SEASONAL AND LONG-TERM CHANGES IN BIOMETRICS OF EEL LARVAE - A POSSIBLE RELATIONSHIP BETWEEN RECRUITMENT VARIATION AND NORTH-ATLANTIC ECOSYSTEM PRODUCTIVITY

The European eel larval life has two stages: the oceanic leptocephalus , in which growth is dependent on food supply, and the metamorphosis g lass eel stage, in which feeding and growth are stopped until immigrat ion in estuarine waters. Data from fisheries and experimental surveys in European estuaries at different periods between the 1930s and the 1 990s showed a simultaneous decrease of mean length and abundance level . A monthly survey at the Vilaine estuary (Bay of Biscay) during 2 yea rs provided a periodical model of seasonal variation in length and wei ght of newly recruited glass eels. Otolith microstructure was used to back-calculate larval stage durations and the timing of transatlantic migration. Glass eels entering the estuary in autumn are longer and mo re numerous than those arriving in summer. They migrated across the oc ean during spring and summer, seasons when plankton production is like ly to reach maximum levels. The proposed hypothesis regarding the dete rminism of recruitment in the eel, on the seasonal and on the yearly r ange, is that leptocephalus growth and glass eel survival are partly d ependent on the plankton production at the oceanic scale. Eel recruitm ent dynamics could provide an integrated figure of large-scale environ ment fluctuations. (C) 1997 The Fisheries Society of the British Isles .