Long-term trends in a northern Gulf of Mexico common bottlenose dolphin (Tursiops truncatus) population in the wake of the Deepwater Horizon oil spill

Abstract

Bottlenose dolphins along the northern Gulf of Mexico continue to be impacted by numerous stressors including harmful algal blooms, infectious
disease epizootics, and oil exposure following the Deepwater Horizon (DWH) oil spill. Studies to assess the potential impacts of the DWH oil spill
on dolphins in the northern Gulf of Mexico were initiated as part of a Natural Resource Damage Assessment and included photographic-identification
(photo ID) surveys to determine abundance, density, and site fidelity in St. Joseph Bay, Florida (SJB). Although significant oiling did not occur in
SJB, long-term data have been collected in this region to provide insight into population-level trends in abundance/density over time. Thus, SJB
dolphins could serve as a reference for comparison to other dolphin populations exposed to DWH oiling. During 2005–2007, the results of photo
ID and telemetry surveys determined seasonal fluctuations in abundance and identified two different dolphin populations in SJB: residents sighted
across multiple seasons and years (St. Joseph Bay Stock), and visitors that were present during the spring and fall and associated with a 2–3fold
increase in abundance (Northern Coastal Stock). The goals for the current study were to compare dolphin abundance, density, and site fidelity, prior
to (2005–2007), during (2010), and post-DWH (2011 and 2013) using photo-ID surveys and a spatially explicit robust-design capture-recapture
(SERDCR) model. The data collected during and post-DWH paralleled previous research in that a low number of individuals with high site fidelity
were sighted across seasons and years (St. Joseph Bay Stock), and abundance/density increased in the fall as a result of an influx of dolphins that
were likely members of the Northern Coastal Stock. However, June and August 2010 abundance (347; 193–498, 95% CI and 394; 288–534, 95%
CI, respectively), density (dolphins/km2) (2.60; 1.36–3.70, 95% CI and 2.55; 1.89–3.29, 95% CI, respectively), and site fidelity patterns were more
similar to previous years’ spring and fall data, with high abundance estimates, increased dolphin density in coastal waters, and elevated numbers of
individuals with low site fidelity in the SJB region. Factors that could have contributed to this increase in abundance include immigration of dolphins
from adjacent estuaries, population growth within the St. Joseph Bay Stock, impacts from DWH oiling, and environmental and/or prey-based cues
that influence movements of the Northern Coastal Stock. The results of this study demonstrate the importance of long-term monitoring to assess
impacts of current and future stressors on the dolphins in the northern Gulf of Mexico.