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Investigation on the effects of different levels of effort and of the closed season in the jig fishery for chokka squid <I>loligo vulgaris reynaudii</I>
Abstract
After effort control, the closed season is the most important management tool used to regulate fishing mortality in the South African jig fishery for chokka squid Loligo vulgaris reynaudii. The dynamics of the stock biomass
on the spawning grounds were modelled in order to assess the effects of current effort levels and the existing closed season on the resource. The model assumes that process error dominates over observation error. The
model has three parameters: g, a parameter which combines natural mortality, emigration and somatic growth; q, the catchability; I, which represents immigration to the spawning grounds. Estimates of the parameters were obtained by fitting the model to catch per unit effort data, and were used to project the stock biomass forward in time to evaluate the impact of various effort levels both with and without a closed season. Results from the study indicate that the biological and economic gains provided by the current closed season are small and that there is little justification for urgent action to curtail current effort levels. However, in view of the apparently high risk associated with the current levels of effort and the great sensitivity of the results to basic model assumptions, both maintenance of the closed season and avoiding increases above the current level of effort are recommended. Further, the level of effort may need to be reduced, and the option of lengthening an existing
closed season to effect this may well prove easier to implement than any attempt to reduce the present number of participants in the fishery.
on the spawning grounds were modelled in order to assess the effects of current effort levels and the existing closed season on the resource. The model assumes that process error dominates over observation error. The
model has three parameters: g, a parameter which combines natural mortality, emigration and somatic growth; q, the catchability; I, which represents immigration to the spawning grounds. Estimates of the parameters were obtained by fitting the model to catch per unit effort data, and were used to project the stock biomass forward in time to evaluate the impact of various effort levels both with and without a closed season. Results from the study indicate that the biological and economic gains provided by the current closed season are small and that there is little justification for urgent action to curtail current effort levels. However, in view of the apparently high risk associated with the current levels of effort and the great sensitivity of the results to basic model assumptions, both maintenance of the closed season and avoiding increases above the current level of effort are recommended. Further, the level of effort may need to be reduced, and the option of lengthening an existing
closed season to effect this may well prove easier to implement than any attempt to reduce the present number of participants in the fishery.
