The biology of and fishery for king mackerel, Scomberomorus commerson (Scombridae), along the southern Mozambique and KwaZulu-Natal coast.
The king mackerel, Scomberomorus commerson, is an epipelagic, schooling predator supporting significant commercial, artisanal and recreational fisheries throughout the coastal waters of its Indo-Pacific distribution. Despite the importance of the species within the South West Indian Ocean, little research has been undertaken on its biology and fisheries on a regional basis over the past 20 years. The aim of this study was to evaluate the fishery and biology of S. commerson in the South West Indian Ocean and identify gaps in information required to ensure its effective management. Catch and effort data for the KwaZulu-Natal recreational S. commerson linefishery were extracted from the National Marine Linefish System in order to assess spatial and temporal trends in abundance. Generalized linear models utilizing the delta method were used to quantify the effect of year, month, region, rainfall and sea surface temperature on CPUE. Biological samples of S. commerson were collected monthly from within KwaZulu-Natal and southern Mozambique. Samples were analysed using standard biological techniques. A per-recruit analysis was conducted using the biological parameters from KwaZulu-Natal and southern Mozambique in order to assess the status of the S. commerson stock and provide management recommendations based on the findings. Long-term trends in CPUE were cyclic in nature with peaks and troughs appearing to be independent of fishing pressure. Seasonal abundance reflects the south-north migration into KwaZulu-Natal waters with short term environmental factors such as sea surface temperature significantly affecting spatial and temporal extent of the migration among regions. S. commerson spawn in southern Mozambique waters from September to January (spring-summer) with males maturing at a smaller size (65.2cm FL) compared to females (82.3cm FL). The overall sex ratio (M: F) was 1:1.36 possibly as a result of linefishing selecting for faster growing, larger females. S. commerson in KwaZulu- Natal and southern Mozambique display rapid growth over the first two years before slowing down considerably after maturity is reached. Females grow faster and live longer compared to males dominating the older and larger size classes, and attaining a maximum observed age of 14 years, although fish probably live up to 20 years. Natural mortality rate was estimated at 0.27 yearˉ¹. Fishing mortality for the combined region was 0.21 yearˉ¹. The per-recruit analyses for the KwaZulu-Natal and southern Mozambique indicated that the fishery is being optimally exploited with a current spawner biomass per recruit at 49% of its theoretical pristine level. Uncertainty with regards to the fishing pressure in southern Mozambique as a result of illegal fishing and fishing sectors targeting smaller S. commerson is a cause for concern. The current recreational daily bag limit of 10 fish.person.dayˉ¹ is considered excessive by many stakeholders. Given the similarity of the recreational ski-boat fishery in southern Mozambique, a reduction in the DBL of S. commerson to five fish pppd in both KwaZulu-Natal and southern Mozambique waters would benefit recreational fishers by more equitable sharing of the catch and potentially by reducing fishing mortality at times when the fish are aggregated and vulnerable to high catch rates. A reduced DBL limit would also reduce the incentive of individual anglers to make large catches and to sell their fish illegally.