Mesopelagic diet as pathway of high mercury levels in body feathers of the endangered Diablotin Black-capped Petrel (Pterodroma hasitata)

The Diablotin Black-capped Petrel (Pterodroma hasitata) is an endangered gadfly petrel found in the western North Atlantic. The species is listed as globally Endangered by the International Union for the Conservation of Nature, and is currently being reviewed for listing under the U.S. Endangered Species Act. Approximately 2,000 pairs of Black-capped Petrels nest at five documented sites on the Caribbean island of Hispaniola, although to date only 120 nests have been located. At sea, the species is distributed over the western north Atlantic, Caribbean Sea, and northern Gulf of Mexico. Across its marine range it is exposed to many conservation threats including, but not limited to, offshore energy development and shipping, associated marine pollution, and commercial fisheries. In April 2018 and May 2019, we collected breast feathers from breeding adults in the Dominican Republic (n = 10) and from non-breeding adults at sea offshore North Carolina, USA (n = 10), respectively. We analysed Total mercury concentrations in whole feathers via nitric-acid extraction. We also examined Black-capped Petrel diet using DNA metabarcoding of fecal samples. Results showed relatively high concentrations of Total mercury compared to other Pterodroma worldwide, with mean concentrations of 30.3 +/- 11.1 ppm dry weight (range: 15.2-53.9 ppm dry weight; n = 20). Diet was dominated by fish species, including a high proportion of mesopelagic groups such as myctophids, and fishes of interest to artisanal and commercial Caribbean fisheries. These results confirm earlier suggestions of high susceptibility to mercury by Black-capped Petrels, likely through the consumption of mesopelagic prey, while also suggesting potential interactions with fisheries.