Statistical correlations between purse-seine catches of chub mackerel off South Africa and selected environmental parameters

Abstract

Annual catches of chub mackerel made by South African purse-seiners have decreased markedly since the mid 1970s, in spite of the species continuing to be abundant in the northern part of the Benguela ecosystem and a powerful year-class having been formed in 1977. To investigate a possible influence of environmental conditions on availability of chub mackerel to purse-seiners, prewhitened residuals of catches were correlated with prewhitened residuals of sea surface temperature and wind for three areas off South Africa. Statistically significant correlations for the two southern areas were only marginally greater than the number that would have been expected to have arisen spuriously. However, catches appear to be significantly related to cool sea surface temperatures and north-westerly winds in the northern area. These responses may indicate the preferred habitat of the prey (cooler water temperatures) and/or availability of the resource to the fishing fleet (north-westerly winds push the upwelling front closer inshore). The best purse-seine catches of the older ages of chub mackerel have followed a south-westerly movement of these age-classes into the fishing grounds. If the extent of this movement is modulated by environmental conditions in the north, there may be benefit in purse-seiners extending their search for older chub mackerel northwards and offshore.