Toxicological estimation of mortality of oceanic sea turtles oiled during the Deepwater Horizon oil spill

Using multiple lines of evidence, we estimated the mortality of oceanic sea turtles that were minimally to moderately oiled by the 2010 BP Deepwater Horizon(DWH) oil spill in the Gulf of Mexico. Using estimates of the oil ingested by oceanic sea turtles and comparing them to toxic endpoints following oil ingestion in turtles and other vertebrate species, we derived an estimated percentage of mortality for oil-exposed oceanic sea turtles. Oil ingestion (mg kg super(-1) BW d super(-1)) in oceanic sea turtles was estimated based on extent of oiling categories assigned by a sea turtle technical working group (STTWG) as follows: non-oiled (0), minimally oiled (1), lightly oiled (2), moderately oiled (3) and heavily oiled (4). Because the STTWG concluded that 100% of heavily oiled turtles would have died from the physical effects of heavy oiling, we limited our assessment of mortality to turtles in categories 0 to 3 and estimated how many of these turtles would have died from ingestion of oil. The estimated mortality was 85% for category 3, 50% for category 2 and 25% for category 1. Visibly non-oiled turtles (category 0) were assigned 0% mortality. To calculate the overall mortality for all turtles, the mortality estimations for categories 0 to 3 were applied to the numbers of turtles observed with different degrees of oiling, as documented by direct capture operations during the DWH spill. We concluded that, overall, approximately 30% of all oceanic turtles in the region affected by the DWH spill that were not heavily oiled would have died from ingestion of oil.