Deepwater Horizon Oil Spill: 2010-2011 Efforts to respond to threats to dolphins along the central west coast of Florida
Much concern surrounded the potential catastrophic impacts of the April 2010 Deepwater Horizon (DWH) oil spill on wildlife and habitats in the Gulf of Mexico.
Long-term Gulf resident “Bracket” and her most recent calf, seen here in September 2010.
The most common cetaceans in inshore waters of the Gulf, bottlenose dolphins, reside in coastal waters and bays, sounds, and estuaries where exposure to oil from the DWH incident was likely to occur. Prior to the DWH spill, little was known about the effects of oil spills on dolphins. There are a number of potential routes by which dolphins may be exposed to oil or associated chemicals such as inhalation, ingestion, and direct contact. Without the ability to predict the extent to which the spill would impact the Gulf coast and associated marine mammals, there was a strong need to collect baseline and control data for dolphin populations that might have been impacted directly or that might serve as comparative populations for those that are directly impacted, so that we can better understand the impacts of oil spills on cetaceans.
With the help of the Morris Animal Foundation’s Betty White Wildlife Rapid Response Fund, we initiated a project to address potential impacts on Gulf of Mexico bottlenose dolphins, specifically targeting stocks off the central west coast of Florida. The targeted stocks included the resident Sarasota Bay dolphin community, for which long-term health and population data were available, and the dolphins inhabiting the coastal waters of the Gulf of Mexico immediately offshore of Sarasota Bay, which likely would be exposed to oil before Sarasota Bay. Baseline data were collected on contaminant exposure, reproductive status, abundance, and distribution patterns of dolphins in these regions. We could not assign a probability to the oil spill spreading to Sarasota. If oil had arrived, then we would have collected exposure-response information that may be applicable for estimation of risks to other Gulf of Mexico cetacean populations. However, since the oil did not reach Sarasota Bay and the surrounding Gulf waters, the results of this study are being used by NOAA as control data for interpretation of data arising from potentially impacted populations elsewhere in the Gulf, such as Barataria Bay, Louisiana.
Sighting locations for “Bracket,” one of several potential long-term resident Gulf animals identified during this project.
Two well-tested approaches were used to obtain information on contaminant exposure, abundance, distribution, and residency patterns of bottlenose dolphins in Gulf of Mexico and Sarasota Bay waters: 1) biopsy sampling, and 2) photo-identification. A total of 61 tissue samples for contaminant exposure assessment were obtained from Gulf dolphins through remote biopsy sampling and from Sarasota Bay resident dolphins through capture-sample-release techniques. Currently, all biopsy samples are being analyzed by NOAA and other collaborators.
Photo-identification surveys were conducted in the Gulf study area during June/July 2010 and August/September 2010, and in Sarasota Bay surveys were ongoing, 10 days each month. Data obtained from the photo-identification surveys were utilized to estimate abundance, and identify distribution and residency patterns. There were no significant changes in overall dolphin abundance between survey periods, or as compared to normal, pre-spill patterns. However, dolphin distribution seemed to vary somewhat from one survey period to the next, with more individuals being sighted closer to the coast during June/July and a higher number of individuals offshore during August/September.
These results suggest that there may be differences in the distribution of Gulf dolphins through the year. Project findings also support the concept of long-term residency for Gulf dolphins in addition to those in Sarasota Bay. Over the past three decades, about 900 individuals have been identified at one time or another in the study area for this project. Of these, 218 were identified from our 2010 photographs, including four individuals first identified as far back as 1980, when the Sarasota Dolphin Research Program initiated systematic photographic identification surveys.
It will be important to monitor the dolphins in oil-impacted bays and elsewhere in the Gulf of Mexico over time (years) to see if health or reproductive problems develop as a result of their exposure to oil and dispersants, either from direct contact or from transfer through the ecosystem. To facilitate accurate interpretation of subsequent data from the oil-impacted areas, it is necessary to have control data from sites that were in the same general region but were spared from the original spill.
The samples collected from Sarasota Bay and associated Gulf of Mexico waters in 2010 are serving as important controls for such comparisons.
