Molecular Analysis of mtDNA Types in Exploited Populations of Spinner Dolphins ( Stenella longirostris )

Stock structure of geographic forms of the spinner dolphin (Srenellu longirosrris) in the eastern tropical Pacific (ETP) is a conservation issue. These animals are incidentally killed during yellow fin tuna purse seining by US and international fleets, and mortality quotas apportioned by stock have been established by legislative act. Four management stocks of spinner dolphins in the ETP are currently defined on the basis of morphology and geographical distribution. However, recent work proposes that two of the stocks (the northern and southern 'whitebelly' spinners) comprise a broad zone of hybridisation or clinal integration between the neighbouring endemic 'eastern' spinner and the more typical spinners residing to the west and around the world in the tropics. I f this hypothesis is true. evidence of current or recent gene flow between the neighbouring 'stocks' should be apparent. To investigate this, we conducted a molecular analysis of mitochondrial DNA (mtDNA) extracted from livers of 151 spinner dolphins from the ETP and 13 spinner dolphins from the Timor Sea. We also analysed mtDNA from 10 spotted dolphins from the ETP. Liver mtDNA from each animal was digested with six restriction enzymes to deduce the individual mtDNA haplotypes. Genetic distances were then inferred within the sample pool by comparison of the individual mtDNA haplotypes in a pair-wise fashion. The results suggest the following three points: (1) The morphologically defined neighbouring eastern forms of ETP whitebelly spinners are not genetically distinct at the level of resolution of our analysis. We found no concordance of mtDNA haplotype with either the stock type of the school or individual morphology. The mean between-form distance was not significantly different from the two mean within-form distances. Thus, introgression is likely occurring (or has recently occurred) and significant genetic interchange can be inferred. (2) Mean within-school mtDNA diversity varied from school to school. and no clustering of certain haplotypes within specific schools was detected. (3) Timor Sea spinner and the spotted dolphins have unique mitochondrial genotypes likely characteristic of their geographic and genetic separation in the case of the spinner dolphins and their speeics-level separation in the case of the spotted dolphins. These findings reinforce the morphologically-based hypothesis of non-uniqueness of the whitebelly form and suggest that relatively more emphasis should be placed on conservation of the endemic and presumably locally adapted eastern form.