Trap selectivity and the effects of altering gear design in the South African rock lobster Jasus lalandii commercial fishery

Abstract

The current trap fishery for the West Coast rock lobster Jasus lalandii in South African waters results in the capture, sorting and release of large numbers of undersized animals. Once removed from the water, they are vulnerable to damage from numerous sources. Even sub-lethal injury may result in a considerable reduction to individual productivity through decreased growth or reproductive potential. Given that the J. lalandii resource is heavily depleted, such wastage may have severe repercussions for the sustainability of the fishery. In an attempt to reduce these losses, 20% of the fishing gear used by the rock lobster industry has been modified to include grids designed to allow undersized rock lobsters (mainly females) to escape the traps before they are hauled. The efficiency of this gear (and two alternatives) was assessed by comparison with standard commercial gear over a range of fishing grounds. Results indicated that, in comparison to standard commercial traps, none of the alternative trap designs would be beneficial to the fishery in the long term, provided that overnight sets remain the most common fishing method. SELECT models were used to evaluate the fishing properties of commercial and
bottom-grid traps relative to those of control (fine-mesh) traps. The results indicated that, given the choice, a rock lobster would preferentially enter a commercial trap, followed by a control trap, with bottom-grid traps being the least attractive. This suggests some level of saturation of control traps, a possibility that is of particular concern because the control trap design is used in a fishery-independent monitoring survey.

Keywords: escape gaps, Jasus lalandii, rock lobster, SELECT models, trap selectivity

African Journal of Marine Science 2002, 24: 37–48