Developing Bioenergetic‐Based Habitat Suitability Curves for Instream Flow Models

Instream flow models link a physical habitat model that predicts flow‐related changes in hydraulics to a biological model that predicts the response of fish to altered velocity and depth. Habitat suitability curves (HSCs) based on frequency of habitat use (fish occurrence relative to available habitat) remain the most widely used biological models in habitat simulations. However, in some contexts fish density may be a poor indicator of habitat quality, leading to biased predictions of optimal flow. We explore the use of bioenergetics to derive mechanistic HSCs based on the fundamental energetics of habitat use. Using flow‐related changes in production of Coho Salmon Oncorhynchus kisutch smolt as reference data to evaluate model predictions, we found that bioenergetic‐based HSCs matched the validation data better than frequency‐based HSCs, which systematically underestimated optimal flows. However, biases remained using bioenergetic HSCs, suggesting that habitat suitability may not be independent of discharge as is often assumed. Declining invertebrate drift concentration, increasing temperature, and density dependence of growth at low flows are potential mechanisms of flow‐related declines in habitat suitability; measuring these effects and incorporating them into flow models is an important step in further improving model predictions, particularly at low flows. Received December 3, 2015; accepted May 11, 2016