Environmental factors regulate the effects of roach Rutilus rutilus and pike Esox lucius on perch Perca fluviatilis populations in small boreal forest lakes.

In this study of 18 small boreal forest lakes, the effects of abiotic and biotic factors (roach Rutilus rutilus and pike Esox lucius) on various population variables of perch Perca fluviatilis were examined. As a single variable, the gillnet catch per unit effort (CPUE) of R. rutilus was negatively related to the mean mass of small (< 200 mm) and the growth rate of young (1-2 years) P. fluviatilis. The mean mass of large (> or = 200 mm) P. fluviatilis was the highest at intermediate CPUE of R. rutilus. Redundancy analysis including environmental factors and P. fluviatilis population variables suggested that 'predation-productivity-humus' gradient affected P. fluviatilis populations by decreasing the CPUE and mean mass of small individuals but increasing these variables of large individuals. The CPUE of R. rutilus and lake area had a negative effect on small and a positive effect on large P. fluviatilis growth rate. In small boreal forest lakes, P. fluviatilis populations are affected by the partially opposite forces of competition by R. rutilus and predation by E. lucius, and the intensity of these interactions is regulated by several environmental factors.