By L. Jorgensen, P. -A. Amundsen, H. -M. Gabler, M. Halvorsen, J. Erkinaro, E. Niemela
Fisheries Research, Volume 41, Issue 2, June 1999, Pages 201-211, ISSN 0165-7836, DOI: 10.1016/S0165-7836(99)00014-4.

In 1979, bullheads (Cottus gobio L.) were registered for the first time in the River Utsjoki (70[degree sign]N, 27[degree sign]E), a large tributary of the River Tana, which is one of the most important Atlantic salmon (Salmo salar L.) rivers in the world. The bullhead was probably introduced into the river by man. The occurrence of bullheads in the River Utsjoki has raised the question of whether their presence may reduce the production of salmon smolts by occupying, e.g., favourable salmon habitats. The spatial distribution of bullheads and salmon parr was investigated by electrofishing in different river habitats and by gillnetting in 13 lakes in the watercourse. Salmon and bullheads were segregated predominantly along the river reaches, and there was an inverse relationship between the densities of the two species at individual localities. Almost all bullheads (98%) were caught within 50 m of a lake, while they were almost completely absent from typical river reaches which were the principal habitat of salmon parr. Different physical conditions seemed to explain the different densities of salmon parr in the different parts of the watercourse, while the presence of bullheads appeared to have little or no influence. Both salmon parr and bullheads inhabited a number of lakes in the watercourse, coexisting with grayling (Thymallus thymallus (L.)), whitefish (Coregonus spp. ) and burbot (Lota lota (L.)), among other species.
Keywords: Salmo salar; Cottus gobio; Spatial distribution; Lotic habitat; Lentic habitat; Fennoscandia

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