Combined Gut-Content and Stable Isotope Trophic Analysis of the Pelagic Stingray (Pteroplaytrygon violacea) from the Western North Atlantic Ocean

The understanding of an organism’s trophic level is vital to understanding the impact that a specific species has on the ecosystem, and trophic relationships are vital for correctly modeling ecosystems and ecosystem effects of fisheries removals. The pelagic stingray is a common bycatch species in the Atlantic pelagic longline fishery and understanding its ecological impacts are necessary to make correct assumptions for pelagic ecosystems. The pelagic stingray is found in sub-tropical and tropical waters worldwide and is thought to inhabit the epipelagic zone of the ocean based on fishery catch records. Very few studies have been done on the diets of the pelagic stingray, most with low sample sizes. For this work, 120 specimens (63 males and 61 females) were collected during commercial pelagic longline fishing operations in the U.S. South Atlantic Bight between August 2008 and December 2009. Diet composition of these stingrays was assessed for variance among seasons, gender, and body lengths (age class proxy). Stomach contents were fixed in formalin, then dissected and quantified to the lowest possible taxonomic level. Preliminary stomach content dissections have shown the major consumed items were teleost species (36.37%), shrimp (28.57%), and squid (23.38%). This was in contrast to previous diet studies from the Pacific Ocean which found mollusks (50%) to be the dominant prey item, followed by Actinopterygiian fishes (19%) and crustaceans (17%). In addition, stable isotope analysis of δ ¹³ C and δ ¹⁵ N was performed on white dorsal muscle to correlate trophic feeding level data and the gut-content analysis. The δ ¹³ C values of the pelagic stingray were recently compared to literature-published values for the stomach content species and values of analyzed swordfish from the region. The results suggest the stingrays were possibly feeding on a different base of the food web. The range in δ ¹⁵ N values showed foraging across several trophic levels.