Gear selectivity and the variation of yield

For constant fishing effort, the variation of yield is generally driven by largely unpredictable changes in annual recruitment. Given the statistics of recruitment fluctuations, yield variability depends on the level of effort and the selection parameters of the fishing gear. This paper examines the significance of the latter, with particular reference to the shape of the selection ogive. An age-structured population model was used to develop the theory of yield variation. Gears are selective on fish length and the model takes account of stochastic growth in each cohort. Data from trawl surveys in the North Sea suggest that the length-at-age distribution is Gaussian with a standard deviation which increases linearly with age. The model is applied to the North Sea haddock fishery using data from quarterly trawl surveys in 1991. The catch analysis is performed in quarterly steps to take account of the within-year growth of young fish. The results show that yield is least variable when the selection range is of the order of 50% of the retention length. Thus “knife edge” selection (zero selection range) is not the optimal harvest strategy when the objective is to minimise yield variability.