An experimental test of metabolic and behavioural responses of benthic fish species to different types of substrate

Behavioural and respiratory responses to different types of substrate were tested in burbot (Lota lota) and stone loach (Barbatula barbatula). Both sp ecies showed distinct diel cycles of activity and respiration rate, with hi ghest values immediately after dusk and lowest during the day. In burbot, m ean respiration rate over a 24-h cycle was 30% higher on pebble substrates compared with cobbles. During daytime, the difference increased to 86%, but no significant differences were found during the night. The results show t hat lack of adequate shelter may substantially affect metabolism and somati c growth rates in benthic fish. These effects occur even when no predator i s actually present and shelter is not essential for survival. The results p rovide evidence that current theories on the effects of substrate and preda tion pressure should be applied to the benthic community with care. Because most of these theories are derived from epibenthic or pelagic model-specie s, where the actual presence of a threat is needed to modify behaviour and thus the specimens' metabolism, their validity for the benthic community se ems to be limited. The availability of adequate shelter may be of more impo rtance than other environmental resources for fish that have a high substra te affinity, even when no predator is actually present.