Quantifying the contribution of small-scale turbulence to the encounter rates between larval fish and their zooplankton prey: effects of wind and tide

It has recently been suggested that small-scale turbulence can increase the encounter rate between planktonic predators and their prey. In this paper, we quantify the contribution of turbulence to contact rates and estimate the potential for misrepresenting larval contact rates when turbulence is not accommodated in interpretations of the larval food resource. This was achieved through simulations evaluating the nature and magnitude of wind-and tide-induced turbulence on the encounter rate between larval fishes and copepod nauplii. These simulations were based on an empirical model describing turbulent energy levels under varying conditions of wind and depth, and on the Simpson-Hunter formulation for depth-averaged turbulence due to tidal motion. The contribution of small-scale turbulence to encounter rates depended strongly on wind and tidal velocities, prey density and distribution, and larval fish swimming speeds. Failure to consider the influence of small-scale turbulence when prey density 5 35 1-' can result in up to an 11-fold underestimation of the frequency of contacts between f ~s h larvae and prey under conditions prevailing at fronts In waters over the northwest European continental shelf during summer, and up to 112 % for larvae at depth 20 m during wind velocities of 5 m S-' These biases increase when prey densities are lower and when prey are heterogeneously distributed. We also show that turbulence may slow the rate of starvation among larval fish because weak, slowswimming individuals are more likely to benefit from turbulence than faster-swimming larvae. We conclude that turbulence, in addition to light and nutrients, may b e an important component of the observed increase in plankton production rates and biomass at tidal fronts and at other upwelling systems.