Drift Concentration Changes with Flow —Consequences for Predicting Flow Needs of Drift Feeding Fish

There is a growing recognition that food requirements of fish need to be considered alongside habitat when assessing flow (discharge) needs of fish. For drift-feeding fish that also requires understanding the relationship between invertebrate drift flux and flow. Drift flux is influenced by water velocity and drift concentration. We examined the relationship between drift concentration and flow in a New Zealand river. We sampled invertebrate drift in a 520 m long reach comprising a run, riffle, deep run/pool sequence on five occasions spread over two natural flow recessions. The sampled flows ranged between 88% of mean annual low flow and 70% of median flow. Drift concentration increased with increasing flow, and varied spatially (laterally and longitudinally). Spatial variation in drift concentration was correlated with lateral and longitudinal velocity gradients. The flow and spatial relationships with drift concentration are consistent with the process of shear-stress-driven entrainment. Flow-related entrainment more than compensated for reduction in drift concentration expected from dilution. Our results have implications for invertebrate dispersal and available food for drift-feeding fish. Instream flow assessments employing drift foraging models that do not account for increasing drift concentration (and drift flux) with increasing flow will underestimate the flow requirements for drift-feeding fish.