Turbulence-enhanced prey encounter rates in larval fish: Effects of spatial scale, larval behaviour and size

Turbulent water motion has several effects on the feeding ecology of larval fish and other planktivorous predators. In this paper, we consider the appropriate spatial scales for estimating relative velocities between larval fish predators and their prey, and the effect that different choices of scales might have on encounter and ingestion rates. Four possible scaling choices have been used in the literature, giving rise to varying estimates of the effect of turbulence on encounter rate. We argue that the correct scale is that based on the larval reactive distance, and that this interpretation is consistent with classical coagulation theory. We then demonstrate that differences in larval search strategy (pause-travel versus cruise search) and behaviour (e.g. reactive distance, swimming speed, pause duration) will lead to substantial differences in estimated encounter rates. In general, small larvae are more likely to benefit from turbulence-increased encounter than larger larvae. Overall ingestion rate probability (= probability of encounter x probability of successful pursuit) is likely to be highest at moderate-high levels of turbulence. In most larval fish habitats, turbulence levels appear to lie in the range in which turbulent intensity has an overall positive effect on larval fish ingestion rate probability. However, experimental data to test the model predictions are lacking. We suggest that the model inputs require further empirical study.