A holistic approach to the exploitation of intertidal stocks: limpets as a case study

The potential ecological consequences of commercially exploiting the limpets Patella argenvillei and P. granatina on the west coast of South Africa were investigated. Standing stocks were quantified and population parameters (growth, mortality and recruitment) determined for both species. These data were used to derive empirical estimates of maximum sustainable yield (MSY) for P. granatina and P. argenvillei (7,4 and 1,5 tons wet foot mass respectively). The population parameters were also used in the development of a length-based simulation model to provide a second estimate of MSY for P. granatina (up to 12,9 tons). Small-scale experimental harvesting showed that both species recovered extremely slowly if all limpets were removed (total clearance). When the harvest was size-selective (partial clearance), with P. granatina >60 mm and P. argenvillei >75 mm being removed, P. granatina failed to recover after 13 months but P. argenvillei recovered within 15 months. Partial and total clearance led to enhanced growth and survival of the remaining limpets. Partial clearance of P. granatina had no effect on other species in the community, but partial and total removal of P. argenvillei resulted in increased cover of sessile filter-feeders and macroalgae as well as a decrease in the cover of encrusting coralline algae. It is concluded that the potential exists for the development of an economically viable and ecologically sustainable fishery. Empirically derived quotas and minimum size limits of 60 mm for P. granatina and 75 mm for P. argenvillei are suggested as the chief mechanisms for regulating a potential fishery. The need for a holistic approach to the management of living (particularly intertidal) resources is stressed.