Searching day and night: diel hierarchical search patterns in a large marine predator that never stops swimming

Abstract

Animals may demonstrate area-restricted search (ARS) behaviour when encountering prey, where they turn frequently to remain in an area where the probability of finding additional prey is relatively high. We would expect optimally foraging animals to adjust the scale of ARS behaviour to maximize potential prey encounters, which is likely a function of prey distribution and will change over diel cycles. While most predators have distinctive diel activity periods (e.g. only hunt at night), others are more variable. Animals that never stop moving may still show diel activity patterns, but how their search mechanisms change throughout the cycle is less clear. We used biologgers to quantify volume-restricted search (VRS or three-dimensional ARS) by great hammerhead sharks, Sphyrna mokarran, and record potential prey encounter rates. Analysis of 736 dives suggested that VRS dives tended to be shorter, with faster swim speeds, than non-VRS dives. VRS behaviour was detected in 73% of dives, with 44% of those exhibiting VRS at two spatial scales (smaller and larger VRS behaviour within one dive). For dives that showed VRS at one spatial scale, the average radius was 24 ± 23 m; for dives that had VRS at two scales, the average radii were 16 ± 15 m and 45 ± 22 m. Of dives with two spatial scales, 84% showed a hierarchical structure, with smaller VRS dives nested within larger VRS dives. Sharks were more likely to search at larger spatial scales and swim faster during the daytime than at night. Despite this, prey encounter rates were similar during the day and night and sharks were more likely to make stronger jerk movements (potential predatory strikes) at night. Great hammerhead sharks adjusted the spatial scale of hierarchical search patterns and optimal swim speeds across the diel cycle, which may reflect trade-offs between maximizing prey encounter rates and minimizing metabolic costs.