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

Abstract

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.

Understanding the population consequences of disturbance

“…Schwacke et al. () used a respiratory metric of health to quantify the effect of the Deepwater Horizon oil spill on vital rates of bottlenose dolphins in the Gulf of Mexico. In the absence of a direct estimate of calf survival, Christiansen and Lusseau () used the fetal length of minke whales ( Balaenoptera acutorostrata ) as a proxy, and investigated how fetal length was associated with female body condition (Christiansen, Víkingsson, Rasmussen, & Lusseau, ).…”

“…However, classes may be subdivided to reflect the different vital rates of disturbed and undisturbed animals (King et al, 2015). Most PCoD applications have used a simple Leslie matrix to predict the trajectory of a population under different scenarios of anthropogenic disturbance (King et al, 2015;New et al, 2014;Schwacke et al, 2017). Matrix models historically assumed that vital rates are simply a function of an individual's age or stage, but integral projection models (IPMs) account for the additional effects of continuously varying traits (such as physical size) on vital rates (Ellner & Rees, 2006).…”

Health Assessments of Common Bottlenose Dolphins (Tursiops truncatus): Past, Present, and Potential Conservation Applications

“…The incorporation of health assessments into existing capturerelease protocols provided the collection of baseline health data across multiple populations, thus allowing for investigation of geographic variability in health parameters (32,42). The methodologies established and samples collected during these earlier projects formed the basis for developing a risk assessment framework to quantify the impacts for dolphins affected by anthropogenic threats (e.g., Deepwater Horizon oil spill) in geographical areas where baseline data were not available (36).…”

The Deepwater Horizon oil spill marine mammal injury assessment

“…These DWH oil exposure-related excess observed strandings (by stock) were then scaled to an estimated total number of excess mortalities using models to correct for carcass beaching and recovery efficiencies, including a carcass drift model (DWHM-MIQT 2015). Finally, Schwacke et al (2017) and DWHMMIQT (2015) used results from all of these analyses to estimate how DWH oil-related injuries combined to impact the trajectory of each stock's population.…”