THE STRUCTURE OF SCULPIN POPULATIONS ALONG A STREAM SIZE GRADIENT

Data on spatial variation of sculpin density, growth and fecundity supported the hypothesis that populations of streamfish are structured by changes in risk of predation and prey availability along a gradient in stream size. Cottus bairdi in warm streams and C. cognatus in cold streams exhibited similar patterns. Sculpins in large streams had faster individual growth rates and higher fecundities than those in small streams, but occur at lower density. The patterns appeared to be persistent and suggest that predation reduces sculpin density in larger streams. Competitive release, variations in prey productivity, and local factors probably contribute to the variation in sculpin growth.