Impacts of Cownose Rays (Rhinoptera bonasus) to northern Gulf of Mexico Shellfish Resources: An Integrated Approach

Past studies have suggested that increases in cownose ray (Rhinoptera bonasus) abundance may pose problems for fisheries management due to their specialized diet of exploitable mollusks. Despite their reported abundance in the northern Gulf of Mexico there are few published studies examining the spatial distribution and diet of R. bonasus from this region, and thus their impact to shellfisheries is currently unknown. We conducted a three-year (2007-2010) integrative study on the spatial and seasonal distribution, habitat use and foraging behavior of cownose rays across the coastal Alabama corridor. Acoustic and satellite tagging of adult cownose rays demonstrated strong connectivity between inshore and offshore waters across the northern Gulf of Mexico, with evidence of multiple seasonal returns to the Mobile Bay estuary. Gillnet data showed adult cownose rays preferred unstructured bottom habitat (sand/mud) over high relief oyster (Crassostrea virginica) reefs and reached highest abundances in spring and fall along Gulf barrier islands. Young-of-the-year (YOY) and juvenile rays, on the other hand, primarily utilized inshore estuarine waters in summer, indicating this area may serve as a nursery ground during warmer months. Food habits of cownose rays varied across ontogeny and space: adult diets were dominated by crustaceans along barrier islands and open waters of the Gulf of Mexico, whereas juvenile and young-of-the-year individuals almost exclusively consumed bivalves (i.e. veneroid clams) in riverine and estuarine areas. Individual cownose ray diets were primarily monotypic (54.3 %) and appeared selective for amphipods (i.e. Haustorius sp.) when hyperabundant along gulf barrier islands, and bivalves in inshore estuaries regardless of ambient densities. Despite the importance of bivalves to rays in estuarine locations, we found no evidence of C. virginica consumption. However, we still encourage continued sampling around potential oyster seeding areas as oyster abundance was relatively low throughout the sampling period. While this study highlights the importance of locality in determining dietary composition of cownose rays, we do not suggest this species is opportunistic or completely mollusk-specialized. Alternatively, we propose cownose rays exhibit a continuum of foraging behaviors between specialization and generalization, depending on locale and prey availability. Given the foraging behavior, space utilization patterns and low fecundity of cownose rays in the northern Gulf of Mexico, predator control programs for this species do not appear necessary in this region.