Foraging ecology of large benthic mesopredators: Effects of myliobatid rays on shellfish resources

Recent studies documenting strong trophic cascades due to apex predator declines have revealed the importance of the second level or "mesopredator" prey populations in transmitting these effects down the foodweb. Because many of these mesopredators are also large and highly mobile, understanding their potential impacts has been difficult but is central to understanding top-down control in marine ecosystems. In this study, I examined the foraging ecology and habitat use of two species of myliobatid rays that filled the niche of large mesopredators in their respective communities, cownose rays ( Rhinoptera bonasus) and spotted eagle rays (Aetobatus narinari ).;Cownose rays were identified as seasonal residents of the northern Gulf of Mexico coast. Ontogenetic habitat partitioning was evident in cownose rays across both time and space and was attributed to predator avoidance and/or life-stage dependent resource needs. Cownose rays exhibited habitat-specific feeding patterns and generally specialized on numerically dominant prey. However, there was no evidence of commercial shellfish consumption by cownose rays within coastal waters of Alabama. Moreover, even when presented with manipulated densities of hard clam (Mercenaria sp.), cownose rays continued to forage on more locally abundant benthic invertebrates. Cownose ray foraging effects may be enhanced in temperate estuaries when compared to subtropical regions as restricted migratory corridors can aggregate these predators and enhance their effects on patchy shellfish beds.;In Bermuda, tagged spotted eagle rays were shown to be highly associated with Harrington Sound, an enclosed inshore lagoon that supported exploitable mollusk species. Spotted eagle ray diet was entirely molluskan and exhibited preference for locally dominant calico clam (Macrocallista maculata ). Enclosure and field exclusion experiments documented foraging preference and impact of spotted eagle rays at relatively high density patches of calico clam. However, given the current population status of clams and lack of patch-depleting behaviors exhibited by spotted eagle rays, rays are unlikely to extinguish clam populations at current levels.;Understanding the impacts of myliobatid rays necessitated the coupling of individual-based methods (e.g. telemetry) with traditional surveys and field experimentation. Future studies of large mobile predator impacts should consider adopting similar integrative approaches.