Do catch-based indices provide a reliable index of annual recruitment for the South African anchovy population

Abstract

The ability of catch-based measures (both catch itself and measures of catch-rate/CPUE such as catch-per-set) to provide reliable indices of abundance of the annual anchovy recruitment on the South African west coast is investigated. Ground truthing is provided by the annual acoustic surveys of this abundance and corresponding estimates provided when this information is utilised in population assessments. The measures considered for the fishery as a whole reflect at most a 30% increase across the turn of the century, way below the some three-fold increase from before to after that time indicated by the surveys and assessment. The same is true for similar measures from catches made close to Robben or to Dassen Island, except when catches within 10 nm of these islands are considered, though surprisingly then this result of a similar increase to the survey results does not extend to catches within 20 nm. Nevertheless the CVs about the catch within 10 nm – recruitment relationships are some 70%, which is hopelessly too large for such measures to provide meaningful and reliable predictions of anchovy recruitment. Thus this study has failed to reveal any evidence to support the suggestion that anchovy catch- related measures might provide meaningful indices of anchovy recruitment abundance.