Pools as refugia for brown trout during two summer droughts: trout responses to thermal and oxygen stress

In non-drought years (1977, 1985), temperatures and oxygen concentrations from 1 to 14 July at the deepest point in each of five pools in Wilfin Beck were similar with ranges of 12-18 degrees C and 7.8-9.8 mg l(-1). Trout Salmo trutta were present in all pools. In drought years (1976, 1983), temperature increased and oxygen concentration decreased as pool size decreased. In the two smallest pools, they were outside the thermal and oxygen limits for trout (ranges for both pools 24-29 degrees C, 1.2-2.5 mg l(-1)), and trout were absent. Values in a medium-sized pool were close to the incipient lethal levels and a few juvenile trout were present in both drought years. The lowest temperatures and highest oxygen concentrations were recorded in the two largest pools (ranges 20-25 degrees C, 3.6-4.8 mg l(-1)) and trout of all ages (0+ to adults) were present in both drought years. In these two pools, both temperature and oxygen concentration decreased from the surface to the deepest point in the pool. Trout preferred lower temperatures near the pool bottom rather than higher oxygen concentrations near the surface, but some fish moved towards the surface at night when the pool cooled slightly. These field results were discussed in relation to lethal values recorded for brown trout in the laboratory, and there was general agreement between field and laboratory values. Trout in the drought years occurred at temperatures close to, or below, the incipient lethal value of 24.7 degrees C (+/- 0.5) and also at the highest oxygen concentrations, but only when these were at temperatures below the incipient lethal value. (C) 2000 The Fisheries Society of the British Isles.