The updated sardine assessment, using the revised stock structure hypothesis

The stock assessment model of the revised South African sardine stock structure hypothesis allows for two stocks (Warm Temperate Sardine and Cool Temperate Sardine), distributed between two areas (the west and south coasts of South Africa, separated at Cape Agulhas). Sardine growth is coast-dependent, and the distribution of length-at-age off the south coast is modified from the south coast growth curve to accommodate the immigration of individuals from the west coast which have grown according to a different growth curve for the first part of their life on the west coast. Catch by stock off the west coast can either be proportional to the numbers of exploitable sardine-at-age by stock off the west coast, or related to alternative pre-specified weightings of CTS to WTS between the region to the north of Cape Point and that between Cape Point and Cape Agulhas. Results show that the new way of modelling coast-dependent – and no longer stock-dependent – growth has only a small impact on the results and the standard deviation about the mean length-at-age off the south coast is not much larger than that off the west coast, despite immigration of sardine of a different mean length from the west coast. Allowing for time-varying growth rates – and not age at which length is zero – does improve the fit to the data. The model alternative which additionally fits to parasite prevalence-at-length data is also included. Some initial alternatives for allowing for different weightings by stock in catches off the south-west and north-west coast are provided for further discussion.