INFLUENCE OF HABITAT COMPLEXITY ON RESISTANCE TO FLOODING AND RESILIENCE OF STREAM FISH ASSEMBLAGES

The structure of fish assemblages in five reaches of a high desert stream in north-central Oregon was determined by snorkeling before and after a summer flash flood and two spring floods. One reach in each of two other streams that were unaffected by the first flood was used as a reference system. Stream reaches varied in habitat complexity as measured by hydraulic retention. Following the floods, hydraulically complex stream reaches lost proportionately fewer fish, had generally higher fish diversities, and had higher fish assemblage similarity than hydraulically simple stream reaches. Fish assemblages were resilient, and certain species such as speckled dace Rhinichthys osculus were exceptionally good at recolonizing disturbed habitats. Successful recruitment of different fish species depended, in part, on flood timing. Young of the year of species that spawn in early spring (e.g., rainbow trout Oncorhynchus mykiss) were more negatively affected by early spring floods than summer floods. Species that spawn later in the season (e.g., cyprinids and catostomids) were more negatively affected by summer flooding. Higher fish diversities in hydraulically complex reaches (lower disturbance intensity) after floods support predictions of the intermediate-disturbance hypothesis and suggest that fish assemblage resistance may be related to overall habitat complexity in these small streams.