Seasonal and Long-Term Changes in the Distribution and Abundance of Demersal Fishery Resources in Continental Shelf Waters off Ghana, West Africa

Abstract

Between 1963 and 1990, the abundance of demersal fishery resources in Ghana’s shelf waters underwent significant changes whereby the relative importance of major species changed in every trawl survey conducted in the area. Triggerfish (Balistes capriscus) dominated this ecosystem for nearly twenty years (from early 1970s to late 1980s) displacing bigeye grunt (Brachydeuterus auritus) as the most abundant species.The density of all demersal species (excluding triggerfish) assessed in bottom trawl surveys decreased from 50 kg ha-1 in 1963-64 to 32.4 kg ha-1 in 1990. The lowest density of 22.5 kg ha-1 occurred between 1973 and 1977. Density of triggerfish was high between 1973 and 1982, reaching a value of 28 kg ha-1 between depths of 30 and 50 m. Its density subsequently declined and by 1990, the species had virtually disappeared from the study area. In the period of decline of triggerfish, the density of rays, soles and cuttlefish increased.The observed changes in relative importance and density of species is attributed in part to the proliferation of the triggerfish in this ecosystem and also to changes in the marine climate over the period in question.