Survival, density, and abundance of common bottlenose dolphins in Barataria Bay (USA) following the Deepwater Horizon oil spill

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Field studies documented increased mortality, adverse health effects, and reproductive failure in common bottlenose dolphins Tursiops truncatus following the Deepwater Horizon (DWH) oil spill. In order to determine the appropriate type and amount of restoration needed to compensate for losses, the overall extent of injuries to dolphins had to be quantified. Simply counting dead individuals does not consider long-term impacts to populations, such as the loss of future reproductive potential from mortality of females, or the chronic health effects that continue to compromise survival long after acute effects subside. Therefore, we constructed a sex-and agestructured model of population growth and included additional class structure to represent dolphins exposed and unexposed to DWH oil. The model was applied for multiple stocks to predict injured population trajectories using estimates of post-spill survival and reproductive rates. Injured trajectories were compared to baseline trajectories that were expected had the DWH incident not occurred. Two principal measures of injury were computed: (1) lost cetacean years (LCY); the difference between baseline and injured population size, summed over the modeled time period, and (2) time to recovery; the number of years for the stock to recover to within 95% of baseline. For the dolphin stock in Barataria Bay, Louisiana, the estimated LCY was substantial: 30 347 LCY (95% CI: 11 511 to 89 746). Estimated time to recovery was 39 yr (95% CI: 24 to 80). Similar recovery timelines were predicted for stocks in the Mississippi River Delta, Mississippi Sound, Mobile Bay and the Northern Coastal Stock.

Section: Dwh-associated Survival Reduction Factormentioning

confidence: 99%

Section: Density-dependent Fecunditymentioning

confidence: 99%

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

Schwacke¹,

Thomas²,

Wells³

et al. 2017

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“…The higher value for western Louisiana is not surprising given that all strandings recovered were from the ocean side of the beaches, and there is very little estuarine habitat including only one small estuarine stock in Lake Calcasieu in far western Louisiana (Waring et al 2015). In contrast, both the West and East regions sustain relatively large BSE stocks, with abundance estimates of > 2000 dolphins for BB in the West region (McDonald et al 2017), and > 3000 dolphins for MS in the East region (Mullin et al unpubl.). However, while the MS BSE stock is relatively large, the sound is a semiopen embayment, and prior observations of seasonal fluctuations in dolphin abundance have led to the hypothesis that members of the adjacent northern coastal stock may periodically enter the BSE waters, where they could potentially die and strand.…”

Section: Discussionmentioning

confidence: 99%

“…Photographic catalogs exist for some BSE populations (McDonald et al 2017) (but no coastal populations) in the study area, and these were searched in cases where adequate dorsal fin photographs existed for the stranded animals (many carcasses were too decomposed for accurate photo-identification). Where a match was made, the stranded animal was assigned a probability distribution of being coastal, p ij~b eta (1, 9999), equivalent to assuming that it is almost certain the animal is not coastal (allowing for a small mis-identification error).…”

Section: Photo-and Genetic-id Datamentioning

confidence: 99%

Where were they from? Modelling the source stock of dolphins stranded after the Deepwater Horizon oil spill using genetic and stable isotope data

Thomas¹,

Booth²,

Rosel³

et al. 2017

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Understanding the source stock of common bottlenose dolphins Tursiops truncatus that stranded in the northern Gulf of Mexico subsequent to the Deepwater Horizon oil spill was essential to accurately quantify injury and apportion individuals to the appropriate stock. The aim of this study, part of the Natural Resource Damage Assessment (NRDA), was to estimate the proportion of the 932 recorded strandings between May 2010 and June 2014 that came from coastal versus bay, sound and estuary (BSE) stocks. Four sources of relevant information were available on overlapping subsets totaling 336 (39%) of the strandings: genetic stock assignment, stable isotope ratios, photo-ID and individual genetic-ID. We developed a hierarchical Bayesian model for combining these sources that weighted each data source for each stranding according to a measure of estimated precision: the effective sample size (ESS). The photo-and genetic-ID data were limited and considered to potentially introduce biases, so these data sources were excluded from analyses used in the NRDA. Estimates were calculated separately in 3 regions: East (of the Mississippi outflow), West (of the Mississippi outflow through Vermilion Bay, Louisiana) and Western Louisiana (west of Vermilion Bay to the Texas−Louisiana border); the estimated proportions of coastal strandings were, respectively 0.215 (95% CI: 0.169−0.263), 0.016 (0.036−0.099) and 0.622 (0.487−0.803). This method represents a general approach for integrating multiple sources of information that have differing uncertainties.

“…After the Deepwater Horizon (DWH) oil spill began on 20 April 2010, determining contemporary survival and abundance estimates for common bottlenose dolphins in various BSEs, including Barataria Bay, in the northern GoM impacted by the spill became an important objective to determine injury for the Natural Resources Damage Assessment (N RDA). This motivated the development of a novel capture-recapture model (McDonald et al 2017, this Theme Section), referred to as a spatial robust-design model, to estimate survival, and abundance for Bara taria Bay using recent photo-identification (photo-ID) data.…”

Section: Introductionmentioning

confidence: 99%

Using salinity to identify common bottlenose dolphin habitat in Barataria Bay, Louisiana, USA

Hornsby¹,

McDonald²,

Balmer³

et al. 2017

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Following the Deepwater Horizon (DWH) oil spill, numerous studies were conducted to determine impacts on common bottlenose dolphins Tursiops truncatus. Common bottlenose dolphins are found in estuarine environments of the northern Gulf of Mexico which vary in salinity, depending on location (e.g. distance to freshwater source), season, and ocean tides. Although common bottlenose dolphins can be found in low-salinity waters (<15 ppt), they cannot tolerate very low salinity for long periods of time. We matched dolphin telemetry data in Barataria Bay, Louisiana (USA), with contemporaneous estimates of salinity to establish a salinity threshold and identify preferred dolphin habitat. Dolphins frequently used areas where salinity was higher thañ 11 ppt, sometimes used areas for short periods of time with predicted salinity of ~8 ppt, and avoided waters with salinities below ~5 ppt. While not a hard boundary, the ~8 ppt threshold can be used to delineate reasonable polygons of preferred dolphin habitat. We temporally averaged the location of the ~8 ppt isohaline from 2005 through 2012 to establish areas of preferred dolphin habitat. In Barataria Bay, the polygon of dolphin habitat encompasses 1167 km 2 , and extends from the bay's barrier islands to approximately half-way through marshes in northern parts of the bay. This polygon of suitable common bottlenose dolphin habitat was then ultimately used to quantify cetacean injury due to DWH oil.