Changes in the density of introduced bullhead, Cottus gobio L., and its impact on juvenile Atlantic salmon, Salmo salar L., densities in a sub-Arctic salmon river in northern Finland

A newly introduced species, the bullhead, Cottus gobio L., was observed for the first time in the River Utsjoki, a tributary of the River Teno, in 1979. The River Teno is one of the most important Atlantic salmon, Salmo salar L., rivers in Northern Europe and its tributary the River Utsjoki is an important salmon spawning and nursery area. The densities of bullhead were lowest in the areas to which the species has spread most recently and highest in the areas downstream of its introduction point. Densities were markedly lower upstream. Statistical analysis showed that the presence of the bullhead had not affected juvenile salmon densities in the River Utsjoki.     

Introduction and dispersion of the bullhead, Cottus gobio L., in a sub-Arctic salmon river in northern Finland

The bullhead, Cottus gobio L., was observed in the River Utsjoki in the far north of Finland for the first time in 1979. This river is a tributary of the sub-Arctic River Teno, which flows into the Arctic Ocean and is one of the most important salmon rivers in northern Europe. The bullhead does not belong to the natural fish fauna of the Finnish river systems flowing into the Arctic. It was probably introduced by anglers as bait or accidentally during the water exchange activities as salmon fry were transferred from Finnish hatcheries to northern Norway. A viable bullhead population has dispersed through the River Utsjoki catchment. In 1995 its range extended from the lowest part of the river to a point 43 km upstream. It is highly probable that it has also entered the mainstem of the River Teno.