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. 2022 Aug;36(4):e13878.
doi: 10.1111/cobi.13878. Epub 2022 Feb 16.

Modeling population effects of the Deepwater Horizon oil spill on a long-lived species

Lori H Schwacke  1 , Tiago A Marques  2 , Len Thomas  2 , Cormac G Booth  3 , Brian C Balmer  1 , Ashley Barratclough  4 , Kathleen Colegrove  5 , Sylvain De Guise  6 , Lance P Garrison  7 , Forrest M Gomez  4 , Jeanine S Morey  1 , Keith D Mullin  8 , Brian M Quigley  1 , Patricia E Rosel  9 , Teresa K Rowles  10 , Ryan Takeshita  1 , Forrest I Townsend  11 , Todd R Speakman  1 , Randall S Wells  12 , Eric S Zolman  1 , Cynthia R Smith  4
Affiliations

Modeling population effects of the Deepwater Horizon oil spill on a long-lived species

Lori H Schwacke et al. Conserv Biol. 2022 Aug.

Abstract
in English, Spanish

The 2010 Deepwater Horizon (DWH) oil spill exposed common bottlenose dolphins (Tursiops truncatus) in Barataria Bay, Louisiana to heavy oiling that caused increased mortality and chronic disease and impaired reproduction in surviving dolphins. We conducted photographic surveys and veterinary assessments in the decade following the spill. We assigned a prognostic score (good, fair, guarded, poor, or grave) for each dolphin to provide a single integrated indicator of overall health, and we examined temporal trends in prognostic scores. We used expert elicitation to quantify the implications of trends for the proportion of the dolphins that would recover within their lifetime. We integrated expert elicitation, along with other new information, in a population dynamics model to predict the effects of observed health trends on demography. We compared the resulting population trajectory with that predicted under baseline (no spill) conditions. Disease conditions persisted and have recently worsened in dolphins that were presumably exposed to DWH oil: 78% of those assessed in 2018 had a guarded, poor, or grave prognosis. Dolphins born after the spill were in better health. We estimated that the population declined by 45% (95% CI 14-74) relative to baseline and will take 35 years (95% CI 18-67) to recover to 95% of baseline numbers. The sum of annual differences between baseline and injured population sizes (i.e., the lost cetacean years) was 30,993 (95% CI 6607-94,148). The population is currently at a minimum point in its recovery trajectory and is vulnerable to emerging threats, including planned ecosystem restoration efforts that are likely to be detrimental to the dolphins' survival. Our modeling framework demonstrates an approach for integrating different sources and types of data, highlights the utility of expert elicitation for indeterminable input parameters, and emphasizes the importance of considering and monitoring long-term health of long-lived species subject to environmental disasters. Article impact statement: Oil spills can have long-term consequences for the health of long-lived species; thus, effective restoration and monitoring are needed.

El derrame de petróleo Deepwater Horizon (DWH) en 2010 expuso gravemente a este hidrocarburo a los delfines (Tursiops truncatus) de la Bahía Barataria, Luisiana, causando un incremento en la mortalidad y en las enfermedades crónicas, y deteriorando la reproducción de los delfines sobrevivientes. Realizamos censos fotográficos y evaluaciones veterinarias durante la década posterior al derrame. Asignamos un puntaje pronóstico (bueno, favorable, moderado, malo, o grave) a cada delfín para proporcionar un indicador integrado único de la salud en general. También examinamos las tendencias temporales de estos puntajes. Usamos información de expertos para cuantificar las implicaciones de las tendencias para la proporción de delfines que se recuperaría dentro de su periodo de vida. Integramos esta información, junto con información nueva, a un modelo de dinámica poblacional para predecir los efectos sobre la demografía de las tendencias observadas en la salud. Comparamos la trayectoria poblacional resultante con aquella pronosticada bajo condiciones de línea base (sin derrame). Las condiciones de enfermedad persistieron y recientemente han empeorado en los delfines que supuestamente estuvieron expuestos al petróleo de DWH: 78% de aquellos evaluados en 2018 tuvieron un pronóstico moderado, malo o grave. Los delfines que nacieron después del derrame contaron con mejor salud. Estimamos que la población declinó en un 45% (95% CI 14-74) relativo a la línea base y tardará 35 años (95% CI 18-67) en recuperar el 95% de los números de línea base. La suma de las diferencias anuales entre el tamaño poblacional de línea base y el dañado (es decir, los años cetáceos perdidos) fue de 30,993 (95% CI 6,607-94,148). La población actualmente está en un punto mínimo de su trayectoria de recuperación y es vulnerable a las amenazas emergentes, incluyendo los esfuerzos de restauración ambiental planeada que probablemente sean nocivos para la supervivencia de los delfines. Nuestro marco de modelado demuestra una estrategia para la integración de diferentes fuentes y tipos de datos, resalta la utilidad de la información de expertos para los parámetros de aportación indeterminable, y enfatiza la importancia de la consideración y el monitoreo de la salud a largo plazo de las especies longevas sujetas a los desastres ambientales. Modelado de los Efectos Poblacionales del Derrame de Petróleo Deepwater Horizon sobre Especies Longevas.

Keywords: análisis de salud; delfín; derrame de petróleo; dolphin; especie longeva; expert elicitation; health assessment; información de expertos; mamífero marino; marine mammal; modelo poblacional; oil spill; population model; slow-living species.

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Figures

FIGURE 1
FIGURE 1
Overview and information flow for population model of bottlenose dolphins in Barataria Bay under baseline and injured scenarios
FIGURE 2
FIGURE 2
Probability of guarded, poor, or grave prognoses across years for cohorts of bottlenose dolphins born prior to the Deepwater Horizon oil spill in 2010 and prevalence for the cohort of dolphins born after 2010 (post, all years combined) in (a) Barataria Bay and (b) Sarasota Bay (fitted curves, predicted values; dashed‐line curves, 95% CIs from generalized additive model with separate smoothing splines for sampling year [Barataria Bay p = 0.02, Sarasota Bay p = 0.09]; vertical dashed lines, postspill cohorts’ 95% binomial confidence intervals)
FIGURE 3
FIGURE 3
Probability distributions of the (a) consensus of the expert elicitation for density‐dependence shape parameter, ρ, for a bay, sound, and estuary population of bottlenose dolphins compared with ρ values from Schwacke et al. (2017) and (b) corresponding mean function of fecundity versus population size for the Barataria Bay dolphin population obtained by sampling values of ρ from the distributions in (a), for which N nominal = 3045 (dotted vertical line), F nominal = 0.24, and F max = 0.34 (dotted horizontal lines)
FIGURE 4
FIGURE 4
Elicited distribution for the probability of recovery over a lifetime for Barataria Bay dolphins exposed to Deepwater Horizon oil
FIGURE 5
FIGURE 5
Median predicted size for the Barataria Bay dolphin population under baseline scenario (blue line) and Deepwater Horizon injury scenario (red line) from 10,000 simulations (shading, 95% CIs; dashed vertical line, years to recovery; dashed arrow, maximum proportional decrease; black dots, population abundance estimates from spatial capture‐recapture (Glennie et al., 2021); solid vertical lines, 95% CIs). The year 0 estimate was used to initialize the population simulations, but the year 9 estimate was not used directly in the population model
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