Management of South African dusky kob Argyrosomus japonicus (Sciaenidae) based on per-recruit models

The dusky kob Argyrosomus japonicus is an important linefish species that is abundant in South African waters between Cape Agulhas and the Mocambique border. The juveniles (<1 070 mm) occur inshore, where they are targeted by surf-zone and estuarine anglers, and the adults are caught mainly beyond the surf by recreational and commercial linefishermen. Yield-per-recruit (Y/R) and spawner biomass per-recruit (SB/R) models were applied to the South African dusky kob stock, with accommodation for different inshore (F-inshore) and offshore (F-offshore) fishing mortality. It is shown that, as a result of high juvenile mortality (F-inshore = 4-6-fold F-SB25 and 6-10-fold F-SB40), dusky kob were exploited far beyond optimal (F-SB40) and threshold (F-SB25) fishing limits, and that SB/R was 1.0 - 4.5% of the pristine value. This is indicative of severe stock depletion and of recruitment overfishing. Based on the bag frequencies of 16 367 angler outings, it is clear that the current recreational bag limit of 10 fish.angler(-1).day(-1) does not limit fishing effort. Three alternative management scenarios (combinations of F-inshore and minimum size at first capture) are discussed, which elevate SB/R above 25%SB/R-F=0, increase Y/R beyond current levels, and equitably apportion Y/R-inshore and Y/R-offshore. Whereas it would take about 41 years for any of the alternative scenarios to attain maximum SB/R, a 3-fold increase is expected after 9 years and a 6.0 - 6.4-fold increase after 20 years. The increase in spawner biomass associated with the alternative strategies is expected to increase both inshore and offshore yield dramatically (as a result of higher recruitment), even though Y/R-inshore would be 69 - 70% of its current value. It is concluded that, because of the susceptibility of A. japonicus to recruitment overfishing, the species may have been overexploited in fisheries throughout its wide distributional range, and that accurate life history data and assessment-based regulations are essential for successful long-term management.