Predator Speeds and the Encounter Rates with Lagrangian Coherent Structures in the Atlantic Ocean.

• Piper Priddy, Marine Science, University of Delaware

• Matt Oliver, School of Marine Science and Policy, University of Delaware

In marine environments, prey is unpredictable, patchy, and unevenly distributed. Unpredictability of prey fields poses a unique challenge for marine predators. One such strategy is predator speed; it is unclear what the optimal cruising speed is for maximizing the encounter rates of nutrient-rich areas. Lagrangian Coherent Structures (LCS) can act as aggregators of nutrients and prey, potentially creating predator-prey hotspots. LCS are boundaries in fluid flow that facilitate the convergence and divergence of particles and are quantified using both Finite-Time Lyapunov Exponents (FTLE) and Dilation Rates (Delta). FTLE measures the rate of separation of two passive particles over a finite time, while Delta measures the separation of a particle from its trajectory and is calculated as the difference between the initial and final patch size. In this study, we utilize sharks as proxies for predators, due to a considerable amount of tracking data, background literature, and varying cruising speeds. The Gulf Stream, a strong western boundary current along the Eastern Atlantic Ocean, was selected due to its strong LCS features (i.e., eddies). We simulate four different predator cruising speeds: stationary, slow, regular, and fast. Statistical analyses compared differences among each predator’s encounter rates with new FTLE and Delta features in the Gulf Stream. Interactions were simulated over a year (2004), which isolated seasonal variations. This allowed for a better understanding of the frequency in which predators with varying cruising speeds encounter potentially nutrient-rich hot spots. Our results indicate that fast sharks have a higher likelihood of encountering new FTLE and Delta features daily compared to slow, stationary, and regular sharks. This suggests that the evolutionary advantage of speed in the Gulf Stream allows faster sharks to more regularly encounter nutrient-rich hotspots. This speed-related interaction could play a significant role in their foraging success and overall survival.