Can WUA Predict the Flow Requirements of Drift-Feeding Salmonids? — Comparison with a Net Rate of Energy Intake Model Incorporating Drift—Flow Processes

We compared a process-based, invertebrate drift- and drift-feeding net rate of energy intake (NREI) model with a traditional hydraulic-habitat model for predicting the flow (discharge) requirements of brown trout in a New Zealand river. The NREI model predicted fish numbers increasing with flow to an asymptote about the MALF (17 m³/s) when run with the common assumption of constant drift concentration with flow. However, drift concentration increased with flow, consistent with passive entrainment. The predicted fish numbers—flow relationship based on flow-varying drift concentration increased with flow through the MALF and beyond. By contrast the WUA—flow relationships indicated maximum WUA at 10–11 m³/s and 22 m³/s, depending on the habitat suitability curves used in the hydraulic-habitat model. WUA appears to underestimate the flow needs of drift-feeding salmonids. The NREI model showed that assessing the flow needs of drift-feeding fish is much more complex than interpreting a WUA—flow relationship based only on physical habitat suitability. The relationship between fish abundance and flow is an emergent property of flow-dependent drift-foraging dynamics interacting with flow-dependent drift concentration and drift (energy) flux, local depletion of drift by feeding fish and flow-related replenishment of drift from the bed and dispersion.