HABITAT PARTITIONING IN SYMPATRIC SHARKS AT FRENCH FRIGATE SHOALS ATOLL

Abstract

Competition and predation are important ecological mechanisms shaping resource utilization patterns in sympatric species. Determining how mobile marine predators partition resources is extremely challenging because of the concealing nature of the ocean, hence most studies of resource partitioning in sharks have focused on diet which can be gleaned from examining the stomach contents of dead individuals or stable isotope analyses of shark tissues. Far less is known about how sympatric sharks partition habitat because of the difficulties associated with quantifying their use of habitat over time. To overcome this challenge, I used passive acoustic monitoring to quantify space and habitat use by sympatric shark species at a remote, protected atoll (French Frigate Shoals: FFS) with near-pristine ecology. I analyzed the movements of 149 individuals over a 2-year period to determine whether and how sympatric sharks partition habitat. The observed patterns of shark habitat use at FFS were consistent with preferred prey distribution, resource partitioning/competitive exclusion and predator avoidance. Some sympatric shark species partitioned habitat along spatial and temporal (seasonal and diel) dimensions, whereas others overlapped possibly by exploiting different prey. Tiger sharks may occupy a keystone role in the shark assemblage at FFS, with their habitat use and prey preferences shaping those of other shark species. A tiger shark preference for foraging on fledging birds and other prey found concentrated in shallow lagoon habitats may restrict grey reef shark use of these habitats because of predation risk, and narrow Galapagos shark diet relative to other locations. This study reveals how apex predators can modify the behavior of subordinate species, leading to the structuring of marine ecosystems.