Isolation and interchange among insular spinner dolphin communities in the South Pacific revealed by individual identification and genetic diversity

Gray's spinner dolphins Stenella longirostris longirostris are found in apparently relatively small and discrete communities around many islands throughout the Pacific. However, the boundaries of these communities, on the scale of a dolphin's lifespan or across generations, are unknown. Here we report a combined demographic and genetic approach to describing the isolation and interchange of insular spinner dolphins among island communities of the Society Archipelago, French Polynesia. Dorsal fin photographs for individual identification and biospy samples for genetic analyses (n = 154) were collected from 6 island communities during 189 small-boat surveys over 3 yr. Capture-recapture analyses at Moorea (our primary study site), based on long-term observations of distinctively marked individuals and microsatellite genotypes (12 loci), indicated a local community of about 150 dolphins. This community appeared relatively closed on an intra-generational scale, as confirmed by resightings of individuals across 15 yr. Surveys around neighbouring islands indicated the presence of similar distinct communities, likely to follow demographic patterns similar to Moorea, with relatively low levels of interchange between communities. Overall, significant differentiation at both mitochondrial and nuclear levels indicated restricted gene flow among neighbouring communities, although some individual movement was documented. High levels of insular mitochondrial DNA (mDNA) genetic diversity (female long-term effective population size [N, f] - 100 000) contrasted with demographic characteristics. No evidence of bottlenecks was found in microsatellite allele frequencies or mtDNA haplotypes, discounting the possibility of a recent founder effect. Instead, we suggest that this genetic pattern is the result of metapopulation structure, based on numerous insular communities evolutionarily connected through male and female gene flow.