Effect of rotational harvesting on the size composition of Cape rock oysters Striostrea margaritacea on the east coast of South Africa

Abstract

A commercial fishery for Cape rock oysters Striostrea margaritacea along the KwaZulu-Natal North Coast follows a 4-year rotational cycle, with each harvest year followed by three fallow years across four harvest zones. We analysed reported harvesting effort and catch information, and fisheryindependent oyster size composition data collected over 18 years to investigate the sustainability of rotational harvesting. Total harvesting effort and catches declined over the study period, but on average, the number of oysters collected per outing increased. Fewer outings in recent years were attributed to incomplete reporting and a progressive loss of access to harvest sites. Generalized linear mixed models (GLMMs) were used to estimate trends in oyster mean size in relation to fishing method (divers and intertidal collectors), harvest zone, 4-year rotational cycles, and months spent in a zone. Oyster mean size increased from north to south along the coast. Oysters caught by divers on newly exploited deeper reefs were initially larger than those caught by intertidal collectors. Mean oyster size decreased monthly during 1-year harvest periods but recovered to pre-harvest size over three fallow years. Results confirmed that the current rotational harvest strategy is well-suited to oyster biology and sustainable at the present level of effort. Improved reporting on harvesting effort and catch are required to verify longer-term spatio-temporal trends in the fishery. More effective stakeholder communication is needed to resolve potential user conflict.