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

摘要

We 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 m(3)/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 similar to 100 m(3)/s, respectively) and the South Platte River, Colorado ( flow at sampling, 7-18 m(3)/s). The midsized-river HSC ascribe lower suitability to water velocities > 0.6 m/s. They predicted WUA peaking at 10-11 m(3)/s, well below the MALF. The WUA-flow relationships for the two large-river HSC were asymptotic at about 22 m(3)/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.