Effects of prey size structure and turbulence on feeding and growth of anchovy larvae

Abstract

Foraging processes in plankton and plank- tivorous fish are constrained by relative prey and predator size and therefore, these are important varia- bles to include in a foraging model. The distribution of prey biomass across different size classes can be char- acterized by a size spectrum slope. We present a for- aging model for anchovy larvae including the most relevant processes such as prey encounter, capture- and pursuit success, all influenced by light, turbulence and prey characteristics. We modelled ingestion rates and specific growth rate by coupling the foraging model with an existing bioenergetic model, and per- formed a sensitivity analysis of prey ingestion in tur- bulent environments assuming either hemispherical or conical perceptive volume. Our results suggest that turbulence has no positive effect because of the low capture ability, small prey size and small visual vol- ume for anchovy larvae. The predicted ingestion is too low to sustain the growth potential of larvae when assuming conical perceptive volume even under prey densities substantially higher than normally found in the field. Ingestion rate is sensitive to the total biomass and the slope of the prey size spectra, specifically because it determines the abundance of prey around the optimal size for the larvae. The model also sug- gests that small larvae benefit from a prey size struc- ture with steep prey size-spectra slope while a large larva benefit from less steep slopes. The model can act as a link between size-spectra measurements from the field and the foraging conditions of larval anchovies.