Abstract
Common bottlenose dolphins (Tursiops truncatus) inhabit bays, sounds and estuaries across the Gulf of Mexico. Following the Deepwater Horizon oil spill, studies were initiated to assess potential effects on these ecologically important apex predators. A previous study reported disease conditions, including lung disease and impaired stress response, for 32 dolphins that were temporarily captured and given health assessments in Barataria Bay, Louisiana, USA. Ten of the sampled dolphins were determined to be pregnant, with expected due dates the following spring or summer. Here, we report findings after 47 months of follow-up monitoring of those sampled dolphins. Only 20% (95% CI: 2.50–55.6%) of the pregnant dolphins produced viable calves, as compared with a previously reported pregnancy success rate of 83% in a reference population. Fifty-seven per cent of pregnant females that did not successfully produce a calf had been previously diagnosed with moderate–severe lung disease. In addition, the estimated annual survival rate of the sampled cohort was low (86.8%, 95% CI: 80.0–92.7%) as compared with survival rates of 95.1% and 96.2% from two other previously studied bottlenose dolphin populations. Our findings confirm low reproductive success and high mortality in dolphins from a heavily oiled estuary when compared with other populations. Follow-up studies are needed to better understand the potential recovery of dolphins in Barataria Bay and, by extension, other Gulf coastal regions impacted by the spill.
Highlights
Our findings from follow-up monitoring studies confirm significant decreases in reproductive success and high mortality rates when compared with other populations not impacted by the spill
This evidence suggests that dolphin reproduction and survival is being impacted by chronic disease, indicating that the effects of the Deepwater Horizon (DWH) oil spill have been long-lasting
Continued studies are needed to further understand the potential recovery trajectory for dolphins in Barataria Bay, as well as other Gulf Coast regions impacted by the spill
Summary
While the health assessment study documented severe and prevalent disease conditions in Barataria Bay dolphins post-spill, the question of how these disease conditions would affect reproductive success and survival remained. Intensive vessel-based monitoring was conducted to determine reproductive success and survival of the dolphins sampled during the 2011 health assessments. Data for analysis of reproductive outcome and survival were synthesized from differing types of vessel-based surveys conducted from September 2011 to July 2015 (see electronic supplementary material, figure S1). To facilitate the variation in time between sampling occasions, time intervals were computed as the fraction of a year, based on months, between two occasions, and wt was raised to the interval length during likelihood computation This scaled wt to a yearly time frame and facilitated modelling annual survival as constant.
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