Silver eel

Abstract: The identification of five stages for female and two stages for male eels Anguilla anguilla using multivariate analysis was carried out on a large sample of individuals collected at six different locations in France. Stages corresponded to a growth phase (stages I and II), a pre-migrant phase (III) and two migrating phases (IV and V). It is likely that an important period of growth triggered silvering through the production of growth hormone (GH) in stage III eels. In migrating eels gonad development, gonadotropin hormone (GTH-II) production and increase of eye surface were similar at all sites. Differences among locations were found in gut regression and pectoral fin length. As variability for these increased with the size of the watershed and values were highest for the most downstream locations, fin length and gut regression may indicate the time since an eel started its migration.

Abstract: The environmental history of American eels Anguilla rostrata from the East River, Nova Scotia, was investigated by electron microprobe analysis of the Sr:Ca ratio along transects of the eel otolith. The mean (±SD) Sr:Ca ratio in the otoliths of juvenile American eels was 5.42 × 10 -3 ± 1.22 × 10 -3 at the elver check and decreased to 2.38 × 10 -3 ± 0.99 × 10 -3 at the first annulus for eels that migrated directly into the river but increased to 7.28 × 10 -3 ± 1.09 × 10 -3 for eels that had remained in the estuary for 1 yr or more before entering the river. At the otolith edge, Sr:Ca ratios of 4.0 × 10 -3 or less indicated freshwater residence and ratios of 5.0 × 10 -3 or more indicated estuarine residence. Four distinct but interrelated behavioural groups were identified by the temporal changes in Sr:Ca ratios in their otoliths: (1) entrance into freshwater as an elver, (2) coastal or estuarine residence for 1y r or more before entering freshwater, and, after entering freshwater, (3) continuous freshwater res- idence until the silver eel stage and (4) freshwater residence for 1 yr or more before engaging in peri- odic, seasonal movements between estuary and freshwater until the silver eel stage. Small (< 70 mm total length), highly pigmented elvers that arrived early in the elver run were confirmed as slow growing age-1 juvenile eels. Juvenile eels that remained 1 yr or more in the estuary before entering the river contributed to the production of silver eels to a relatively greater extent than did elvers that entered the river during the year of continental arrival.

Abstract: The main energetic stores at the silver eel stage were studied by analysing muscle fat concentrations and hepatosomatic indices in female silver eels from various habitats in Sweden. Muscle fat concentrations varied both within and between localities and lean eels with muscle fat concentrations <20% occurred at all study sites. Furthermore, no correlation could be found between muscle fat content and internal or external maturation indices, neither was the relative liver size related to the maturation process, as the correlation between the hepatosomatic and gonadosomatic indices was very weak. Consequently, it was concluded that silvering and the spawning migration may begin also at low muscle fat concentrations. However, most of the energy reserve is stored as muscle fat in eel, and it is highly unlikely that female silver eels with such low fat contents, as were observed occasionally in this study, will ever recruit to the next generation. Therefore, it is suggested that the maturation process in eel is more flexible than previously recognized, and that this process might be temporarily arrested and feeding resumed during the first part of the migratory phase.