Responses of juvenile salmonids to spring floods

For stream-dwelling fishes, flooding is a major agent of mortality and displacement, especially when these events coincide with sensitive stages in the life history of an organism. Spring floods often parallel spawning, hatching and emergence of many fishes and are perhaps the most significant and consistent source of disturbance in small streams. Habitat complexity may ameliorate the effects of flooding by providing refuge for fish. In this study, we quantified the effects of spring floods on the downstream movement of juvenile salmonid fishes (< 40 mm, TL) in two Wisconsin stream reaches of different physical complexity (each defined by the amount of large woody debris present in the channel). Sampling from drift nets positioned across the stream channel, we collected over 1000 samples throughout the spring snowmelt period. In general fish were collected much more often at night and in mid-channel locations. We also found an effect of flooding, but surprisingly fish movement was negatively associated with stream flow. Finally, fish movement was much greater (three-fold) in the open reach than it was in the complex reach. There are several possible explanations for the patterns that were observed, but the most plausible is that fish movement was volitional and that fish were more inclined to leave the open reach than they were to leave the complex reach during this life stage.  Our results do not support the view that the fish are forced out of open habitats rather they choose to leave a less preferable habitat.