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 m(3) 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 (Q (43)) and only 15 mesohabitats at Q (95). 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.