MANAGEMENT ASPECTS
(Convener: Wickstrom; Rapporteur: McGovern)
24. Alternative climbing media for elver passes in Denmark were reviewed by Dahl. He found plastic material (Enkamat) with its long life and ease of maintenance suitable to incorporate into eel passes. It has been applied in Danish rivers and streams of various types to assist efficient upstream migration of eels. Glasseels preferred finemeshed climbing material and larger eels preferred coarse material.
25. Legault presented two papers on the establishment of elver ladders in France and on factors that may influence ascending eel selectivity. The design consisted of modular units with resting platforms. The ability of eels to climb vertical obstacles decreases with age. The success of these ladders was evident from the large number of yellow eels and elvers utilizing them in the upstream movement.
26. Upstream light barriers were successful in deflecting eels away from the intake points of power stations (Hadderingh). The effectiveness of this system is influenced by river flow and water turbidity. Deflected eels should have an alternative route to circumvent the power plants thereby reducing the mortalities recorded.
27. The production of cultured eel in Italy is increasing both from intensive and extensive rearing units (Saroglia). The expansion of the industry is limited by the availability of elvers and of juvenile eels. Local supplies to the farms are supplemented with imports of elvers and young yellow eels mainly from France. The price of eel is stable as the market can still expand and looks good for the future. Environmental conditions and natural water temperatures allow aquaculture without the need of expensive heating or water recirculation systems.
BIOLOGICAL ASPECTS
(Convener; Wickstrom; Rapporteur: McGovern)
28. Electrofishing as a sampling device was evaluated by Costa et al., on eel populations in very small rivers. It was recorded that repeated fishing is necessary before any trend in population size could be calculated. Accuracy of population estimates favoured the weight mean capture/recapture and the mixed method of analysis.
29. Preliminary results from the use of video analysis of eel otoliths were presented by Doering and Ludwig. Fourier shape descriptors of otoliths were proposed to be suitable as an objective measure of age. The usefulness of this method as a tool in age determination will be reviewed when the study is completed.
30. The directions and depths of migrating silver eels were recorded by Tesch using acoustic pressure sensitive transmitters. Vertical movement in the pelagic zone indicated a dual pattern associated with sunrise and sunset. It was observed that eels did not rest on the sea floor regardless of tidal movements and that the recorded active migration was necessary to reach the spawning grounds.
31. The influence of temperature on sexual differentation from the glasseel stage was studied under culture conditions by Holmgren et al. Variable growth rates were recorded for test batches and for individual eels which changed with time, but culturing conditions were not optimal. The ungraded eel developed into bimodal length frequency populations. At least 90% of the eels which reached the length of 25 cm were male.
32. The differences in number and structure of arrested growth lines were used to characterize the habitat of yellow eels in the Vilaine River, France (Mounaix). It was noted that morphometric relations between eels and otolith structure might be suitable as habitat indicators. Double, large lines were considered as annual arrested marks in both riverine and estuarine samples, and simple single bands were recorded only in riverine eels.
