How Do You Feed Trout in Individual-Based Models?

Mechanistic individual-based models (IBMs) of salmonid fishes are being used in a variety of applications, including forecasting the effects of resource management activities and other environmental alterations.  One challenge for these applications is the strong influence of food availability on real-world and model outcomes, combined with uncertainty about the use of alternative feeding modes by the fish.  While drift feeding is often the dominant feeding mode for salmonids, in many lotic environments fish may feed from the benthos or on other prey that are not delivered by moving water.  Does this alternative need to be included in individual-based models?  We present one example where an IBM that allows only drift feeding conflicts with population-level field observations.  We also contrast the abilities of a drift-only model and one with multiple feeding modes to match results for individual growth in a field experiment.  Inclusion of an alternative to drift feeding yielded a better fit to individual growth data and produced consistent calibration values for drift concentration that matched field observations.  Including multiple feeding modes in some individual-based models of stream salmonids may be appropriate, but doing so adds complexity that can make both model calibration and understanding model results more challenging.