Larval fish feeding and turbulence: A case for the downside

Theory states that small-scale turbulence decreases pursuit success of plan ktonic predators by advecting the encountered prey from the reactive zone o f the predator during the pursuit event. We tested the quantitative predict ions of a previously published model describing this phenomenon in larval c od by videorecording particle motion and feeding behavior of larval cod (8. 7-12.3 mm) preying on copepods in a laboratory tank. Fluid motion shared ch aracteristics with that in the ocean, i.e., intermittent, logarithmically d istributed, average particle-particle velocity difference proportional to s eparation distance(1/3). Estimated bulk dissipation rates were 0-2 X 10(-8) m(2) s(-3) and similar to those commonly experienced by larval cod in natu re (e.g., located at 30 m during winds of ca. 7 m s(-1)). Owing to the inte rmittent nature of turbulence, we related individual predation events to lo cal, instantaneous relative velocities instead of bulk averages. Pursuit su ccess decreased significantly with relative velocity and the observations a pproximated the predicted effect of turbulence on pursuit success. Nonlinea r and counteracting effects of turbulence on pursuit success and encounter may partly explain the contradictory observations of how turbulence affects larval fish feeding, growth, and survival in the sea.