IMPACT OF ENVIRONMENTAL PERTURBATION ON A TROPICAL FISH COMMUNITY

Tissawewa is a tropical reservoir in which annually recurring water le vel fluctuations are the main source of environmental variations. Aspe cts of a major perturbation studied related to (i) a progressive water level decrease that ultimately caused the reservoir to (ii) dry compl etely and a radical change of environment after refilling. The first a spect caused a decrease in the abundance of small pelagic fish but an increase in large bottom-dwellers. Main causes responsible for these c hanges are a decrease in the food available to the fish community resu lting in weight loss and increased mortality of smaller bodied species . Also, the concentration of suspended silt and seston increased, prob ably damaging fish gills, thereby adversely affecting oxygen intake of the fish. Apparently, bottom-dwellers are better adapted to these con ditions than pelagic species. The second aspect caused the composition of the food source to change, resulting in diet shifts and changes in relative abundance of some fish species (often compensated for by an inverse change of another species of the same feeding guild). In contr ast with the annually recurring minor perturbations, the major perturb ations caused the species composition and size structure of the fish c ommunity to change.