Can Weighted Useable Area Predict Flow Requirements of Drift‐Feeding Salmonids? Comparison with a Net Rate of Energy Intake Model Incorporating Drift–Flow Processes

Abstract

AbstractWe compared a process‐based invertebrate drift and drift‐feeding net rate of energy intake (NREI) model and a traditional hydraulic‐habitat model (using the RHYHABSIM [River Hydraulics and Habitat Simulation] software program) for predicting the flow requirements of 52‐cm Brown Trout Salmo trutta in a New Zealand river. Brown Trout abundance predicted by the NREI model for the constant drift concentration–flow scenarios were asymptotic or linear, depending on drift concentration, increasing through the mean annual low flow (MALF; 17 m3/s). However, drift concentration increased with flow, consistent with passive entrainment. The predicted fish abundance–flow relationship based on flow‐varying drift concentration increased logistically, and more steeply, with flow through the MALF and beyond. Predictions for the relationship between weighted useable area (WUA) and flow were made for three sets of drift‐feeding habitat suitability criteria (HSC) developed on three midsized and one large New Zealand river (flow at sampling was 2.8–4.6 m3/s and ~100 m3/s, respectively) and the South Platte River, Colorado (flow at sampling, 7–18 m3/s). The midsized‐river HSC ascribe lower suitability to water velocities > 0.6 m/s. They predicted WUA peaking at 10–11 m3/s, well below the MALF. The WUA–flow relationships for the two large‐river HSC were asymptotic at about 22 m3/s. Overall, WUA appears to underestimate the flow needs of drift‐feeding salmonids. The NREI model showed that assessing flow needs of drift‐feeding fish is more complex than interpreting a WUA–flow relationship based only on physical habitat suitability. The relationship between predicted fish abundance and flow is an emergent property of flow‐dependent drift‐foraging dynamics interacting with flow‐dependent drift concentration and drift flux, local depletion of drift by feeding fish, and flow‐related replenishment of drift from the bed and dispersion. It is time that the principles and predictions of drift–NREI models influence assessments of habitat capacity and instream flow needs of drift‐feeding fish.Received April 10, 2015; accepted November 10, 2015 Published online April 27, 2016