Genetic analysis of stock structure of blue shark (Prionace glauca) in the north Pacific ocean

The blue shark (Prionace glauca) is globally distributed, occupying the pelagic, open ocean in temperate and tropical waters. It is the most abundant shark in the north Pacific and is exploited in several fisheries. Catch size and sex composition in fisheries and research data, along with tagging and distribution patterns, have been used to infer the existence of distinct north and south Pacific stocks. The present study consisted of a comprehensive survey of nuclear genetic diversity at microsatellite loci within and among six regions encompassing the north Pacific blue shark distribution to rationalize current management practice which assumes a single stock. We tested microsatellite diversity at 14 loci in 786 tissue samples for east-west population differentiation, examined regional samples for evidence of population mixture, and estimated historical and contemporary effective population sizes (Ne). The results strongly supported the existence of a single population of blue shark in the north Pacific. Historical and contemporary Ne values between 4500 and 5500 and the resultant low ratio of effective to census size (Nc) does not reflect recent population perturbation. However, it does highlight that the population may be more vulnerable to reduced reproductive success arising from natural or fishing mortality coupled with environmental change than total abundance would imply.