No abstract

–  Modelling‐governing patterns of European eel (Anguilla anguilla L.) distribution of four eel size classes (<150, 150–300, 300–450 and >450 mm) in the Frémur basin (northwest France) was done using artificial neural network (ANN) techniques and ecological profiles. Our results demonstrate the high predictive power of the ANN models. Some macro‐ and microscale factors, such as distance from the sea, depth and flow velocity, have the most significant influence on the models. Influence of distance from the sea appears to be very different from the spatial organisation usually described in river systems. In fact, the general tendencies of total eel densities according to the distance from the sea showed that densities increase weakly upstream. Another outcome was the variations in habitat preference according to the eel size, even if this species is spread over practically every type of microhabitat. Small eels were mainly found in shallow habitats with strong abundance of aquatic vegetation, whereas large eels tend to be found in intermediate to high depth with small to intermediate abundance of aquatic vegetation. Finally, we hypothesise that European eels change behaviour and microhabitat characteristic preference around a size of 300 mm.

Sicyopterus lagocephalus post-larvae migrating in Mascarene Archipelago rivers, La Réunion Island, provide an important food source to local populations and fishing activity has a socioeconomic impact. Improved knowledge of the life traits and the characterisation of post-larval stages should enable a better understanding of physiological changes triggering migratory behaviour. It would also help to explain the recruitment processes to managers and to implement conservation measures for stock management. Sicyopterus lagocephalus is an amphidromous fish: adults live in rivers, but larvae are carried to the sea after hatching. The present study characterised the metamorphosis and post-larval stages of Sicyopterus lagocephalus during river recolonisation on La Réunion Island. The results show that cranium reorganisation and pectoral fins transformation are part of the first and major transformations and are completed less than 2 weeks after freshwater colonisation (post-larval stages PL1 and PL2). These transformations, mobilising all the available calcium, seem to be critical for survival in their new environment. Individuals then progressively acquire pigmentation, complete scalation and the fork at the end of the caudal fin disappears (juvenile stages J1 and J2). The end of the recruitment phase takes place 21 to 28 days after entering the river mouth, when territorial behaviour appears.

Anguillid glass eels were sampled between October 2000 and October 2001 in an estuarine goby-fry traditional fishery of Réunion Island (21°S, 56°E), Mascarene Islands, western Indian Ocean. Recruitment occurred from November to April. Among the 633 specimens collected, 4 species were identified by biometric measurements coupled with number of vertebrae (61.9% of the specimens were Anguilla marmorata, 19.4% A. bicolor bicolor, 18.3% A. mossambica and 0.3% A. nebulosa labiata). A. mossambica had the shortest total length at recruitment (51.2 ± 2.7 mm), compared to A. marmorata (53.3 ± 2.5 mm) and A. bicolor bicolor (54.0 ± 2.1 mm). Most juvenile pigmentation corresponded to the glass eel stage (VA to VB). We extracted 34 otoliths and visualized their microstructure by SEM. The short-finned A. bicolor bicolor had the shortest leptocephalus stage (46.2 ± 5.8 d) and age at recruitment (79.8 ± 7.7 d). The long-finned glass eels had the same age at recruitment (120.2 ± 24.7 and 123.6 ± 17 d for A. marmorata and A. mossambica respectively) and the same leptocephalus stage duration (96.9 ± 26.4 and 102.1 ± 17.2 d for A. marmorata and A. mossambica respectively). Otolith readings and sampling dates showed that A. mossambica hatched about 2 mo earlier than A. marmorata. Their identical early life histories should imply adjoining spawning grounds, whereas A. bicolor bicolor must spawn in a distinctive location. Hypotheses for spawning area locations are discussed as a function of the region's oceanic circulation.