The recognition of the vulnerability of marine species to environmental conditions throughout their life-cycle has broadened the scope of investigations on factors influencing their population dynamics and demographics. The European eel Anguilla anguilla (Linnaeus, 1758) stock
is declining rapidly due to overfishing, pollution, habitat degradation, diseases, and oceanic conditions. We analyzed arrival waves of glass eels collected in the Bay of Biscay and the western Mediterranean area during the period 2001–2003. We observed significant differences in length,
weight, and condition between Atlantic and Mediterranean samples, and among arrival waves within sites. All samples were screened for genetic variation using ten allozyme and six microsatellite loci. We observed a pattern of genetic patchiness among arrival waves, namely a highly significant
genetic differentiation without a temporal grouping of samples. Although natural selection and gene flow could also play a role, we suggest that the pattern observed results from the high variance in reproductive success in each spawning season. A low effective population size might have contributed
to the current decline in the abundance of European eel. A precautionary approach to fisheries should be implemented in order to preserve maximal genetic potential to cope with changing anthropogenic and environment pressures.
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