Oil in the Marine Environment

Deepwater Horizon Oil Spill and Marine Mammals

The Deepwater Horizon Oil Spill and
Marine Mammals

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 58 stocks, 38 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.

Cetacean species and stocks in the Gulf of Mexico

Sperm whale (listed as endangered under the Endangered Species Act)
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 – 33 stocks)
Fraser’s dolphin
Pilot whale, short-finned

Baseline information for these species and stocks, as well as for other marine mammal species and stocks, is limited. For example, abundance estimates for only 3 of the 58 Gulf of Mexico stocks (or stock groups) listed in the National Marine Fisheries Service's 2009 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, 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

Current information regarding oil spill effects on marine mammals is limited. Marine mammals may have been and may continue to be affected by the oil itself or by activities during the response and recovery phases (e.g., vessel traffic, noise, use of dispersants, seismic surveys around the wellhead, and clean-up activities).

Potential behavioral responses of concern include such things as -

  • displacement of animals from prime habitat
  • disruption of social structure (e.g., pods, mother-calf pairs)
  • changing prey availability and foraging distribution and/or patterns
  • changing reproductive behavior/productivity
  • changing movement patterns or migration

Potential physical/physiological effects of concern include such things as -

  • irritation, inflammation, or necrosis of skin
  • chemical burns of skin, eyes, nares, mucous membranes
  • inhalation of toxic fumes with potential short- and long-term respiratory effects (e.g., inflammation, pulmonary emphysema, infection)
  • ingestion of oil (and dispersants) directly or via contaminated prey (or contaminated vegetation, in the case of manatees), leading to inflammation, ulcers, bleeding, possible damage to liver, kidney, and brain tissues
  • stress from presence of vessels, aircraft, noise, handling (animals captured)
  • complications of the above may lead to dysfunction of immune and reproductive systems, physiological stress, declining physical condition, and death

Observations of Impacts from Previous Spills and Studies

General observations

  • The limited information available on the effects of oil exposure on marine mammals from the Exxon Valdez oil spill, other oil spills, and a limited number of controlled studies suggests that some marine mammal species may be more vulnerable to exposure to oil than others.
  • In addition, the effect of oil on marine mammals depends heavily on the nature of the oil and the type and duration of exposure.
  • Cetaceans (whales and dolphins) may be able to detect oil but do not always avoid it.
  • The skin of at least some cetaceans appears to be relatively resistant to effects from short-term exposure (hours). The effects of longer exposures are unknown.
  • Pulmonary emphysema was a relatively common finding in sea otters exposed to toxic fumes after the Exxon Valdez spill.
  • Brain lesions were observed in harbor seals examined after the Exxon Valdez spill.
  • Determining cause of death for marine mammals, particularly for cetaceans, during an oil spill can be difficult. For example, not all animals found dead necessarily died from exposure to oil. Gray whales found after the Santa Barbara spill were initially thought to have died from the spill, but that conclusion was reversed after examination of the whales. Similarly, the large number of dead, stranded gray whales observed after the Exxon Valdez spill could not be linked to the spill, and the increased observations of strandings have been attributed, at least in part, to the increased search effort associated with the spill.
  • Alternatively, not all animals that are exposed and become ill or die are likely to be detected and documented. Two of the killer whale pods occurring in Prince William Sound prior to the Exxon Valdez spill declined by 33 and 40 percent after the spill. One of those pods has not reproduced successfully since then and is expected to become extinct as the last individuals in that pod age and die. The other pod has not fully recovered but has not continued to decline. Although the cause of death of the whales that disappeared could not be confirmed, the close association of their loss with the spill suggests that the spill was the primary factor.
  • Assessments of spill effects must include adjustments to account for potential sources of bias.
  • Scientists know very little about the possible effects of oil on manatees. They may be particularly vulnerable to ingestion of oil if oil adheres to or otherwise contaminates the shallow-water plants that they depend on for food.
  • Scientists know very little about the effects of dispersants on marine mammals.
  • Baseline data (i.e., data characterizing the status—defined broadly to include abundance, composition, health and condition of individuals, etc.—of potentially affected populations before a spill occurs or before they encounter oil) are critical for assessing impacts.

Deepwater Horizon Oil Spill

The Deepwater Horizon oil spill presents 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 being released from the Deepwater Horizon wellhead is 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 include 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. Led by the National Marine Fisheries Service in coordination with the Fish and Wildlife Service, the Wildlife Branch developed protocols for stranding response and sample collection, as well as protocols for ground-based and vessel-based clean-up operations to minimize harm to wildlife.

Surveillance and various operational crews and the public were able to report injured and stranded wildlife to the Wildlife Branch via the Wildlife Hotline (1-866-557-1401) and other means. The Wildlife Branch continues to provide 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 are 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 is 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 assessing oil spill-related losses to natural resources and services and restoring those losses to baseline conditions (i.e., conditions that would exist had the spill not occurred). The cost of assessment and restoration is provided either by the party responsible for the losses or, if a responsible party does not exist or is unable or unwilling to provide compensation, by the Oil Spill Liability Trust Fund established under this Act.
Assessment efforts for marine mammals have involved and will continue to involve a variety of surveillance and monitoring activities in the Gulf of Mexico.

  • The National Marine Fisheries Service, the U.S. Fish and Wildlife Service, and the state of Florida conducted helicopter surveys in coastal and deepwater habitats affected by the spill. This information was used to direct response efforts and will be used to assess marine mammal movements and behavioral responses. Agencies also conducted fixed-wing aircraft surveys during the spill period and will continue to fly these surveys periodically for longer-term assessment purposes.
  • The National Marine Fisheries Service conducted marine mammal vessel surveys along the continental shelf and slope, targeting sperm whales (an endangered species) and Bryde’s whales. Researchers tagged whales to track their movements and diving patterns, took biopsy samples for various purposes including assessment of contaminant loads, and deployed stationary acoustic recorders as a complementary means of detecting whales, particularly those that are difficult to detect at the surface. The National Marine Fisheries Service plans to continue conducting periodic vessel surveys for longer-term assessment of marine mammal status and movements, as well as effects on prey species.
  • In coastal waters, researchers from the National Marine Fisheries Service and the National Ocean Service have been tracking bottlenose dolphins using photo-identification techniques and collecting biopsy samples to determine sex, stock identification, and contaminant loads. The agencies plan to continue these assessments over time, with additional emphasis on monitoring population-level health effects.
  • Researchers from the U.S. Geological Survey have been tracking daily movements of several manatees tagged before the spill to monitor movements and behavior, as well as possible contact with contaminated water and vegetation.

Under the Natural Resource Damage Assessment (NRDA) requirements of the Oil Pollution Act of 1990, federal and state agency and tribal trustees must determine what would be required to restore injured resources and services to their pre-spill conditions and to compensate the public for interim losses. The trustees must develop a restoration plan, seek compensation from the responsible party to implement the plan, and monitor restoration activities to ensure restoration has occurred.

The injury assessment and restoration planning phase of NRDA was officially started on 29 September 2010. During this phase, trustees assess the nature and amount of injuries to the environment and develop a restoration plan. This will be followed by a restoration phase during which the trustees will work with the public to implement and monitor the effectiveness of restoration projects.

Research Coordination

The Marine Mammal Protection Act requires permits or other authorizations for all research, assessment, and enhancement activities that may take marine mammals. This includes scientific research, the import or export of marine mammal parts, photography, rehabilitation, public display, capture from the wild, or other activities that may intentionally or incidentally affect marine mammals. Permits are issued by the National Marine Fisheries Service for whales and dolphins and the Fish and Wildlife Service for manatees, pinnipeds, and polar bears.

Many researchers are involved in research to help assess the spill’s impact on marine mammals, and the Marine Mammal Commission encourages additional research. Many of these researchers already have permits for work in the Gulf or on species potentially affected by the spill. However, coordination of research activities is critical, not only to reduce any additional stress on affected marine mammals but also to make the best use of information gained from such research.

The Marine Mammal Commission has encouraged the Permits Office of the National Marine Fisheries Service to coordinate research being conducted by permit holders already authorized or seeking authorization to work on Gulf marine mammals and to solicit specific details regarding planned research. Such coordination will help the Service ensure that necessary research is being conducted, that unnecessary redundancy is being avoided, and that the added stress from research projects is being appropriately managed.

Congressional Hearings

Various Senate and House committees have held dozens of hearings on the spill, focused on investigation of the cause of the explosion as well as response efforts. On 10 June 2010, the Marine Mammal Commission testified before the House Subcommittee on Insular Affairs, Oceans, and Wildlife 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.

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:

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

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

NOAA Southeast Region Natural Resource Damage Assessment (NRDA) activities and case documents related to the Deepwater Horizon spill:

National Marine Fisheries Service Office of Protected Resources Gulf of Mexico Oil Spill Web site:

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

Fish and Wildlife Service Permits Web site (manatees):



(2011). Deepwater Horizon Oil Spill and Marine Mammals. Retrieved from http://www.eoearth.org/view/article/51cbf0477896bb431f6a107e


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