Assessment of the South African sardine resource using data from 1984-2010: further work towards a base case single stock hypothesis

Abstract

The sardine assessment is extended to consider three hypotheses for the presence of two (west and east) stocks separated at Cape Agulhas. The first hypothesis of two completely separated stocks is dismissed, as it implies that an unrealistically low proportion of the recruits for the eastern stock are detected in the recruitment survey. The second hypothesis of movement of recruits from west to east stocks, the extent of which varies from year to year, is more successful in terms of matching the data for realistic values of detectability of eastern recruits in the recruitment survey. However the proportion moving from year to year is only weakly correlated with high biomass or recruitment in the western stock that year. The third hypothesis is a variant of the second for which the data are less informative because the “dividing line” between west and east recruits is taken to be unknown. This (in the one of several possible forms implemented thus far) produces similar results to the second hypothesis. It is proposed that because of the limited time available now to complete the OMP revision, and further issues still to be resolved relating to future projections under two stock hypotheses in relation to the distribution of catching, that future work in finalising this OMP process be restricted to the second hypothesis (specifically the variant which assumes movement from 1994 onwards onwards when the recruit survey was extended further eastwards).