Low reproductive success rates of common bottlenose dolphins Tursiops truncatus in the northern Gulf of Mexico following the Deepwater Horizon disaster (2010-2015)

Abstract

Following the Deepwater Horizon (DWH) oil spill, reproductive success rates in 2 northern Gulf of Mexico (GoM) bottlenose dolphin stocks exposed to oil were evaluated for 4 yr during and after the spill (2010 to 2015) in efforts to assess population-level reproductive health. Pregnancy was determined from either (1) ultrasound examinations of the reproductive tract during capture-release health assessments, or (2) endocrine evaluations of blubber tissue collected from dart biopsies of free-ranging dolphins. Follow-up photo-identification was then used to track the status of pregnant females and any associated neonatal calves for a minimum of 1 yr after the initial pregnancy detection (IPD). For all pregnant females observed following IPD, individuals seen with a calf (reproductive success) and without one (reproductive failure) were recorded. The resulting estimated reproductive success rates for both GoM stocks (19.4%; 7/36) were less than a third of those previously reported in other areas not impacted by the spill (i.e. Sarasota Bay, FL; Indian River Lagoon, FL; and Charleston Harbor, SC) using similar techniques (64.7%; 22/34). We also evaluated the relationships between reproductive success and 13 potential covariates, including stock, ordinal date, progesterone, cortisol, thyroid hormone concentrations, leukocyte count, lung health score, and total body length. Among these, the results only provide strong evidence (Bayes factor >20) of a relationship between reproductive failure and the total leukocyte count covariate. The high reproductive failure rates measured in both GoM stocks following the DWH oil spill are consistent with mammalian literature that shows a link between petroleum exposure and reproductive abnormalities and failures.