects of the fishery on such species.
10. Desaunay and co-workers had examined R. Vilaine estuary (France) glass eel otoliths for the last two years, according to 1991 Working Party guidelines. Immigration was continuous, lengths and weights following similar seasonal trends with maxima in December-January and minima in May-June. Average ages varied from 7 to 10 months. The study confirmed previous hypotheses on seasonal and biometric variations and the range of the larval period. Interannual variation was observed also both in biometry and age of glass eels sampled in spring.
11. Further studies by the above group were carried out on glass eels from the Adour, Vilaine and Somme estuaries. Biometric and otolith structure data and estimates of larval stage duration were collated to see if later immigration times at more northerly latitudes correlated with longer migration pathways over the continental shelf. No significant correlations were found and it was suggested that local or regional variations in factors such as fluvial stimulation, currents and tides were major factors influencing entry to freshwater. More hydrographic data of appropriate spatial and temporal scales are needed.
Appelbaum queried the interpretation of otolith rings but the authors referred to supporting evidence from Japanese eel and other fish species. It was estimated that 10–25 otoliths could be processed per week from extraction to final reading by the preparation and SEM methods used.
12. Donnelly et al. assessed the glass eel populations in the Shannon Estuary, Ireland, with a view to the possibility of developing a fishery for inland restocking and to help compensate for declining recruitment. Immigration occurred at the beginning of February. Patterns of migration and biometric change were similar to those in comparable estuaries. Improvements in methods for sampling, predicting runs and capturing elvers were discussed.
13. White & Knights gave a verbal presentation of preliminary data on work on the Rivers Severn and Avon, England. Elver numbers and freshwater stocks appeared to have fallen, elver runs showing some slight increases between 1991 and 1993. However, no data were available from the elver catching stations. Experimental fishing suggested an elver fishery yield of about 7.5 t in 1991. Questionnaire surveys of fishermen in subsequent years gave estimates lower than those expected. Migration of elvers and juveniles has been studied using up to 24 weir/sluice fixed traps on 15 tidal and upriver sites. Elvers predominate at the tidal limit between April and October, but very few reach these sites compared to the number caught in the fishery. Possible effects of the fishery and hydrographic factors are still under study. Catches per trap per night fall markedly with distance upstream, larger and older eels predominating to an increasing extent. Further work on the effects of barriers and on the use of eel passes and restocking will be presented at a later date.
14. Legault studied upstream migration using a ladder pass at the Ville Hatte dam 10 km from the sea in Brittany, France. Peak migrations occurred in 1992 in July and October (total 215 000, mostly > 1 g, range 0.3 – 350 g). More Europe-wide studies of fluvial migration using pass traps on migration barriers were urged.
15. Female American eels predominate at northerly latitudes in the St. Lawrence River and Lake Ontario, Canada. The number of recruits trapped at the Cornwall dam, which gives access to Lake Ontario, have declined by 95% between 1985 and 1992. Hodson presented a review of catch data which showed patterns in decline very similar to those at some European sites. The possibility that changes in Atlantic currents could be important was discussed but lack of data prevented any firm conclusions.
Concern was expressed that major declines in female eels could lead to endangering the species in N. America. Loss of habitat and migration barriers (especially pollution and hydro dams) could be implicated. The erection of physical barriers preceded major declines by too many years to correlate with the onset of the decline but pollution is of particular concern. Contamination of eels by chlorinated compounds, although now falling, has been demonstrated and food chain transfer to Beluga whales is suspected. Bioaccumulation in flesh and ovaries and contaminant-related pathologies occur in eels, these could affect the success of silver eel migration and breeding.
Provision of eel ladders, control of fisheries and further research were recommended. Ensuing discussion centred around common features between the North American and European eel recruitment situations and the need for similar and concerted actions.
