Impact of environmental perturbation on a tropical fish community

Abstract

Tissawewa is a tropical reservoir in which annually recurring water level fluctuations are the main source of environmental variations. Aspects of a major perturbation studied related to (i) a progressive water level decrease that ultimately caused the reservoir to (ii) dry completely and a radical change of environment after refilling. The first aspect caused a decrease in the abundance of small pelagic fish but an increase in large bottom-dwellers. Main causes responsible for these changes are a decrease in the food available to the fish community resulting in weight loss and increased mortality of smaller bodied species. Also, the concentration of suspended silt and seston increased, probably damaging fish gills, thereby adversely affecting oxygen intake of the fish. Apparently, bottom-dwellers are better adapted to these conditions 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 contrast with the annually recurring minor perturbations, the major perturbations caused the species composition and size structure of the fish community to change.