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 turbule nt (epsilon = similar to 7.4 x 10(-8) m(2) s(-3)) laboratory environme nts 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 foo d 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 highe r in turbulent water than in calm water, but the difference was not si gnificant. Interspecific differences in swimming and pausing behavior were related to differences in prey search strategy used by the two sp ecies (cod: pause-travel; herring: cruise). We used a newly developed search model for pause-travel predators in calm and turbulent environm ents to compare encounter rates for predators using cruise and pause-t ravel search strategies. Encounter rates for cod and herring larvae, e stimated with respective search models, were similar in calm and low t urbulence water; at high turbulence levels, the pause-travel model pre dicts higher encounter rates than does the cruise model. In terms of p rey encounter rate, cod larvae benefit more from turbulent motion than do herring larvae. However, aspects of larval behavior other than pre y search strategy (e.g. prey capture success) need to be examined expe rimentally before the overall effects of turbulence on larval fish fee ding rates can be fully evaluated.