Stock structure of dusky flathead (Platycephalus fuscus) to inform stocking management

Abstract

Delineating stocks and quantifying population structure are critical for the management of exploited populations, but the stock structure of many species remains unclear. In New South Wales (NSW), Australia, a marine stock enhancement program for dusky flathead (Platycephalus fuscus) is developing, and knowledge of stock structure is required to inform enhancement strategies, as well as to support broader fisheries management. A combination of mitochondrial and microsatellite markers was used in this study to evaluate structuring among fish from eight estuaries, spanning the majority of the NSW coast. Pairwise comparisons of mitochondrial haplotypes revealed that seven of the eight estuaries were genetically homogeneous (St Georges Basin was distinct). Moreover, analysis of molecular variance showed that 97% of sequence diversity occurred within estuaries. The results from microsatellite markers were almost identical to those for mitochondrial DNA, with St George’s Basin having the only distinct genotype, and within estuary variation accounting for 99% of the genotype variation. Isolation by distance analysis showed that <5% of genetic variation was due to distance. These results indicate that dusky flathead forms a single stock across NSW, where mixing most likely occurs during early life phases and through limited adult migration. The outlying estuary, St Georges Basin, likely suffers from limited connectivity due to a constricted entrance, and it is possible that stocking may improve local genetic diversity.