Management of Argyrozona argyrozona (Pisces: Sparidae) in South Africa based on per-recruit models

Abstract

Carpenter Argyrozona argyrozona is an endemic sparid that constitutes an important component of the South African linefishery between Cape Agulhas and Port Alfred, where it exists as two stocks; one on the central Agulhas Bank and the other on the eastern Agulhas Bank. Spawner biomass-per-recruit (SB/R), fecundity-per-recruit (Egg/R) and yield-per-recruit (Y/R) models were used to model both South African carpenter stocks. Owing to the allometric relationship between annual fecundity and individual size, Egg/R ratios were between 40% and 74% of SB/R at equivalent fishing mortality (F). Egg/R ratios account for allometric increases in fecundity with size/age, and are therefore regarded as better estimators of reproductive potential. It is shown that the current length at first capture (Lc) of 250mm TL and F (at M = 0.1) will reduce Egg/R to 6.41% of the pristine value in the eastern Agulhas Bank population and to between 6.06% and 14.15% on the central Agulhas Bank, indicating that both stocks are heavily overfished. An increase in Lc from 250mm to 350mm TL and a 70% reduction in commercial fishing effort is recommended to attain a target reference point of 40% Egg/RF=0. Bag frequencies indicate that a reduction in daily bag limit from 10 fish person−1 day−1 to four fish person−1 day−1 would effect an equivalent reduction in recreational fishing mortality. The trawl bycatch of carpenter is only 3% of the reported line catch, consequently restrictions to this fishery are not recommended.