Mesohabitat use by bullhead (Cottus gobio)

Habitat composition and connectivity within a stream vary with changing flows but the influence of changing flow on habitat use by fish is not well understood. Meso- and microhabitat surveys were used to investigate habitat use by bullhead (Cottus gobio Linnaeus) in response to discharge variation in a small tributary of the Upper Severn, England. Mesohabitat mapping surveys were carried out over a range of summer flows (0.016–0.216 m3 s−1) and were coupled with direct underwater observations (snorkelling) of fish location. Five mesohabitat types—glides, runs, riffles, chutes and pools—were present in the reach at all flows surveyed and ‘backwaters’ were found at three flows. The macro-morphology of the reach comprised six riffle–pool sequences divided into 27 mesohabitats with the maximum diversity (23 mesohabitats) at intermediate flows (Q43) and only 15 mesohabitats at Q95. Despite low numbers of fish (N = 78), bullhead displayed a strong association (51% of the fish) with glides—relatively deep habitats having high rates of velocity increase with flow. However, 54% of the fish were observed in two large, persistent mesohabitats, a glide (34%) and a pool (20%), both located below a faster flowing mesohabitat. Habitat use curves based upon micro-habitat data showed bullhead favoured low velocities (<0.30 m s−1), depths less than 0.30 m and a cobble substratum. This study illustrates the value of cross-scale investigations in linking fish ecology, flow and physical habitat variability and suggests mesohabitat size, persistence and arrangement may influence fish distribution.