An attempt to rearrange available calendar-year biostatistics on the Cape horse mackerel stock off Namibia on a reproductive-cycle basis, and the VPA results

Abstract

Updated estimates of catch-at-age based on Bulgarian, Romanian and Soviet data are presented for the Cape horse mackerel Trachurus trachurus capensis stock off Namibia (ICSEAF Divisions 1.3, 1.4 and 1.5). These were used, in combination with recalculated mass-at-age and fishing effort data, to estimate the fishable biomass and Total Allowable Catch (TAC) levels corresponding to the F 0.1 strategy by means of Virtual Population Analysis (VPA). In addition to normal calendar-year periods, split-year periods constituting a tentative attempt to adapt the data to the actual reproductive cycle of the species in the region were used to analyse the monthly biological and catch and effort data. The precision of the VPA results obtained with this split-year data set did not appear to justify advocating a switch from the calendar-year basis in use at the time. Over the 12 years 1978–1989, the stock biomass remained fairly stable at 1 500 000–2 000 000 tons, permitting annual catches in the range of 500 000–600 000 tons. A decreasing trend in stock biomass estimates from 1984 was more clearly discernible when the calendar-year data set was employed. Finally, provisional estimates of catch-at-age for 1989 were also calculated and are included.