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The Deepwater Horizon Oil Spill and Marine Mammals

The Marine Mammal Commission has been monitoring closely all aspects of the Deepwater Horizon oil spill, including response, assessment, and restoration efforts. The Commissioners, staff, and scientific advisors are in frequent contact with other federal and state agencies and non-governmental stakeholders with expertise on marine mammals and marine ecosystems, and the potential effects of oil spills on marine mammals. Although oil from the Deepwater Horizon spill continues to be seen in certain areas of the Gulf, federal and state resource agencies have shifted their focus to injury assessment and restoration planning. Determining the short-term and long-term effects of the spill on the ecosystem, including marine mammals, is a primary goal of assessment efforts. This information will be used to determine liabilities for losses to natural resources and ecological services as a result of the oil spill and to guide restoration efforts to ensure that resources are returned to baseline conditions. Baseline conditions are those that would exist had the spill not occurred.

The Commission has actively supported response and assessment efforts, provided information to help understand the potential effects of the spill, and developed recommendations for long-term assessment of effects. To monitor the distribution of marine mammals in the oil spill area, the Commission funded the initial deployment of passive, high-frequency acoustic recording equipment by researchers from Scripps Institution of Oceanography working in collaboration with the National Marine Fisheries Service’s Southeast Fisheries Science Center. In June 2010 the Marine Mammal Commission testified before Congress regarding the Deepwater Horizon oil spill and its effects on marine mammals. The Commission’s testimony summarized potential short-term and long-term effects, how these effects will be assessed, and the likely impact of oil and gas activities on marine mammals in the Gulf and elsewhere. In August 2011 the Commission submitted to Congress a statement of research needs to assess the long-term effects of the spill on marine mammals. The Commission will continue to work with federal and state resource agencies and non-governmental stakeholders to review response and assessment efforts. The Commission also will continue to track and facilitate efforts to recover and restore the Gulf ecosystem.

Marine Mammals at Risk in the Gulf of Mexico

Stock assessment reports compiled by the National Marine Fisheries Service identify 21 marine mammal species under its jurisdiction that occur in the Gulf of Mexico, comprising 57 stocks, 37 of which are bottlenose dolphin stocks. In addition, the Fish and Wildlife Service is responsible for one species of marine mammal that occurs in the Gulf of Mexico, the Florida manatee.

Marine mammal species and stocks in the Gulf of Mexico

Sperm whale*
Bryde's whale
Killer whale
Cuvier's beaked whale
Atlantic spotted dolphin
False killer whale
Blainville’s beaked whale
Pantropical spotted dolphin
Pygmy killer whale
Gervais’ beaked whale
Striped dolphin
Dwarf sperm whale
Bottlenose dolphin (oceanic)
Spinner dolphin
Pygmy sperm whale
Bottlenose dolphin (continental shelf)
Rough-toothed dolphin
Melon-headed whale
Bottlenose dolphin (coastal – 3 stocks)
Clymene dolphin
Risso's dolphin
Bottlenose dolphin (bay, sound, estuary – 32 stocks)
Fraser’s dolphin
Pilot whale, short-finned
Manatee, West Indian (Florida subspecies)*
* listed as endangered under the Endangered Species Act

Baseline information for these species and stocks is limited. For example, abundance estimates for only 5 of the 57 Gulf of Mexico stocks (or stock groups) listed in the National Marine Fisheries Service's 2010 stock assessment reports meet the Service's own standards for acceptable precision (i.e., a coefficient of variation equal or less than 0.3 for the best population estimate). The lack of information will make it difficult to determine what changes have occurred as a result of the spill, if any, in population size, distribution, habitat use, and other aspects of marine mammal demography and ecology.

Potential Effects of Oil Spills on Marine Mammals

All marine mammal stocks in the Gulf may have been, or may still be, affected by the Deepwater Horizon spill. All effects are initially manifested at the individual level, and must lower the individual’s probability of survival or reproduction to affect the population. The effects may be direct (e.g., contact with oil or dispersants, interactions with response activities) or indirect (e.g., degradation of habitat, reduced availability of prey).

Potential physiological or behavioral effects of concern include:

  • injury, lesions, disease, or death through external or internal contact
  • physiological dysfunction of the immune, reproductive, or other vital system
  • injury from response activities, such as vessel strikes, interactions with booms or other response equipment, or noise introduced into the marine environment
  • disturbance or disruption of significant biological behaviors, including foraging, reproduction, or resting
  • displacement from primary habitat
  • disruption of social organization
  • reduced prey or seagrass availability through prey displacement or reduction in biomass of prey or seagrass
  • ecosystem changes harmful to marine mammals via hypoxia or anoxia or harmful algal blooms
  • reduction in status involving individual fitness, population vital rates (reproduction and survival rates), and/or population abundance and trends

Observed Impacts from Previous Spills and Studies

The available information on the effects of oil on marine mammals is limited. Current understanding is based primarily on information from the Exxon Valdez oil spill, other oil spills, a small number of controlled exposure studies, simulations and in vitro studies, and observed effects of exposure on non-marine mammal species. Available information does not allow for a prediction of the severity of either short- or long-term effects of the Deepwater Horizon spill on marine mammals. However, available information does provide ample evidence that exposure to oil can harm marine mammals. For example:

  • inhalation of specific volatile organics from some types of oil can cause respiratory irritation, inflammation, or emphysema
  • ingestion of oil may cause gastrointestinal inflammation, ulcers, bleeding, diarrhea, or maldigestion
  • certain inhaled and ingested chemicals in oil also may damage organs such as the liver, kidney, adrenal glands, spleen or brain; cause anemia, cancer, congenital defects, and immune system suppression; or lead to reproductive failure
  • chemical contact may cause skin and eye irritation; inflammation; burns to mucous membranes, mouth and nares; or increased susceptibility to infection
  • oil mixtures can physically foul the baleen of mysticete whales, which is used for filtering food
  • increased vessel and air traffic may disrupt foraging, habitat use, daily or migratory movements, and behavior; increased vessel traffic also increases the risk of vessel strikes
  • noise from seismic surveys or other response-related activities may cause disturbance or displacement, hearing loss (temporary or possibly permanent), or other physical injury to marine mammals
  • booms and skimmers to contain and collect surface oil, and in-situ burning to remove it, also may affect marine mammals both through direct interaction and displacement from habitat; residue from burns puts additional chemicals into the air, posing inhalation risks
  • oil spills may alter the marine environment through reductions in biomass of marine mammal prey or seagrass (forage for manatees), shifts in prey or seagrass distribution, or contamination of prey or seagrass

Deepwater Horizon Oil Spill

The Deepwater Horizon oil spill presented a number of unique challenges, as compared to other spills in U.S. and international waters:

  • Amount: The amount of oil that escaped from the Deepwater Horizon wellhead was unprecedented, amounting to approximately 4.9 million barrels (205.8 million gallons) over the course of 86 days. The estimated flow rate at its peak was between 35,000 to 60,000 barrels (1.47 to 2.52 million gallons) a day, which was roughly equal to the amount spilled by the Exxon Valdez tanker every week. The spill required a massive response effort involving 13 federal agencies, 5 states, residents of local communities, volunteers, contractors, expert consultants, non-governmental organizations, and industry.
  • Movement: Modeling of oil spill trajectories is normally done based on the movement of surface currents, winds, tides, and factors that affect the ocean surface. However, predicting how the oil from this spill would spread was a challenge for modelers as the source of the release was deep underwater (at the wellhead), large amounts of oil were found to be remaining in the water column, and little information was available regarding how oil travels and weathers at depth.
  • Type: The oil released from the Deepwater Horizon wellhead was a mixture of both Louisiana sweet crude oil and other oil products (methane, ethane, and propane). The actual composition of the oil is a critical determinant of its potential effect on the Gulf ecosystem. Among other things, the composition of oil determines its toxicity, physical characteristics, and the rate that it weathers.
  • Dispersants: Up to 15,000 gallons of chemical dispersants were applied daily at the wellhead at the height of the spill, and dispersants also were applied on the surface of the water in the earlier stages of the spill. More than 1.8 million gallons of dispersants were applied over the course of the spill, both at the surface and subsurface. Responders use dispersants to reduce the surface tension of the oil, which means that it is more easily fragmented into smaller particles. Smaller particles have a larger surface-to-volume ratio and are therefore more amenable to degradation by microbes (e.g., bacteria) in the ocean. Fragmented oil also weathers more quickly and is less likely to form large slicks that cover and contaminate shorelines.
  • Clean-up and containment: In addition to the direct effects of oil, clean-up and containment operations also may have affected marine mammals. Clean-up operations included a range of techniques such as containment of oil in booms, skimming of oil at the ocean surface, and in-situ burning. Clean-up operations also involved a large number of vessels and aircraft in coastal and pelagic habitats. The final stages of containment of the well involved seismic surveys of the area around the wellhead to detect leaks from other parts of the well. Clean-up and containment activities had the potential to disturb marine mammals, possibly displacing them from important feeding or reproductive grounds or other important habitat.
  • Baseline information: As noted previously, limited baseline information on the status and health of marine mammals in the Gulf of Mexico may significantly confound before-and-after comparisons needed to determine the full effects of the spill.

Response and Assessment Activities

The government's response to affected wildlife (e.g., mammals, sea turtles, and seabirds) followed procedures established for this spill by the Wildlife Branch of the Deepwater Horizon Unified Command. Under this structure, the National Oceanic and Atmospheric Administration (NOAA) and the Fish and Wildlife Service worked with the Oiled Wildlife Care Network to coordinate the Gulf marine mammal stranding network, revise NOAA’s marine mammal response guidelines to address Gulf species, train stranding responders regarding hazardous materials and chain-of-custody protocols, and distribute sampling supplies.

Surveillance and various operational crews and the public were able to report injured and stranded wildlife to the Wildlife Branch via the Wildlife Hotline and other means. The Wildlife Branch provided daily summary reports of stranded wildlife in the area affected by the oil spill on the Restore the Gulf web site, and the locations of stranded animals were posted on the NOAA GeoPlatform web site. More detailed information on marine mammal and sea turtle strandings, as well as general information on response activities and effects of oil on marine mammals and sea turtles, were posted on NOAA's Office of Protected Resources Gulf of Mexico oil spill web site.

The Oil Pollution Act of 1990 was passed after the Exxon Valdez oil spill and addresses liability for losses to natural resources and services caused by the discharge or substantial threat of discharge of oil. Designated trustees (federal, tribal, and state agencies) are charged with conducting a Natural Resource Damage Assessment after an oil spill or other discharge event. The assessment consists of collecting and analyzing information to evaluate the nature and extent of injuries resulting from the incident. Trustees then determine the restoration actions needed to bring injured natural resources and services back to baseline conditions and make the environment and public whole for interim losses. BP and the other responsible parties are then expected to pay the costs of natural resource damages (including the costs of assessing such damage) and compensate the public for lost services derived from those natural resources.

To assess damages and plan restoration, the Trustees must compare the best available baseline information on conditions before the spill against information collected during and after the spill. Assessment efforts for marine mammals have involved and will continue to involve a variety of surveillance and monitoring activities in the Gulf of Mexico, such as:

  • photo-identification and biopsy sampling of bottlenose dolphin populations at selected estuarine sites (Barataria Bay, Louisiana; Chandeleur Sound, Louisiana; Mississippi Sound, Mississippi; and St. Joseph Bay, Florida)
  • large-vessel pelagic research cruises to—
    • visually assess and photo-document marine mammal contact with oil and occurrence of marine mammals in oiled areas
    • deploy satellite tags and collect biopsy samples from Bryde’s whales, sperm whales, and other marine mammals in offshore waters
    • collect habitat information including surface hydrographic data, temperature profiles, salinity, dissolved oxygen, and acoustic echo-sounder backscatter information to characterize water column productivity and prey resources, and
    • deploy low and mid-frequency passive acoustic monitoring buoys
  • aerial surveys to estimate abundance and assess distribution of marine mammals in oil-affected areas, document locations of marine mammals in distress, and describe their physical and/or behavioral reactions if and when they came into contact with oil
  • live capture-release health assessment studies of bottlenose dolphins in Barataria Bay, Louisiana, and Sarasota Bay, Florida, to determine sub-lethal and chronic health impacts
  • genetic analyses of biopsy and stranding samples for species identification, sex determination, and/or stock structure
  • manatee tracking data analysis, and
  • prey and seagrass sampling

In October 2010 the Trustees confirmed damage and injury to natural resources as a result of the spill and issued a notice of intent to begin planning restoration activities. Planning and implementation of restoration activities likely will take several years and require integration and analysis of multiple types of information. These include measures and comparisons of the ecological, biological, geophysical, chemical, and oceanographic conditions in the Gulf, both pre- and post-spill, and/or modeling of conditions where pre- and/or post-spill information is not available.

Restoration activities that may benefit marine mammals include not only clean-up of the spilled oil, but also (1) basic assessment of the marine mammal stocks in the Gulf, and (2) reduction of other human-related risk factors in the Gulf, such as noise from seismic surveys, vessel traffic, sonar and military activities; fishery interactions; disturbance from tourism and illegal feeding; harmful algal blooms and anoxic zones.

Determining the respective roles of human-related risk factors and their interactions with the spill is a substantial but important challenge. In 2010, prior to the spill, unusually high numbers of bottlenose dolphins began to strand in the northern Gulf. Strandings continued to occur in high numbers throughout the spill and through spring and summer 2011. NOAA has declared these deaths to be an Unusual Mortality Event (in accordance with the Marine Mammal Protection Act), and is coordinating the investigation of these mortalities (pre- and post-oil spill) with ongoing Natural Resource Damage Assessment activities where the data needs of these two processes coincide.

Priorities for assessing the spillís long-term effects on Gulf marine mammals

Responding to stranded animals and assessment of immediate effects of oil are of the utmost priority during or immediately after a spill. However, the likelihood of detecting certain impacts decreases with time and the utility and value of certain types of research declines accordingly. Therefore, the Commission gives high priority at this time to assessment of long-term effects, including:

  • assessing the health status of stranded or live-captured animals
  • assessing oil spill-related changes in the ecosystem leading to a potential reduction in prey availability
  • evaluating other ecosystem changes that are harmful to marine mammals and that may have been exacerbated by the spill (e.g., harmful algal blooms, hypoxia or anoxia), and
  • determining the extent to which exposure to oil and/or response activities leads to a reduction in status involving individual fitness, population vital rates (survival and reproduction), and population abundance and trends.

Implementation of the needed research will require resources beyond those currently available, as well as improved infrastructure (e.g., research vessels, aircraft, and laboratories), more trained personnel, better sampling methods, and refined analytical tools to detect and assess the effects of exposure to oil. Coordination of public and private research activities is critical to focus on the most important topics, achieve collaboration to the greatest degree possible, develop a weight-of-evidence approach for detecting effects, and avoid unnecessary duplication of effort. Collaboration and partnerships among the involved federal, state, and local agencies, industry, non-governmental organizations, research institutions and organizations, and the public also should help maximize the benefits of limited resources and minimize the effects of research activities on marine mammals.

Future research strategies and capacity

The extent to which we can learn more about the spill’s effects on marine mammals, as well as the effects of other human-related factors, will depend largely on our ability to improve research strategies and capacity in the Gulf. The Deepwater Horizon oil spill provided a sobering indicator of the shortcomings of our current research and management approach for marine mammals in the Gulf of Mexico. Those shortcomings can be grouped under five key topics, as follows:

  • Stock assessment: Stock assessment information is necessary to provide the baseline against which changes in the status of a stock can be measured, and includes information on stock structure, movement patterns, abundance, and trends. Improving the capacity of NOAA and the Fish and Wildlife Service to conduct thorough stock assessments should be a high priority.
  • Stranding program: Stranded marine mammals are a major source of information used to manage marine mammal stocks in U.S. waters. In the Gulf, stranding networks played a key role during the spill by monitoring coastal areas for stranded animals, collecting tissues for various types of analyses, and caring for live-stranded animals and moving them to facilities that could provide the necessary care. The Gulf’s stranding networks must be further developed and supported to assist with the tracking of the long-term effects of the oil spill.
  • Health assessments: The health of individual animals can be an important indicator of adverse effects from natural or anthropogenic risk factors in the ocean or coastal environment, including exposure to oil, dispersants, and response activities.
  • Environmental studies: The northern Gulf is a dynamic and heavily industrialized area. Large-scale changes in community structure or prey abundance caused by the oil spill or other human-caused or natural disturbances can affect the carrying capacity or distribution of marine mammal populations. Research also is needed to better understand marine mammal/prey dynamics. Evaluating the adverse effects of these many factors, managing the activities that cause them, and mitigating their impacts on the Gulf ecosystem, including marine mammals and their prey, will require a major commitment.
  • Cumulative effects: The status of the Gulf’s marine mammal populations will vary not as a function of any single risk factor, but rather as a function of all of them, including the interactions between those factors. At the least, marine mammal research and management should be capable of identifying the marine mammal stocks in the Gulf and assessing their relative abundances and trends to determine if, when, and where they are being exposed to cumulative effects that hinder their potential to grow and recover.

Web Sites for Further Information

U.S. Government Web site on Gulf of Mexico oil spill response and restoration activities:

NOAA Office of Response and Restoration Web site on Deepwater Horizon Oil Spill Response:

NOAA Damage Assessment, Remediation, and Restoration Program’s Gulf Spill Restoration web site (including Natural Resource Damage Assessment work plans):

National Marine Fisheries Service Office of Protected Resources Gulf of Mexico oil spill web site:

U.S. Fish and Wildlife Service Web site on Deepwater Horizon Oil Spill Response:

Environmental Protection Agency Response to the BP Oil Spill in the Gulf of Mexico web site:

Oiled Wildlife Care Network Blog (includes archived postings regarding Deepwater Horizon response activities):

Bureau of Ocean Energy Management, Regulation and Enforcement BP/Deepwater Horizon oil spill library and reading room:

National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling web site and report:

U.S. Coast Guard/Bureau of Ocean Energy Management, Regulation and Enforcement Joint Investigation Team web site and report:

Gulf of Mexico Research Initiative (BP funded research on effects of the Deepwater Horizon oil spill and related topics):

Gulf of Mexico Sea Grant Programs Deepwater Horizon Oil Spill Research and Monitoring Activities Database:

National Marine Fisheries Service Permits web site (whales and dolphins):

Fish and Wildlife Service Permits web site (manatees):




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