Refuge-use strategies of stream fishes in response to extreme low flows

Low flows were simulated in an artificial stream to determine the refuge-use strategies of two benthic New Zealand fish species, Canterbury galaxias Galaxias vulgaris and upland bullies Gobiomorphus breviceps , and to investigate how refuge availability and the rate of flow recession affect the type and incidence of refuge use. When riffles dewatered, upland bullies tended to move to runs, whereas Canterbury galaxias showed a stronger propensity to burrow into the substratum. Both species showed a strong and consistent tendency to move upstream when emigrating from riffles. Burrowing was more frequent on coarse substrata and during rapid flow recessions. The incidence of surface stranding increased with the rate of flow recession, but only when interstitial refugia were unavailable, and was higher on gravel than on cobble substrata. The effect of rate of flow recession and substratum size on the probability of stranding depended upon the type of refuge-use strategy adopted by the fishes. Rate of flow recession affected upland bullies more than Canterbury galaxias, whereas substratum size affected Canterbury galaxias more than upland bullies. These results suggest that the impact of disturbance is contingent upon species-specific refuge-use strategies, which result from interplay between refuge availability, the nature of the disturbance and species' behaviours and morphologies. When component species adopt contrasting refuge-use strategies, disturbance events may not consistently favour one species over another but rather inflict species-specific mortality that varies both temporally and spatially.