Murky waters: Searching for structure in genetically depauperate blue threadfin populations of Western Australia

• New genetic variants in Eleutheronema tetradactylum from Western Australia identify cryptic population structure.
• Migrants, or individuals with a recent ancestry of migration, comprise about 20% of each population.
• Year-class cohorts of recruits were more genetically similar to adults sampled in the same area.
• Recent co-ancestry, rather than high levels of gene flow best explain the genetic similarity of populations.
• The same fine-scale stock structure, found elsewhere in the species range, appears to exist also in Western Australia.

The blue threadfin (Eleutheronema tetradactylum) is an exploited fishery species in southeast Asia and Australia. Demographic studies have revealed fine-scale stock structure throughout the Australian coastline, with demographically isolated populations separated by only tens of km. Similarly, population genetic analysis revealed fine-scale structure across most of its Australian range with important implications for fisheries management. However, in northern Western Australia, genetic stock structure analysis showed a contradictory lack of structure. In the present study, one mtDNA marker and a suite of five microsatellite loci were used to further investigate the stock structure of Western Australian blue threadfin populations. By increasing sample sizes from previously investigated areas: Roebuck Bay (n = 93 adults) and Eighty-mile Beach (n = 92 adults and 163 recruits from two settlement cohorts), we were able to detect subtle genetic differentiation that was previously obscured by low levels of genetic polymorphism. Therefore, the same fine-scale stock structure that has been observed elsewhere in this species also appears to exist in Western Australia. This has clear ramifications for a revised management strategy that incorporates the fine scale structuring of northwest Western Australian stocks of the blue threadfin.