The development and performance of a length-based stock assessment of Foveaux Strait oysters (Ostrea chilensis, OYU 5) in southern New Zealand, and application to management

The iconic Foveaux Strait oyster fishery has been fished for over 150 years, and in this time has survived several mortality events due to the haplosporidian parasite Bonamia exitiosa. Disease mortality has been the principal driver of oyster abundance since 1985 and the subsequent low fishery exploitation rates have had little effect on future stock size. Because of recurring disease mortality, stock assessments that provide predictions of future stock size under different levels of disease mortality and harvest are critical to effective management and stakeholder engagement. We describe the development of an integrated Bayesian length-based stock assessment model, which includes fishery and fishery independent abundance indices, and some information on biological parameters. Projections of future stock size have been corroborated by subsequent surveys of the oyster populations. Simulations to evaluate model performance show growth parameters can be estimated close to true values when the tag-recapture and length frequencies data concur, but process error CV for abundance indices cannot be estimated reliably. Improvements to the model to better estimate future stock size, and to predict disease mortality are discussed.