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

L Thomas,J Litz,A Hohn,Pe Rosel,Cg Booth,Lh Schwacke

doi:10.3354/esr00754

Abstract

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 33:253-264 (2017) - DOI: https://doi.org/10.3354/esr00754 Where were they from? Modelling the source stock of dolphins stranded after the Deepwater Horizon oil spill using genetic and stable isotope data L. Thomas1,*, C. G. Booth2, P. E. Rosel3, A. Hohn4, J. Litz5, L. H. Schwacke6 1Centre for Research into Ecological and Environmental Modelling (CREEM), University of St Andrews, The Observatory, Buchanan Gardens, St Andrews KY16 9LZ, UK 2SMRU Consulting Europe, New Technology Centre, North Haugh, St Andrews KY16 9SR, UK 3National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, 646 Cajundome Blvd., Lafayette, LA 70506, USA 4National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, 101 Pivers Island Rd, Beaufort, NC 28516, USA 5National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, FL 33149, USA 6National Oceanic and Atmospheric Administration, National Centers for Coastal Ocean Science, 331 Fort Johnson Road, Charleston, SC 29412, USA *Corresponding author: len.thomas@st-andrews.ac.uk ABSTRACT: 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. KEY WORDS: Genetic assignment · Stable isotope ratios · Bayesian hierarchical model · Data integration · Deepwater Horizon · Stranding Full text in pdf format Supplementary material PreviousNextCite this article as: Thomas L, Booth CG, Rosel PE, Hohn A, Litz J, Schwacke LH (2017) Where were they from? Modelling the source stock of dolphins stranded after the Deepwater Horizon oil spill using genetic and stable isotope data. Endang Species Res 33:253-264. https://doi.org/10.3354/esr00754 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 33. Online publication date: January 31, 2017 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2017 Inter-Research.

Highlights

The explosion on the Deepwater Horizon (DWH) drilling platform on 20 April 2010 resulted in the release of millions of barrels of oil into the northernGulf of Mexico
Large numbers of bottlenose dolphins were found stranded after the spill within the spill footprint, and a significant weight of evidence indicated that a large portion of these strandings were caused by exposure to petroleum-associated chemicals from the DWH spill (Venn-Watson et al 2015a,b, DHNRDAT 2016)
The model allows us to have a different distribution of the probability of being coastal for each animal, and to make inferences about dolphins that do not have additional information by sampling at random from those that do, i.e. we assume that animals are ‘exchangeable’

Summary

Introduction

The explosion on the Deepwater Horizon (DWH) drilling platform on 20 April 2010 resulted in the release of millions of barrels of oil into the northernGulf of Mexico (nGoM). Part of the assessment involved using the strandings to scale the number of mortalities by stock, for those stocks for which more direct estimates of mortality from mark−recapture studies were not available (DHNRDAT 2016, McDonald et al 2017, this Theme Section). This requires accounting for the proportion of recorded strandings from each stock, the probability that an animal from each stock that died would strand at a given location (which differs between stocks) and the probability that a stranding at that location would be discovered and reported to the stranding network. We focus only on the first question: What proportion of the recorded strandings came from each stock? The remaining questions are addressed, and an overview of the NRDA for bottlenose dolphins is given, in DHNRDAT (2016)

Methods

Results

Conclusion