This study followed the response of a population of exotic crayfish, Orconectes virilis, during a large restoration project in Fossil Creek, Arizona where stream flow was restored as part of a dam decommissioning, and non-native fishes were removed from part of the river to recover native fishes. It was predicted that crayfish would increase with restored flow and that increases would be higher where exotic fish were removed due to a release from competition and predation. Results indicated that crayfish densities rapidly increased, with at least a two-fold greater increase in areas where exotic fishes were removed relative to where they remain. A smaller scale, manipulative experiment tested how crayfish, at different densities, affected algae, detritus, and macroinvertebrates to determine if there were detectable densities below which crayfish effects would be minimal. Effects on algae, leaf litter and detritus were minimal when crayfish were maintained at 1 m-2, which is slightly higher than current maximum densities (0.76 m-2). In contrast, at higher densities (3 m-2) crayfish strongly reduced leaf litter and macroinvertebrates, important food resources for native fish. Growth rates were high enough during the study that projected crayfish densities could reach the level of the high-density treatments within 0.8 – 2.3 years.
