Genetic consequences in the southern African endemic seabream Polysteganus undulosus (Sparidae) after eight decades of overfishing

Abstract

The Critically Endangered seventy-four seabream Polysteganus undulosus, a slow-growing sparid that forms spawning aggregations off South Africa, faced heavy exploitation from 1910 until a fishery moratorium was put in place in 1998. Utilising temporal samples from 1962/1963 (mid-collapse) and 2005/2006 (post-collapse), we assessed genetic diversity at six microsatellite loci. Amplification success for archived samples was low (43%), necessitating a rarefaction approach, revealing a 40% decrease in allelic diversity. Significant genetic differences between recent and archived samples confirmed the impact of overfishing. Simulation studies indicated that missing genotypes did not affect these tests, validating the genetic differences. Using a coalescent-based approach, a 10-fold decrease in effective population size (Ne) was estimated over this 43-year period (Ne = 43.88 to 4.76). Simulations provided corrected Ne of 36.86, accounting for missing genotypes that were responsible for inflated values. Assuming 25% of pristine levels in the 1960s, pristine Ne values ranged from 147 to 176 in 1910, suggesting a 96.8–98.3% genetic decline over 88 years of exploitation. This study emphasises severe genetic consequences of overfishing on P. undulosus diversity and effective population size. The results provide vital genetic baseline data for recovery assessment and conservation efforts, as well as an analytical framework to evaluate the stock decline using partial genotyping data gathered from degraded archived samples.