Effects of residual flow and habitat fragmentation on distribution and movement of bullhead (Cottus gobio L.) in an alpine stream

The present study investigates the population structure and dynamics of Cottus gobio in three adjoining, hydrologically very different stream sections that are affected by water abstraction (residual flow). During the one-year investigation from October 1997 to September 1998 the selected stretches were repeatedly sampled by electrofishing. Multiple mark-recapture method was applied to determine fish density along the individual stretches and the individual marking technique (VI-tag) was used to record the migratory activity of individual fish. Stretches with intermittent flow (section A) are temporarily colonised by adult Cottus gobio from downstream sites only at higher water levels. These stretches are completely dry for about half of the year and do not enable populations to become established. Small populations persist in those transitional stretches that are intermittently isolated from other surface waters (section B). However, population size is less than one-quarter the size of those in directly adjoining downstream stretches with permanent water flow (section C). In addition, bullheads in section B have considerably poorer conditions. Changes in discharge led to increased migratory activity over the entire study area, whereby the radius of activity of the bullhead was largely (88%) restricted to distances of < 150 m. Elevated discharges mostly triggered upstream longitudinal and lateral dispersion. When the water drops to low water level and large areas in these sections dry out, the bullheads have to withdraw into deep pools. This might be the main reason for the lack of the probably less mobile juvenile stages. The results clearly demonstrate that habitat fragmentation resulting from water abstraction has drastic effects on all life stages of Cottus gobio in respect to habitat availability, spatial distribution, movement, fitness and survival. These effects can be seen at an extremely fine scale and are revealed in the significant differences in abundance and condition of Cottus gobio between the short (94 m-188 m) adjacent sections. In order to maintain and restore ecological integrity of running waters, the preservation of intact flow conditions is one of the key issues for river management.