Genetic evaluation of the unknown contribution of stocked fish in angler catches: a case study using Mulloway Argyrosomus japonicus

Stock enhancement involves the augmentation of wild populations with hatchery-reared recruits. Stock enhancement generally also includes a postrelease monitoring program which tracks stocked individuals within the fishery, and this relies on having a means to identify the likely origin of recaptured fish (e.g., physical, otolith, or genetic tags). This study reports the application of sibship analysis to retrospectively infer the origin of Mulloway (Argyrosomus japonicus) within stocked estuaries, when other means of identification were not available. Eight cohorts of Mulloway were stocked into two estuaries, across a seven-year period, but only some of the fish released during the program were physically marked with chemical otolith stains. Fish were sampled from stocked estuaries (mostly through an angler-based sampling program) and genotyped for six microsatellite loci, alongside 129 fish sampled from nonstocked estuaries. The presence of multiple sibs within the mixed populations in stocked estuaries was used to infer the origin of captured fish, against a background of sibship for known-origin individuals (verified by otolith marks) and sibship levels within unstocked estuaries. The analysis suggested hatchery-reared fish could have contributed 9% of individuals sampled from the augmented populations (7% when corrected for background sibship). The proportion of fish inferred to be of hatchery origin decreased with size (likely due to mortality and migration), and the expected contribution rates for hatchery-reared fish differed among cohorts. The results highlight that sibship analysis may be useful for retrospective genetic evaluation of stocked estuaries.