Population genetic structure of the Japanese eel Anguilla japonica: panmixia at spatial and temporal scales

Since the 1970s, the population of the Japanese eel Anguilla japonica has dramatically declined in East Asia. Consequently, conservation and resource management of this species are urgently required. However, the population genetic structure of this species, in temporal and spatial scales, is still poorly understood. We used 8 polymorphic microsatellite DNA loci to investigate its genetic composition. For cohort analysis, juvenile (glass) eels were collected yearly between 1986 and 2007 from the Danshui River, Taiwan; for arrival wave analysis, glass eels were collected monthly from Fulong Estuary, Taiwan; and for spatial analysis, glass eels were collected from Taiwan, China, Korea and Japan. Genetic differentiation among annual cohorts, arrival waves and spatial samples was very low; a significant difference was observed among annual cohorts and spatial samples, but not among arrival waves. However, specific temporal or spatial scale patterns were not seen in either pairwise genetic comparisons or the phylogenetic tree of all samples. Occasional genetic variations among samples occurred randomly, but a stable lasting genetic structure could not be formed. The isolation by distance (IBD) test showed no evidence of genetic structuring at the spatial scale, and the results of the isolation by time (IBT) test were insignificant among arrival waves. Genetic heterogeneity over a 21 yr time scale showed marginal significance, potentially reflecting a genetic drift in the Japanese eel. Our results suggest the existence of a single panmictic population of Japanese eel in East Asia. Therefore, the Japanese eel should be considered as a single management unit for conservation.