Although generally limited to coastal waters of South America, South Africa, and New Zealand, dusky dolphins (Lagenorhynchus obscurus) show high potential for dispersal over large distances. In New Zealand, photographic identification data indicate a seasonal shift in residency of dolphins between Kaikoura and the Marlborough Sounds as well as changes in group size and behavior. The effect of this seasonal variation on the genetic structure of New Zealand’s dusky dolphins was examined by sequencing a 473–base pair fragment of the mitochondrial DNA control region for 169 individuals from 4 regions along the New Zealand coast. A neighbor-joining phylogeny and an analysis of molecular variance did not support genetic subdivision among regions (ϕsr = −0.041, P = 0.13). However, nested-clade analysis demonstrated significant evidence for contiguous range expansion and fragmentation along the New Zealand coast. Seasonal movement patterns from Kaikoura to either Otago or the Marlborough Sounds and West Coast are presented as an alternative explanation of nested clade results. New Zealand–wide diversity indices and rate of substitution among sites were used to estimate effective female population size. Lineages-through-time analysis was used to test hypotheses of population growth. Structure of the neighbor-joining phylogeny, the nested haplotype network, and results of the lineages-through-time analysis suggest that the New Zealand dusky dolphin population underwent at least 1, if not 2, historical population expansions.
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