Using salinity to identify common bottlenose dolphin habitat in Barataria Bay, Louisiana, USA

Abstract

Following the Deepwater Horizon (DWH) oil spill, numerous studies were conducted to determine impacts on common bottlenose dolphins Tursiops truncatus. Common bottlenose dolphins are found in estuarine environments of the northern Gulf of Mexico which vary in salinity, depending on location (e.g. distance to freshwater source), season, and ocean tides. Although common bottlenose dolphins can be found in low-salinity waters (<15 ppt), they cannot tolerate very low salinity for long periods of time. We matched dolphin telemetry data in Barataria Bay, Louisiana (USA), with contemporaneous estimates of salinity to establish a salinity threshold and identify preferred dolphin habitat. Dolphins frequently used areas where salinity was higher than ~11 ppt, sometimes used areas for short periods of time with predicted salinity of ~8 ppt, and avoided waters with salinities below ~5 ppt. While not a hard boundary, the ~8 ppt threshold can be used to delineate reasonable polygons of preferred dolphin habitat. We temporally averaged the location of the ~8 ppt isohaline from 2005 through 2012 to establish areas of preferred dolphin habitat. In Barataria Bay, the polygon of dolphin habitat encompasses 1167 km2, and extends from the bay's barrier islands to approximately half-way through marshes in northern parts of the bay. This polygon of suitable common bottlenose dolphin habitat was then ultimately used to quantify cetacean injury due to DWH oil.