Foraging effects of cownose rays (Rhinoptera bonasus) along barrier islands of the northern Gulf of Mexico

Large mobile predators are hypothesized to fulfill integral roles in structuring marine foodwebs via predation, yet few investigations have actually examined the foraging behavior and impact of these species on benthic prey. Limited studies from the Cape Lookout system implicate large schooling cownose rays (Rhinoptera bonasus) in the devastation of patches of commercially harvested bay scallop via strong density-dependent foraging behavior during migrations through this estuary. However, despite the extensive Atlantic range of R. bonasus, the pervasiveness of their patch-depleting foraging behavior and thus impact on shellfisheries remains unknown outside of North Carolina waters. To further understand the potential impacts of cownose rays on benthic prey and the role of bivalve density in eliciting these impacts, we conducted exclusion and manipulation experiments at two sites in the northern Gulf of Mexico frequented by rays during spring migrations. Despite a correlation in ray abundance with haustorid amphipod (primary natural prey) density at our study sites, we were unable to detect any effect of rays on amphipod densities. In addition, through manipulation of predator access, we determined the main cause of mortality to manipulated patches of hard clams was predation by smaller predators such as Callinectes sapidus and not cownose rays. While cownose rays consume hard clam in other parts of their range, we suggest rays along northern Gulf of Mexico barrier islands may prefer foraging on smaller and thinner-shelled bivalves (e.g., Donax sp.), as well as more abundant amphipod crustaceans. We caution that these preferences may have reduced our ability to detect effects of rays on manipulated prey, and thus future impact experiments should strongly consider the local diet of these predators and explore novel techniques to estimate effects on small crustaceans. Further synchronized experimentation along basin-wide scales may elucidate the environmental factors that determine the severity of cownose ray foraging impacts across their range.

► We quantify the foraging effects of cownose rays on natural and manipulated prey.
► Field experiments on sandflats indicate cownose rays have no detectable impacts.
► Smaller predators appear to more negatively impact shellfish along barrier islands.
► We suggest potential latitudinal variation in the foraging impacts of cownose rays.