Changes in the Size Structure of Cichlid Populations of Lake Malaŵi Resulting from Bottom Trawling

Abstract

A bottom trawl fishery, primarily for cichlids, was started in 1968 in southern Lake Malaŵi. Since then a dramatic change has occurred in the catch of cichlids from one dominated by large species to one dominated by small species. Rates of change in abundance and mean length were much greater in the southern end of the southeast arm (area A) which has been fished for a longer period using trawls with 25-mm codend mesh than in the central region of the southeast arm (area B) which has been fished for a shorter time using trawls with 38-mm codend mesh. The changes in area A suggest heavy fishing mortality with little recruitment for species over 200 mm maximum length, heavy fishing mortality with limited recruitment for the medium-sized species (180–200 mm maximum length), and light fishing mortality and excellent recruitment for the small species (140–180 maximum length). Changes in area B showed the same trends but were considerably less drastic. The number of species caught has declined over 20% in the heavily fished areas. The remaining common species have greatly expanded their distribution. The change in the relative abundance of large and small species in the catch, and the decline in the number of species, appear to be stabilizing. These changes are probably the result of the size-selective nature of the codend mesh of the trawl net resulting in excessive fishing mortality for the larger species.