Genetic patchiness among recruits in the European eel Anguilla anguilla

Heterogeneity in genetic composition among recruits of marine species is mostly due to a large variance in reproductive success mediated by oceanographic processes. Temporal genetic variation in a population of the European eel was quantified over 2 time scales among glass eel (1) interannual samples (cohorts), and (2) intra-annual samples within cohorts ('arrival waves'). A total of 789 glass eels comprising 11 different arrival waves were collected at Den Oever in The Netherlands over the period 2001 to 2003. All samples were screened for genetic variation using 10 allozyme and 6 microsatellite loci. The main result from this study is the highly significant genetic differentiation among arrival waves, despite the low FST values (FST = 0.0036). Heterogeneity in genetic composition was observed both among cohorts and among samples within cohorts. Genetic differentiation partitioned within cohorts was more than 10-fold the differences among cohorts. Genetic heterogeneity is likely to result from a large variance in the contribution of individuals to each cohort determined by genetic drift. Although natural selection and gene flow could also play a role in the observed genetic pattern, we suggest that large variances in reproductive success are a contributing factor to the recruit differentiation. If only a subset of the adults contribute to the new recruits, effective population size in European eel might be much lower than the census size. A low effective population size combined with fluctuating oceanic conditions might have contributed to the current dramatic decline in abundance of European eel.