A STRUCTURED APPROACH TO USING BIOLOGICAL AND ENVIRONMENTAL PARAMETERS TO FORECAST ANCHOVY RECRUITMENT

Simulation studies have indicated that a valuable increase in mean annual yield could be achieved in the South African anchovy, Engraulis capensis, fishery if below-average recruitment could be forecast at the start of the fishing season, 6 months before recruitment can be reliably measured, This paper reports on initial investigations into biological and environmental indicators which could be used to make such forecasts. Valid estimates of recruitment strength in the species are available from 1985 to 1992 and the environmental and biological indicators of interest have been monitored for periods starting from between 1984 and 1988, to the present. Most of the data were obtained from monitoring on anchovy spawner biomass surveys which have been undertaken each November since 1984. A conceptual model of the factors influencing the recruitment process was constructed and the available data were examined for empirical evidence of the importance of each event included in the model. Fluctuations in copepod biomass and production on the spawning grounds, the incidence of oocyte atresia in adult females, the incidence of southerly winds at Cape Point and the distance offshore of the 16 degrees C isotherm at Cape Columbine were associated with fluctuations in anchovy recruitment. It is suggested that particular attention should be paid to monitoring of these variables to investigate further these associations. If persist ent relationships are found, it may be possible to construct an expert system to forecast recruitment from observed values of these variables at the start of a fishing season. A preliminary set of three decision rules is presented.