Genetic assignment to stock of stranded common bottlenose dolphins in south eastern Louisiana after the Deepwater Horizon oil spill

Degradation of marine ecosystems is an increasing problem and extends beyond nearshore coastal waters with significant human development. However, measuring ecosystem damage and decreased ecosystem function can be difficult. Marine mammals have often been recommended as indicators for evaluating ecosystem health. Between March 2010 and July 2014, a significant cetacean unusual mortality event occurred across the northern Gulf of Mexico, where multiple demographically independent populations of common bottlenose dolphins Tursiops truncatus occur adjacent to one another. Some populations are fairly small and restricted to small habitat areas, while other populations have higher abundances and cover broader geographic ranges. An integral component to determining the impact of this event on these populations is identifying what percentage of each population the stranded animals comprise. We applied genetic assignment test methods to stranded dolphins from southeastern Louisiana to determine the proportion of dead dolphins that came from the local estuarine population versus the population found in adjacent coastal waters. Forty-one microsatellite loci were genotyped in 156 live dolphins sampled to represent the 2 potential stocks of origin and in 131 dead stranded dolphins of unknown origin. Both classical assignment tests and genetic stock identification methods indicated that approximately 6 to 7% of the sampled stranded dolphins originated from the Western Coastal Stock and the remainder from the smaller, estuarine stock in Barataria Bay, Louisiana.