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.

Quantifying injury to common bottlenose dolphins from the Deepwater Horizon oil spill using an age-, sex- and class-structured population model

“…There was a significant increase in the number of dolphin strandings spatially and temporally associated with the spill (Venn-Watson et al 2015b), and a high prevalence of adrenal and lung lesions in dolphin carcasses recovered within the spill footprint (Venn-Watson et al 2015a). Longitudinal photo-identification studies followed pregnant females to document birth outcome and showed the rate of reproductive failures was over 2-fold higher for stocks exposed to DWH oil than would be expected based on unexposed reference populations , Kellar et al 2017. The official ending of the UME on 31 July 2014, and follow-up assessments of live dolphins 3 and 4 yr post-spill suggest that the health of nearshore dolphins has improved , this Theme Section).…”

“…Field studies conducted to assess injury to nearshore common bottlenose dolphins Tursiops truncatus following the Deepwater Horizon (DWH) oil spill demonstrated poor health (Schwacke et al 2014), reproductive failure , Kellar et al 2017, this Theme Section), and increased mortality that contributed to the largest and longest lasting cetacean Unusual Mortality Event (UME) on record in the northern Gulf of Mexico (Litz et al 2014). Adverse health effects documented in live dolphins after the spill included lung disease, poor body condition, and an impaired stress response indicative of injury to the adrenal pathway and consistent with adrenal insufficiency (Schwacke et al 2014).…”

“…However, the prevalence of moderate to severe lung disease and impaired stress response continues to be higher than expected (as of summer 2014; see Smith et al 2017), indicating there are lingering health effects for the dolphin cohorts exposed to DWH oil that will likely continue to affect survival. Likewise, the rate of reproductive failure remained elevated through spring 2015 when the final Natural Resource Damage Assessment (NRDA) bottlenose dolphin surveys were conducted (Kellar et al 2017).…”

The Deepwater Horizon oil spill marine mammal injury assessment

“…), which is consistent with the increased susceptibility of perinates to in utero Brucella infection (Colegrove et al 2016). Kellar et al (2017, this Theme Section) provide a deeper ana lysis of the reproductive failures seen in bottlenose dolphins in Louisiana, Mississippi, and Alabama, including a synthesis of hormone data from remote biopsy samples and surgical biopsies from dolphins sampled via capture−release health assessments, as well as vessel surveys of animals in photo-ID studies.…”

“…To establish post-spill population vital rates (i.e. survival and reproductive success), the NRDA science team conducted photo-ID surveys and performed mark− recapture analyses in Barataria Bay (Kellar et al 2017, this Theme Section) and Mississippi Sound (DWHMMIQT 2015, DWH NRDA Trustees 2016, Kellar et al 2017). In addition, the NRDA science team analyzed bottlenose dolphin spatial preferences , this Theme Section) and the density of dolphins within different habitat strata (e.g.…”