Competitive habitat displacement of brown trout by Siberian sculpin: the role of size and density

Abstract

Habitat competition in brown trout Salmo trutta and Siberian sculpin Cottus poecilopus was investigated by varying density, fish size, and species composition in stream channels providing areas of different substratum particle sizes. In allopatry, both small (52 ± 4 mm LT) and large (86 ± 6 mm LT) brown trout exhibited strong preference for the intermediate (8–11 cm diameter) and large (17–21 cm) gravel substrata. There was a tendency for more brown trout to occupy finer (2–4 cm) substrata with increasing density, in particular for large brown trout. Also, more small brown trout were observed on finer substrata when tested with large brown trout, suggesting interspecific competition for restricted space. Both small (56 ± 6 mm LT) and large (88 ± 10 mm LT) Siberian sculpin preferred the large gravel in all tests, and did not change their substratum preferences much with increasing densities, suggesting higher tolerance for ‘crowding’. The large Siberian sculpin preferred the coarser substratum, and the largest individuals were consistently found on it. In sympatry with large Siberian sculpin, habitat displacement of brown trout occurred, indicative of interspecific competition. A higher proportion of small and large brown trout occupied the finer substrata than in allopatry. Habitat selection by large Siberian sculpin appeared to be unaffected by species composition and density. Small Siberian sculpin were displaced to finer substrata when tested with large Siberian sculpin, suggesting intraspecific competition. The results indicate that Siberian sculpin are potential habitat competitors for young brown trout.