Environmental Influences on Agulhas Fish Catch

Abstract

This study considers environmental influences on a small fishery along the east (Agulhas) coast of South Africa using catch data. Environmental influences are studied via seasonal analysis of SeaWifs chlorophyll and related variables, composite analysis of atmospheric reanalysis data with respect to high and low catch years, principal component analysis (PCA) of ocean and atmosphere reanalysis data, statistical analysis of de-trended fish catch with respect to climatic indices, and composite analysis of depth-longitude hydrographic sections over the Agulhas shelf. The analyses show that increased fish catch follows a spell of warmer, saltier shelf waters and dry weather guided by large scale climatic factors. Composite differences indicate a decrease of rainfall, soil moisture and river run-off along the East coast during years with higher fish catch that is related to an atmospheric trough over Madagascar and southerly winds in the Mozambique Channel. Coastal winds are northwesterly and induce downwelling through anticyclonic vorticity. A multiple regression of environmental variables onto fish catch at 0-1 year lead time accounts for 50% of variance in the period from 1964 to 2007 using PCA scores that refer to changes in currents, salinity and air pressure in the southwest Indian Ocean. There is an association between the North Atlantic Oscillation, regional anticyclonic winds and the Agulhas Current that deserves further study.