Foraging with Trawlers May Blur the Boundaries Between Estuarine and Coastal Stocks of Common Bottlenose Dolphin (Tursiops truncatus) off South Carolina

Abstract

In the western North Atlantic, common bottlenose dolphins (Tursiops truncatus) that forage in association with shrimp trawlers in ocean waters are widely assumed to be members of coastal rather than estuarine stocks. To test this assumption, I analyzed photo-identification (photo-id) images of common bottlenose dolphins collected by Justin Greenman in 2011 from North Carolina to Florida during surveys conducted aboard the South Carolina Department of Natural Resources Research Vessel Lady Lisa. I developed a dorsal fin catalog of 254 individual dolphins and compared this catalog to 14 regional photo-identification catalogs from North Carolina to Florida. I created a dataset with sightings records for matched dolphins seen 10 or more times (n=18) to examine their stock identities, based on criteria described in National Marine Fisheries Service (NMFS) Stock Assessment Reports (SARs). Only two of the 18 dolphins were sighted only in coastal waters, and four had the majority of their sightings in estuarine waters but also traveled into coastal waters to forage in association with trawlers. Two dolphins were first observed foraging in association with shrimp trawlers dating back to 1998. Overall, I found that dolphins from multiple stocks forage in association with shrimp trawlers in coastal waters of South Carolina. In addition, it appears that this behavior may attract estuarine dolphins into coastal waters outside their typical stock boundaries. The movement of dolphins across stock lines to engage in this foraging behavior raises concerns about our understanding of bycatch in this fishery. Misassignment of bycatch to the wrong stock could be particularly harmful to small stocks of bottlenose dolphins.