A REGIONAL-SCALE HABITAT SUITABILITY MODEL TO ASSESS THE EFFECTS OF FLOW REDUCTION ON FISH ASSEMBLAGES IN MICHIGAN STREAMS

In response to concerns over increased use and potential diversion of Michigan's freshwater resources, and the resulting state legislative mandate, an advisory council created an integrated assessment model to determine the potential for water withdrawals to cause an adverse resource impact to fish assemblages in Michigan's streams. As part of this effort, we developed a model to predict how fish assemblages characteristic of different stream types would change in response to decreased stream base flows. We describe model development and use in this case study. The model uses habitat suitability information (i.e., catchment size, base-flow yield, and July mean water temperature) for over 40 fish species to predict assemblage structure in an individual river segment under a range of base-flow reductions. By synthesizing model runs for individual fish species at representative segments for each of Michigan's 11 ecological stream types, we developed curves describing how typical fish assemblages in each type respond to flow reduction. Each stream type-specific, fish response curve was used to identify streamflow reduction levels resulting in adverse resource impacts to characteristic fish populations, the regulatory standard. Used together with a statewide map of stream types, our model provided a spatially comprehensive framework for evaluating impacts of flow withdrawals on biotic communities across a diverse regional landscape.