Trout bioenergetics as a process-based tool to estimate ecological risk in a regulated river

Bioenergetics models produce quantitative flow-ecology relationships that summarize changes in habitat and food resources from altered flows. We used a drift-foraging bioenergetics model to quantify the net rate of energetic intake (NREI) for trout above and below a water diversion. NREI is reduced by >95% below the water diversion in July–September, when up to 98% of unimpaired flows are diverted. We then used a risk-based approach to estimate the maximum diversion rate, expressed as a percentage of unimpaired flow, that would produce NREI values that are not significantly lower than values under unimpaired flows throughout a 62-year period. NREI decreased with increased precent-of-flow diversion rates in low-flow months (July–September). Diversion rates of 16% in July and 9% in August and September would maintain NREI within the range of unimpaired flow conditions. In higher flow months, May–June, increasing diversions brought estimated instream flows closer to the peak NREI flow, leading to the assessment that increased diversions would increase NREI. Bioenergetic models can be used to develop protective flow rates at times of the year when fish growth and production would be high under unimpaired flows, which often coincides with when water is diverted. Our study is the first to develop protective percent-of-flow diversion rates for holistic flow management using a quantitative process-based and fish-centric ecological metric.