Individual-based simulations of larval fish feeding in turbulent environments

We used an individual-based model coupled to a realistic turbulence flow field to assess the effects of a wide range of turbulence levels on encounter rate (E), pursuit success and ingestion rate in fish larvae. We parameterized the model for larvae of Atlantic cod Gadus morhua and evaluated how the geometry of their prey search volume (hemisphere, wedge) affected feeding rates. We then compared model outputs with feeding rates for cod larvae in previous laboratory and field studies. Search volume is smaller and E is lower for wedge searchers than hemisphere searchers. However, as turbulence increases, larvae encounter more prey but pursuit success decreases exponentially (relative to calm water), yielding a dome-shaped relationship between turbulence and ingestion rate. These results are robust to search volume geometry (wedge or hemisphere). The increase in ingestion rates at moderate turbulence for wedge searchers (relative to calm water rates) was higher than for hemisphere searchers. However, model results derived using hemisphere geometry are consistent with previous laboratory and field observations of cod larvae in turbulent environments. Cod larvae observed in a field study on Georges Bank in 1993-1994 could feed at rates which corresponded with observed growth rates if they behaved as hemispherical searchers and consumed a diet consisting of relatively large prey (e.g. copepodites and adults of Pseudocalanus); if they used wedge-shaped volumes, their ingestion and growth rates would have been ca. 80 % lower, resulting in higher mortality. Models like those developed here will increase future understanding of factors affecting larval feeding rates and dietary composition.