Assessment of the South African chokka squid Loligo vulgaris reynaudii: Is disturbance of aggregations by the recent jig fishery having a negative impact on recruitment?

Abstract An assessment of the South African chokka squid Loligo vulgaris reynaudii fishery is undertaken. Two fisheries catch chokka: the directed jig and the trawl fishery. Chokka is a by-catch in the latter fishery, which targets primarily Cape hake ( Merluccius spp.) and Agulhas sole ( Austroglossus pectoralis ). A population dynamics model is fitted to the jig and trawl CPUE indices, and two biomass indices from scientific surveys, by means of an observation-error estimator. The dynamics are modelled for two periods: January–March and April–December. Parameters estimated are the annual recruitment R and the catchability coefficients corresponding to each biomass index; a composite somatic growth and mortality parameter, g , is fixed externally. Within this approach, two models for annual recruitment are considered: (A) recruitment is constant above a biomass threshold, below which it starts to decline; (B) recruitment depends on jig-induced fishing mortality, larger values of which have an increasing negative impact on reproductive success. The parameters estimated for model A have wide confidence intervals and the model is unable to fit the decline in the early years of the trawl CPUE time-series. A better fit is achieved for the approach taken in model B. The linkage in this model of declines in recruitment to disturbance caused by the jig fishery is, however, somewhat ad hoc, and a basis for an independent test of this link is suggested. Model B estimates the current biomass to be heavily depleted. Stochastic projections under the assumption that the current effort level is maintained, show that the risk of the spawning biomass falling below 20% of its pristine level over the next 10 years is close to 90%. Effort needs to be cut to 2/3 of that at present to achieve a substantial reduction in this level of risk. Some alternative scenarios that are investigated, such as that of a non-linear relationship between jig CPUE and biomass, give more pessimistic results.