Migrations of spiny rock-lobsters, Jasus Lalandii, at Luderitz : environmental causes, and effects on the fishery and benthic ecology
A dispute arose in the Luderitz rock-lobster, Jasus lalandii, fishery as to whether declines in CPUE were due to changes in rock-lobster migration patterns or a reduction in fishable biomass. Rock-lobster migrations were studied at two sites by estimating in situ density in the 10-12m and 15-20m depth zones. Water temperature and dissolved oxygen concentration profiles were obtained at several sites at approximately monthly intervals. Observations over a period of six years indicated that rock-lobster underwent seasonal in- offshore migrations and that marked interannual variation in those migration patterns did, indeed, occur. The movements and their interannual variation were directly related to the inshore presence of water with very low dissolved oxygen concentrations. CPUE was closely related to the density of legal sized rock-lobster on the fishing grounds (observed by SCUBA diving) and hence it is feasible that at least a part of the decline in CPUE at Luderitz was due to an alteration in the "normal" pattern of in-offshore migration. The hypothesis that environmental change caused the major decline in rock-lobster catches at Luderitz was examined by time series analysis of 21 years of CPUE, wind, SST and sea-level data. Luderitz rocklobster CPUE was found to be negatively correlated with the southerly component of summer wind stress at Diaz Point six years previously. On this basis it was hypothesised that variable settlement of puerulus larvae is an important factor driving changes in recruitment to the fishable population. The mechanism may be increased mortality of larvae as they attempt to cross the continental shelf (due to greater northward and offshore drift during periods of strong southerly wind). The dramatic interannual changes in rock-lobster density provided an opportunity to carry out a "natural" experiment on the interaction between rock-lobsters and the associated benthic species assemblage. It was found that, although there were changes in the benthos (notably an increase and subsequent decline in whelk density), these were unrelated to variation in rock-lobster density. In addition, no relationship between per capita food availability and rock-lobster growth on four grounds, was apparent. As regards temporal changes in food supply and subsequent effects on rock-lobster growth rates; there was no temporal reduction in food supply at the main study site which could be related to the reduction in CPUE since 1988.
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