Competition for Food Between Redside Shiners (Richardsonius balteatus) and Rainbow Trout (Salmo gairdneri) in Two British Columbia Lakes

Abstract

The distribu-tion, movements, behaviour and food of trout and shiners in Paul and Pinantan Lakes were studied to determine the items and mechanisms of interspecific competition between them. Data from recent years were compared with data for years when trout alone inhabited the lakes. No interspecific aggression was observed. The possibility that the two species were competing for space was discounted. Stomach contents of shiners in Pinantan Lake revealed a marked qualitative diurnal food cycle. In Paul Lake, shiners have drastically reduced the Gammarus population relative to its pre-shiner abundance, forcing trout, as well as the shiners themselves, to shift their diets to other foods. This overgrazing was caused by the concentration of large numbers of shiners over the shoals where Gammarus are also present in their highest concentrations, and the ability of shiners to pursue food deeper into the weeds and to graze an area more thoroughly than trout. In Pinantan Lake, shiners have apparently reduced the density of Daphnia to a point where trout are unable to feed on them as rapidly as in pre-shiner years. The ability of both species to utilize many types of food tends to reduce the intensity of competition. The study demonstrates how false implications may arise from an appraisal of competition not initiated until after the effects of competition have been observed. If observations had not been made on Paul Lake until after competition had been observed, the importance of Gammarus as an item of competition would probably have been overlooked and the whole competitive relationship misconstrued. Environmental factors and behaviour were shown to be important influences on the dynamics of competition. The physical and biological environment and the distribution and behaviour of competitors may be in states of continual flux in which case the niches of the competitors cannot be considered constant.