U.S. Marine Mammals: Status, Threats and Conservation

Approximately one-half of the world's marine mammal species occur within the territorial waters of the U.S. These include a variety of pinnipeds and cetaceans as well as the sea otter, polar bear, and manatee. Currently, all species are separated into over 150 management stocks (with "stock" defined by the Marine Mammal Protection Act (MMPA) as a group of marine mammals of the same species or smaller taxa in a common spatial arrangement, that interbreed when mature), with some species comprising multiple stocks.

Two marine mammal species once found in U.S. waters have been hunted to extinction: the Caribbean monk seal which lived throughout the Caribbean Sea, including the waters of southern Florida; and the little-known sea mink which inhabited northern New England shores. One sub-species, the north Atlantic gray whale, likely disappeared by the 17th century, presumably a victim of early whaling. Hunting exterminated the Guadalupe fur seal from southern California waters by approximately 1825; however, this species still survives further south on Guadalupe Island (Mexico).

The Problem

• While the Endangered Species Act (ESA) and the MMPA, and other conservation efforts, have generally succeeded in protecting or in promoting the recovery of marine mammal species in U.S. waters, there are many ongoing problems and emerging threats. Several species are nearing extinction while some are in decline. Still others, while not considered endangered, are at numbers well below historical levels.
• Of particular conservation concern are the following:
• Northern right whale [U.S./Canada Atlantic coast]: Currently numbering around 300 animals; the species has failed to recover from historical whaling despite decades of protection; continues to be threatened by ship strikes and fisheries gear entanglement.
• Northern right whale [eastern north Pacific]: Virtually exterminated by whaling; overall numbers are unknown but this population is considered to be perilously close to extinction.
• Orca [Southern Resident population]: Currently numbering around 83 animals; this population (which resides in the coastal waters of Washington and British Columbia) was listed as endangered in 2005; major threats include contaminants, reduced food availability, and disturbance.
• Beluga whale [Cook Inlet, Alaska]: Hunting during the 1990s reduced this population from over 1000 animals to less than 400; while hunting virtually ended in 1999 whale numbers, surprisingly, have not increased. The Cook Inlet beluga whale population now appears to be stable with approximately 375 individuals.
• West Indian manatee [Florida]: Currently numbering around 3300 animals, the Florida manatee faces a number of problems including recreational boating strikes, habitat loss, disturbance, and coastal development; conservation requirements are well documented but the chronic lack of public and political will has undermined many protection efforts.
• Northern sea otter: While sea otters in Alaska repopulated most of their range after being virtually eliminated by commercial hunting, they have recently been in rapid decline and may now number between 55 to 69% less than they numbered in 1976; increased orca predation has been suggested as the main factor, a response to vastly reduced numbers of the orca's preferred prey, Steller sea lions and harbor seals.
• California or southern sea otter: Almost hunted to extinction by the fur trade; now numbers around 3000 animals and may be increasing after a period of decline between 1994 and 2000; the species faces substantial risks from oil spills, contaminants, algal blooms, and disease-causing organisms from sewage and agricultural and street run-off.
• Hawaiian monk seal: Numbers around 1200 animals; this endangered species is being impacted by entanglement in marine debris, human disturbance, and an apparent reduction in food, among other factors; overall numbers have been in gradual decline since the late 1980s despite extensive recovery program efforts.
• Steller sea lion [western Alaska]: Numbers have declined by around 80% since the mid-1970s; commercial fisheries and/or an ocean-climate shift are considered to be the most probable causes through their impacts on prey availability or prey species composition.
• Polar bear: Due to the risk of becoming endangered in the foreseeable future, the polar bear was recently listed as threatened under the ESA; human-induced climate change with the subsequent decline in Arctic sea ice is the main threat facing the species.

The Causes

• Commercial hunting: Globally, this has been the single most important factor in the historic decline of marine mammals. Various species found in U.S. waters, such as the sea otter, northern fur seal, right and blue whales, and Hawaiian monk seal, have not recovered from the effects of past hunting though other factors in some cases have also played a role.
• Fisheries: Great strides have been made in recent years to reduce the incidental take (i.e., bycatch) of marine mammals by U.S. fishing fleets though relatively high numbers of, for example, harbor porpoise, common dolphin, pilot whales, beaked whales, California sea lions and harbor seals are still killed each year. Entanglement in discarded fishing gear also continues to be a problem, notably for the endangered Hawaiian monk seal. Declines in forage fish because of commercial fishing is a prime suspect, either as cause or contributor, in the decline or recovery failure of various Alaskan pinniped populations. While the changes in Alaska are now an area of intense scientific research, there has been little evidence to show that reduced food due to commercial fishing has been a problem in other U.S. waters, though few studies have been conducted.
• Environmental pollution: Pesticides and industrial chemicals and by-products are considered to be having some impact on at least some species and populations, notably in terms of reproductive fitness and disease susceptibility. However, linking pollutants with marine mammal health impacts has proven to be extremely difficult due, in large part, to ethical and logistical considerations involved with marine mammal research and the many complexities associated with understanding the effects of chemical exposure. Levels of contaminants in orcas, southern sea otters, and inshore populations of east coast and Gulf of Mexico bottlenose dolphins are, nonetheless, considered to be high enough to impact these species’ health.
• Disease and biotoxins: Marine mammal die-off events appear to have increased worldwide since the late 1980s, with disease and algal biotoxins emerging as important factors. Mortalities can be high and, in some cases, have affected endangered species. Affected marine mammals in U.S. waters over this period include humpback whales, bottlenose dolphins, California sea lions, and manatees. A surprisingly high frequency and variety of diseases have been diagnosed in California sea otters, a factor hindering their recovery, and in bottlenose dolphins in the Indian River Lagoon, Florida.
• Disturbance and noise: The escalation in a wide range of coastal and offshore activities including shipping, fisheries and recreational boating, minerals mining, oil and gas exploration and drilling, and aquaculture has meant a considerable increase in ambient ocean noise and disturbance levels facing marine mammals. The overall effects of this on the behavior, distribution and, ultimately, the health of individual populations have been difficult to quantify but could be a significant source of stress. Of rising concern is the testing and deployment of intense, low-frequency sonar by the U.S. Navy which has been linked to strandings of beaked whales and dolphins.
• Climate change: The effects on ocean systems associated with a warming climate remain difficult to predict though the largest changes are expected to occur in polar regions. Suitable sea ice conditions are required, for example, by polar bears for hunting and by ringed seals as pupping habitat. Changes to arctic and sub-arctic ice-edge systems will alter the abundance, density, or distribution of marine mammal food and are likely to have severe ramifications for such species as gray and beluga whales, walruses, and bearded seals. A natural climatic regime shift is generally considered an important factor in the decline of various Alaskan seal and sea lion populations but may be indicative of the types of impacts to be expected from human-induced effects on global climate.
• Other impacts: There are other impacts and factors that have particular relevance for specific species and regions. Ship strikes are an important obstacle preventing the recovery of north Atlantic right whales, and the projected increase in high speed vessel traffic (e.g., ferries, container vessels) may have implications for this and other small, vulnerable, whale populations. Recreational boats in Florida continue to injure and kill high numbers of manatees despite the development of reduced speed and boat no-entry zones in critical areas. Catastrophic oil spills remain a constant threat for, notably, the fur-bearing marine mammals. Inbreeding depression, the result of small population size, may be playing a role in the decline of the Hawaiian monk seal or recovery failure of the right whales by, for example, reducing juvenile survival or female reproductive ability.

The Context

• There are substantial uncertainties over the longer term effects of commercial fishing and food reduction, environmental contaminants, increases in disturbance and underwater noise, and climate change on U.S. marine mammals. And while much has been learned about at least some individual risk factors, little can be said about effects associated with the interaction of multiple stressors. Understanding the impacts of cumulative interactions is likely to become increasingly important in the future protection of the nation's marine mammals.
• Public opinion, dedicated scientific research, and the legislative, legal, and public education activities of non-governmental wildlife and conservation organisations have been responsible for the remarkable advances in marine mammal protection on the part of the U.S. However, there is increasing concern over the ability and determination of the nation to provide adequate protection for a rapidly increasing number of threatened and endangered species. Sharply divergent world views among special interest groups and across political affiliations and regions have often undermined unified approaches to addressing the loss in biodiversity and other ongoing environmental issues. Threats to many species - including those that are marine - are expected to escalate in tandem with a growing U.S. population and its ever-expanding requirements for energy, transportation, food, recreation, and resources.

Further Reading

Marine Mammal Commission.  Annual Reports to Congress.  Marine Mammal Commission, Silver Spring, MD.

National Marine Fisheries Service Stock Assessment Reports (SARS) by Species/Stock

Twiss, J.R. and Reeves, R.R. (eds.) 1999. Conservation and Management of Marine Mammals. Smithsonian Institution Press, Washington, DC.