An analysis of yield variability from three harvesting strategies in the South African anchovy fishery, under conditions of randomly fluctuating recruitment success

Abstract

An age-structured population model was used to examine the variability of yield created by randomly fluctuating recruitment success in the South African anchovy Engraulis capensis stock. Three different harvesting strategies were examined: (1) constant age-specific pattern of fishing mortality, (2) constant quota with effort limitation and (3) annual quota adjustments by means of an F msy procedure. Variable recruitment was generated by a stochastic stock-recruit relationship, and mean yields, mean spawner biomasses and probability distributions of yield were calculated at each exploitation level after 150 projections from the population structure of anchovy in 1981. Under conditions of constant fishing mortality, the variability of yield increased continuously as the exploitation rate was increased. The maximum average yield (MAY) is the stochastic equivalent of the maximum sustainable yield (MSY) of deterministic models, but an attempt to sustain this MAY by a constant quota resulted in a very high risk of stock collapse. An Fmsy policy based on pre-season adult biomass resulted in more variable yields than were obtained at the equivalent constant fishing mortality at age. Stability of yield therefore demanded the acceptance of lower average yields than could be attained in the long term by quota adjustment. It was considered that the South African purse-seine industry could not cope with the wide fluctuations of yield necessary to attain MAY. Specific management policies sufficiently robust to withstand both fluctuating recruitment success and the uncertainties in the parameters of working population models would be required.