Assessment of the movement behaviour of dusky kob Argyrosomus japonicus along the coast of South Africa using 39 years of tag-recapture data from a voluntary tagging project

Abstract

The dusky kob Argyrosomus japonicus (family Sciaenidae) is a large-sized estuarine-dependent fish of high fisheries importance, with only ∼1.3% of the pristine stock remaining due to overexploitation, habitat loss and mismanagement. Its conservation is complicated by the species’ cryptic morphology and range overlap with congeners. Understanding movement patterns is critical for effective management. Historical catch-per-unit-effort data suggest annual adult spawning migrations from waters off the Western Cape and Eastern Cape to off KwaZulu-Natal, South Africa; however, contemporary acoustic telemetry data indicate higher adult residency than generally assumed. Therefore, this study analysed 39 years (1984–2022) of tag-recapture data from the Oceanographic Research Institute’s Cooperative Fish Tagging Project (24 520 tag-releases, and 1 600 tag-recaptures). After taxonomic screening, 18 488 tag-releases were retained for the dataset, of which 77% were juveniles, 18% were sub-adults, and 5% were adults. Of 1 420 recaptures, 80% were juveniles, 17% sub-adults, and 4% adults. Mean movement distances were 16 km for juveniles and 43 km for sub-adults, with only 18% and 34% of those groups, respectively, displaying movements of >100 km. Among 53 adults (mean movement of 114 km), 55% moved >100 km, but only 2 of these adults displayed eastward movement consistent with a spawning migration to the coast of KwaZulu-Natal. These results, combined with prior studies, support emerging paradigms of partial migration and the existence of highly resident subpopulations of A. japonicus within a metapopulation structure. High residency and limited connectivity among coastal zones across size classes (life stages) suggest that management should prioritise spatially explicit measures, such as estuary-specific protection, to conserve local populations and prevent further declines of this imperilled species.