Evaluating fish stock changes from the fragmentary catch and effort data of the Angolan fisheries

Abstract

The procedure followed to evaluate in standard units the fishing effort of two different groups of purse-seiners fishing non-contemporaneously for the horse-mackerelTrachurus trecae (Cadenat) in Southern Angola is described in detail. Firstly, by comparing contemporaneous catches, differences in fishing ability within each group were evaluated as efficiency factors relative to the group mean efficiency. The efficiency factors of the two groups were then related by means of an empirical predictive equation of the efficiency factor given the purse-seiner gross metric tonnage, and expressed relative to the efficiency of an average 75 metric ton purse-seiner. The series catch-per-standard-purse-seiner-per-day in each month of the period 1962–71, although incomplete, shows seasonal and cyclic variations in horse-mackerel abundance, probably of an environmental nature. It is curious, however, that the period of 6 years of the cyclic variation is equal to the maximum age of the species in Angola; a somewhat similar coincidence was observed in the South-African maasbanker Trachurus trachurus L.