Encounter rates and swimming behavior of pause-travel and cruise larval fish predators in calm and turbulent laboratory environments

We observed the feeding and swimming behavior of freely swimming cod (Gadus morhua) and herring (Clupea harengus) larvae in calm and turbulent (epsilon = similar to 7.4 x 10(-8) m(2) s(-3)) laboratory environments at limiting and satiating abundances of Acartia tonsa prey. Attack position rates (a measure of prey encounter rate in unsatiated larvae) were significantly higher in turbulent than in calm water at low food abundances for two size groups of cod. The difference in cod attack position rate between calm and turbulent water was much less when prey was more abundant. Attack position rates of herring larvae were higher in turbulent water than in calm water, but the difference was not significant. Interspecific differences in swimming and pausing behavior were related to differences in prey search strategy used by the two species (cod: pause-travel; herring: cruise). We used a newly developed search model for pause-travel predators in calm and turbulent environments to compare encounter rates for predators using cruise and pause-travel search strategies. Encounter rates for cod and herring larvae, estimated with respective search models, were similar in calm and low turbulence water; at high turbulence levels, the pause-travel model predicts higher encounter rates than does the cruise model. In terms of prey encounter rate, cod larvae benefit more from turbulent motion than do herring larvae. However, aspects of larval behavior other than prey search strategy (e.g. prey capture success) need to be examined experimentally before the overall effects of turbulence on larval fish feeding rates can be fully evaluated.