A perfect storm of genetic drift and divergence may prevent the rebuilding of the gemfish (Rexea solandri) stock on the east Australian coast

Abstract

The potentially negative effects of genetic drift are expected to be minor in fisheries species because they are numerically abundant, fecund and able to move freely in ecosystems that lack barriers to gene flow. Here we investigate the operation of genetic drift in a fisheries species that has experienced a dramatic decrease in population size. On the east coast of Australia between 1977–1988 the peak catch of gemfish (Rexea solandri) was approximately 5000 tonnes pa before a 95% decline with little subsequent recovery. To test for genetic drift on the east coast and to investigate population structure, individuals from six locations were genotyped with SNP (n = 2197) and microsatellite (n = 13) loci and were assayed for sequence variation in the mtDNA control region. Genetic divergence between the two populations (east and west) was high; FST for SNP loci was 0.439, G’’ST for microsatellite loci was 0.195 and mtDNA haplotypes were fixed. There was no evidence of interbreeding or gene flow between populations despite spatial overlap. The genetic diversity of the overexploited east population was 10% lower and allelic richness was 28% lower than the west population based on microsatellite loci, and there were significantly fewer SNP alleles in the east compared to the west population. Tajima’s D for SNP loci from the east population was high (1.826) providing support for a sudden population contraction. Genetic effective population size was 486 (95% CI; 278–1779) in the east compared to 1487 (95% CI; 592–infinite) for the west population. We conclude that genetic drift has occurred within the east population and speculate that this has disrupted adaptation, which may explain the resistance of the population to recovery from overharvesting and may indicate a reduced capacity to respond to environmental change. Similar combinations of genetic and genomic approaches may reveal the operation of genetic drift in other harvested species, emphasising the urgency to preserve naturally occurring animal populations which are essential for human food security.