Effects of environmental parameters on the distribution of bullhead Cottus gobio with particular consideration of the effects of obstructions

1. The bullhead Cottus gobio is a small-sized fish that occurs in stony streams and lakes in Switzerland. Its numbers have declined and its distribution has become narrower over the last 30 years, most probably due to chemical pollution and/or structural habitat deterioration. and at present it is classified as a potentially endangered species. 2. This study evaluated whether chemical parameters and/or structural habitat heterogeneity have an effect on the occurrence and density of bullhead in a river system, in order to assess the potential usefulness of this species as an indicator for these parameters. Ten sites were chosen, where chemical parameters were measured monthly over a period of 2 years. Habitat heterogeneity was assessed with respect to river width and maximum depth. Correlations were drawn between the density of bullhead, the chemical water constituents and the structural habitat diversity. The longitudinal distribution of bullhead in the whole river system was determined and all obstructions with a height of at least 10 cm were investigated. In two sites, the total population density of bullhead older than 0 + was assessed using both multiple removal collecting and a mark-recapture method. 3. Structural habitat diversity appeared not to be important for bullhead, and the only chemical parameter that correlated with the density of this species was dissolved organic carbon. 4. Obstructions with a height of 18-20 cm were impassable for upstream movement of bullhead and thus acted as migration barriers. The longitudinal connectivity of the main river was interrupted by 48 such obstructions, which were all human-made. 5. Multiple removal collecting of bullhead resulted in small catching probabilities of 0 . 17 and 0 . 34. The total estimated population densities of bullhead were high, with 118 and 327 in 100 m(2). The mark-recapture method revealed even higher estimated densities, of 121 and 444 in 100 m(2) 6. It is concluded that the bullhead is a useful indicator species for determining to what extent the longitudinal connectivity of a river is interrupted by obstructions. For restoration of rivers. it is recommended that any human-made obstructions with a height of 20 cm or above are removed, starting at confluences with joining rivers and moving upstream.