Fish Response to Velocity-Related Flow Modifications in a Warm-Water Stream

Abstract

Recent developments in the field of stream restoration have led to multiple strategies for improving and restoring a variety of fish habitats across spatial and geographical scales. These habitat modifications often aim to increase overall fish abundance or species diversity through the installation of in‐stream structures. Historically these structures were common in cold water streams for improvement of salmonid habitats, but as stream restoration projects are needed in smaller and warm water streams it is important to understand and quantify the effects of in‐stream structures on other fishes. Velocity is thought to be an important parameter in determining fish habitat choices and hence essential for successful restoration of aquatic ecosystems. This project attempted to change velocity profiles using in‐stream structures and determine if these changes attracted fishes. In this project solid blocks were installed perpendicular to the primary flow direction at ten sites located in runs and riffles in a warm water stream in Northern Michigan in order to create velocities similar to those experienced by fishes in pool habitats. The modified sites and two pools were surveyed by snorkeling over a one‐month period to determine fish use of the newly created habitats. The physical flow conditions were measured using an Acoustic Doppler Velocimeter and a Marsh‐ McBirney flow meter. Fishes did not utilize the artificially created habitats despite large reductions of local velocity behind the blocks. Current speeds behind the blocks were actually lower than those measured in pools and in some cases were opposite in direction to the main channel current. Pools and modified sites showed significant differences in mean velocity, velocity fluctuations, turbulent kinetic energy, and turbulence intensity. Velocity gradients which developed at the edges of the blocks may have been a barrier to fish access to the modified sites, since they were much larger than those occurring in natural pools. Results suggest that several physical characteristics of flow patterns are crucial in describing the complexity of fish habitats and that velocity alone does not provide enough information to determine the suitability of an area for fishes.