Modeling rule-based behavior: habitat selection and the growth-survival trade-off in larval cod

Environmental variation can cause significant fluctuations in the survival of larval fish and plankton. Understanding these fluctuations is critical for developing more accurate fisheries models, which are needed for both scientific and socioeconomic research. Growth, survival, and dispersal of marine planktonic larvae rely strongly on their behavior. Larval fish change their vertical positioning due to strong vertical gradients in light, temperature, predation pressure, and prey availability. Here, we explore how various behavioral rules predict vertical distribution, growth, and survival of larval cod (Gadus morhua) in a numerical model. The rules determine the trade-offs between larval growth, feeding rate, and predation rate, including their dependence on gut fullness and bodymass. We evaluated the survival through size classes for different rules and random behavior and comparedmodel predictions with observed larval distributionpatterns. The rules predicted the correct average depthpositionwith larval size, but failed to predict the timing of the observed vertical distribution pattern. However, model simulations revealed significant increases in survival for larval and juvenile cod with active behavior compared with larvae with random behavior. Behavior was important across all sizes of fish, and this study illustrates the value or added information of incorporating behavior in biophysical models.