Marine mammal

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Main article: Aquatic mammal
A humpback whale (Megaptera novaeangliae), a member of infraorder Cetacea of the order Cetartiodactyla.
A leopard seal (Hydrurga leptonyx), a member of the clade Pinnipedia of the order Carnivora.

Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They do not represent a distinct biological grouping, but are instead unified by their reliance on the aquatic environment for feeding. The level of dependence on the aquatic environment varies considerably between species. For example, cetaceans are completely dependent on the marine environment for all stages of their life, whereas seals feed in the ocean but breed on land, and polar bears can live comfortably on land.

Marine mammal adaptation to an aquatic lifestyle vary considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Likewise, their diet ranges considerably as well; some may eat zooplankton, others may eat small fish, and a few may eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations. Their role in maintaining ecosystems makes them of particular concern considering 23% of marine mammal species are currently threatened.

Marine mammals were first hunted by aboriginal peoples for food and other resources. They were also the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as whales and seals. Commercial hunting lead to the extinction of Steller's sea cow and the Caribbean monk seal. After commercial hunting ended, some species, such as the gray whale and northern elephant seal, have rebounded in numbers; conversely, other species, such as the North Atlantic right whale, are critically endangered. Other than hunting, marine mammals can be killed as bycatch from fisheries, where they become entangled in fixed netting and drown or starve. Increased ocean traffic causes collisions between fast ocean vessels and large marine mammals. Habitat degradation also threatens marine mammals and their ability to find and catch food. Noise pollution, for example, may adversely affect echolocating mammals, and the ongoing effects of global warming degrades arctic environments.

Taxonomy[edit]

Marine mammals vary greatly in size and shape
A polar bear (Ursus maritimus), a member of family Ursidae.
A sea otter (Enhydra lutris), a member of family Mustelidae.
California sea lions (Zalophus californianus), members of the family Otariidae.
A West Indian manatee (Trichechus manatus), a member of order Sirenia.

Marine mammals form a diverse group of 129 species that rely on the ocean for their existence.[1][2] They do not represent a distinct biological grouping, but rather are unified by their reliance on the aquatic environment for feeding.[3] The level of dependence on the aquatic environment for existence varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life, seals feed in the ocean but breed on land, and rhinoceroses can feed on land and in water.[3] Twenty three percent of marine mammal species are threatened.[4][5]

Classification of extant species[edit]

Evolution[edit]

Mammals have returned to the water in many separate evolutionary lineages, namely: Cetacea, Sirenia, Desmostylia, Pinnipedia, Kolponomos (marine bear), Thalassocnus (aquatic sloth), and Enhydra lutris (sea otter); the eutriconodont Ichthyoconodon might have also been marine in habits. Five of these lineages are extinct (Desmostylia; Kolponomos; Thalassocnus, Dyskritodon, Ichthyoconodon).[2] Despite the diversity in morphology seen between groups, improving foraging efficiency has been the main driver in the evolution in these lineages.[7] Today, fully aquatic marine mammals belong to one of two orders: Cetartiodactyla or Sirenia

A skeleton of Thalassocnus from the Muséum national d'histoire naturelle in its presumed swimming pose.

Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the Artiodactyla (even-toed ungulates).[8][9] The term Cetartiodactyla reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the hippopotamuses. Use of the order Cetartiodactyla, instead of Cetacea with parvorders Odontoceti and Mysticeti, is favored by most evolutionary mammalogists working with molecular data[10][11][12][13] and is supported the IUCN Cetacean Specialist Group[14] and by Taxonomy Committee[6] of the Society for Marine Mammalogy, the largest international association of marine mammal scientists in the world. Some others, including many marine mammalogists and paleontologists, favor retention of order Cetacea with the two suborders in the interest of taxonomic stability.

Fossil evidence indicates the sea otter (Enhydra) lineage became isolated in the North Pacific approximately 2 Mya, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris. The sea otter evolved initially in northern Hokkaidō and Russia, and then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 Mya, respectively, the sea otter is a relative newcomer to marine life. In some respects, though, the sea otter is more fully adapted to water than pinnipeds, which must haul out on land or ice to give birth.[15]

The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian, Prorastomus, was found in Jamaica, unlike other marine mammals which originated from the Old World (such as cetaceans[16]). The first known quadrupedal sirenian was Pezosiren from the early Eocene.[17] The earliest known sea cows, of the families Prorastomidae and Protosirenidae, are both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures.[18] The first members of Dugongidae appeared by the end of the Eocene.[19] At this point, sea cows were fully aquatic.[18]

Adaptations[edit]

Further information: Aquatic locomotion
The anatomy of a dolphin showing its skeleton, major organs, and body shape

Marine mammals have a number of physiological and anatomical features to overcome the unique challenges associated with aquatic living. Some of these features are very species specific. Marine mammals have developed a number of features for efficient locomotion such as torpedo shaped bodies to reduce drag; modified limbs for propulsion and steering; tail flukes and dorsal fins for propulsion and balance.[20] Marine mammals are adept at thermoregulation using dense fur or blubber to reduce heat loss; as well as circulatory adjustments to conserve their body temperature (counter-current heat exchangers); torpedo shaped bodies, reduced appendages, and large size to prevent heat loss.[20]

Marine mammals are able to dive for long periods of time. Both pinnipeds and cetaceans have large and complex blood vessel systems which serve to store oxygen to support deep diving. Other important reservoirs include muscles, blood, and the spleen which all have the capacity to hold a high concentration of oxygen. Other features include bradycardia (reduced heart rate), and vasoconstriction (shunts most of the oxygen to vital organs such as the brain and heart) also assist with extended diving and oxygen deprivation.[20] If oxygen is depleted, marine mammals can access substantial reservoirs of glycogen that support anaerobic glycolysis of the cells involved during conditions of systemic hypoxia associated with prolonged submersion.[21][22][23]

Sound travels differently through water therefore marine mammals have developed a number of ways to ensure effective communication, prey capture, and predator detection.[24] The most notable adaptation is the development of echolocation in whales and dolphins.[20]

Marine mammals have evolved a number features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinniped and odontocetes are designed specifically to capture fish and squid. In contrast, baleen whales have evolved baleen plates to filter feed plankton and small fish from the water.[20]

Polar bears, otters, and fur seals have fur, one of the defining mammalian features, that is long, oily, and waterproof in order to trap air to provide insulation. In contrast, other marine mammals – such as whales, dolphins, porpoises, manatees, dugongs, and walruses – have lost long fur in favor of a thick, dense epidermis and a thickened fat layer (blubber) in response to hydrodynamic requirements.[25] Wading and bottom-feeding animals (such as manatee) need to be heavier than water in order to keep contact with the floor or to stay submerged. Surface-living animals (such as sea otters) need the opposite, and free-swimming animals living in open waters (such as dolphins) need to be neutrally buoyant in order to be able to swim up and down the water column. Typically, thick and dense bone is found in bottom feeders and low bone density is associated with mammals living in deep water.[25] Some marine mammals have retained four weight-bearing limbs (such as polar bears and otters) and can walk on land like fully terrestrial animals.[25]

Distribution and habitat[edit]

Bottlenose dolphin at Dolphin Reef, Eilat, Israel

Marine mammals are widely distributed throughout the globe, but their distribution is patchy and coincides with the productivity of the oceans.[20] Species richness peaks at around 40° latitude, both north and south. This corresponds to the highest levels of primary production around North and South America, Africa, Asia and Australia. Total species range is highly variable for marine mammal species. On average most marine mammals have ranges which are equivalent or smaller than one-fifth of the Indian Ocean.[4] The variation observed in range size is a result of the different ecological requirements of each species and their ability to cope with a broad range of environmental conditions. There is a high degree of overlap between marine mammal species richness and areas of human impact on the environment which is of concern.[1]

Most marine mammals, such as seals and sea otters, inhabit the coast. Seals, however, also use a number of terrestrial habitats, both continental and island. In temperate and tropical areas, they haul-out on to sandy and pebble beaches, rocky shores, shoals, mud flats, tide pools and in sea caves. Some species also rest on man-made structures, like piers, jetties, buoys and oil platforms. Seals may move further inland and rest in sand dunes or vegetation, and may even climb cliffs.[26]:96 Most cetaceans live in the open ocean, and species like the sperm whale may dive to depths of −1,000 to −2,500 feet (−300 to −760 m) in search of food.[27] Sirenians live in shallow coastal waters, usually living 30 feet (9.1 m) below sea level. However, they have been known to dive to −120 feet (−37 m) to forage deep-water seagrasses.[28] Sea otters live in protected areas, such as rocky shores, kelp forests, and barrier reefs,[29] although they may reside among drift ice or in sandy, muddy, or silty areas.[30]

Ecology[edit]

Diet[edit]

All cetaceans are carnivorous and predatory. Toothed whales mostly feed on fish and cephalopods, followed by crustaceans and bivalves. Some may forage with other kinds of animals, such as other species of whales or certain species of pinnipeds.[26]:169[31] One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish.[32] Coralling is a method where dolphins chase fish into shallow water to catch them more easily.[32] Killer whales and bottlenose dolphins have also been known to drive their prey onto a beach to feed on it.[33][34] Other whales with a blunt snout and reduced dentition rely on suction feeding.[35]

Baleen whales use their baleen plates to sieve plankton, among others, out of the water; there are two types of methods: lunge-feeding and gulp-feeding. Lunge-feeders expand the volume of their jaw to a volume bigger than the original volume of the whale itself by inflating their mouth. This causes grooves on their throat to expand, increasing the amount of water the mouth can store.[36][37] They ram a baitball at high speeds in order to feed, but this is only energy-effective when used against a large baitball.[38] Gulp-feeders swim with an open mouth, filling it with water and prey. Prey must occur in sufficient numbers to trigger the whale's interest, be within a certain size range so that the baleen plates can filter it, and be slow enough so that it cannot escape.[39]

Sea otters have dexterous hands which they use to smash sea urchins off of rocks.

Otters are the only marine animal capable of lifting and turning over rocks, which it often does with its front paws when searching for prey.[40] The sea otter may pluck snails and other organisms from kelp and dig deep into underwater mud for clams.[40] It is the only marine mammal that catches fish with its forepaws rather than with its teeth.[41] Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest which they use to store collected food to bring to the surface. This pouch also holds a rock that is used to break open shellfish and clams.[42] There, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth.[43][44] The marine otter mainly feeds on crustaceans and fish.[45]

Pinnipeds mostly feed on fish and cephalopods, followed by crustaceans and bivalves, and then zooplankton and warm-blooded prey (like sea birds).[26]:145 Most species are generalist feeders, but a few are specialists.[46] They typical when hunt non-schooling fish, slow-moving or immobile invertebrates or endothermic prey in groups. Solitary foraging species usually exploit coastal waters, bays and rivers. When large schools of fish or squid are available, pinnipeds hunt cooperatively in large groups, locating and herding their prey. Some species, such as California and South American sea lions, may forage with cetaceans and sea birds.[26]:168

The polar bear is the most carnivorous bear, and its diet primarily consists of ringed (Pusa hispida) and bearded seals (Erignathus barbatus).[47] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[48] The polar bear's most common hunting method is still-hunting:[49] The bear locates a seal breathing hole using its sense of smell, and crouches nearby for a seal to appear. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear also hunts by stalking seals resting on the ice. Upon spotting a seal, it walks to within 90 m (100 yd), and then crouches. If the seal does not notice, the bear creeps to within 9 to 12 m (30 to 40 ft) of the seal and then suddenly rushes to attack.[50] A third hunting method is to raid the birth lairs that female seals create in the snow.[49] They may also feed on fish.[51]

A dugong feeding on the sea-floor

Sirenians are referred to as "sea cows" because their diet consists mainly of sea-grass. When eating they ingest the whole plant, including the roots, although when this is impossible they will feed on just the leaves.[52] A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat algae when seagrass is scarce.[53]

Keystone species[edit]

Further information: Keystone species

Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain benthic (sea floor) herbivores, particularly sea urchins, in check. Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. North Pacific areas that do not have sea otters often turn into urchin barrens, with abundant sea urchins and no kelp forest.[54] Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems,[55] and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California[56] However, some kelp forest ecosystems in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors.[56] The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and estuaries.[56]

An apex predator, the animal at the top of the food chain, affects prey population dynamics and defense tactics (such as camoflauge).[57] The polar bear is the apex predator within its range.[58] Several animal species, particularly Arctic foxes (Vulpes lagopus) and glaucous gulls (Larus hyperboreus), routinely scavenge polar bear kills.[59] The relationship between ringed seals and polar bears is so close that the abundance of ringed seals in some areas appears to regulate the density of polar bears, while polar bear predation in turn regulates density and reproductive success of ringed seals.[60] The evolutionary pressure of polar bear predation on seals probably accounts for some significant differences between Arctic and Antarctic seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.[59] The fur of Arctic pups is white, presumably to provide camouflage from predators, whereas Antarctic pups all have dark fur.[59]

Whale pump[edit]

Main articles: Whale pump and Whale fall
"Whale pump" – the role played by whales in recycling ocean nutrients[61]

A 2010 study considered whales to be a positive influence to the productivity of ocean fisheries, in what has been termed a "whale pump". Whales carry nutrients such as nitrogen from the depths back to the surface. This functions as an upward biological pump, reversing an earlier presumption that whales accelerate the loss of nutrients to the bottom. This nitrogen input in the Gulf of Maine is "more than the input of all rivers combined" emptying into the gulf, some 23,000 metric tons (25,000 short tons) each year.[61] Whales defecate at the ocean's surface; their excrement is important for fisheries because it is rich in iron and nitrogen. The whale faeces are liquid and instead of sinking, they stay at the surface where phytoplankton feed off it.[61][62]

Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life. Evidence of whale falls in present-day and fossil records shows that deep sea whale falls support a rich assemblage of creatures, with a global diversity of 407 species, comparable to other neritic biodiversity hotspots, such as cold seeps and hydrothermal vents.[63] Deterioration of whale carcasses happens though a series of three stages. Initially, moving organisms such as sharks and hagfish, scavenge the soft tissues at a rapid rate over a period of months, and as long as two years. This is followed by the colonization of bones and surrounding sediments (which contain organic matter) by enrichment opportunists, such as crustaceans and polychaetes, throughout a period of years. Finally, sulfophilic bacteria reduce the bones releasing hydrogen sulphide enabling the growth of chemoautotrophic organisms, which in turn, support other organisms such as mussels, clams, limpets, and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging 185 species per site.[64]

Interactions with humans[edit]

Threats[edit]

Exploitation[edit]

Men killing northern fur seals on Saint Paul Island, Alaska in the 1890s.

Marine mammals were hunted by coastal aboriginal humans historically for food and other resources. These subsistence hunts still occur in Canada, Greenland, Indonesia, Russia, the United States, and several nations in the Caribbean. The effects of these are only localized, as hunting efforts were on a relatively small scale.[20] Commercial hunting took this to a much greater scale and marine mammals were heavily exploited. This led to the extinction of the Steller's sea cow and the Caribbean monk seal.[20] Today, populations of species that were historically hunted, such as blue whales (Balaenoptera musculus), and the North Pacific right whale (Eubalaena japonica), are much lower compared to their pre-whaling levels.[65] Because whales generally have slow growth rates, are slow to reach sexual maturity, and have a low reproductive output, population recovery has been very slow.[24]

A number of whales are still subject to direct hunting, despite the 1986 moratorium ban on whaling set under the terms of the International Whaling Commission (IWC). There are only two nations remaining which sanction commercial whaling: Norway, where several hundred common minke whales are harvested each year; and Iceland, where quotas of 150 fin whales and 100 minke whales per year are set.[66][67] Japan also harvests several hundred Antarctic and North Pacific minke whales each year for scientific research (in accordance with the moratorium).[65] However, the illegal trade of whale and dolphin meat is a significant market in some countries.[68]

Historical and modern range of northern sea otters

The most profitable furs in the fur trade were those of sea otters, especially the northern sea otter which inhabited the coastal waters between the Columbia River to the south and Cook Inlet to the north. The fur of the Californian southern sea otter was less highly prized and thus less profitable. After the northern sea otter was hunted to local extinction, maritime fur traders shifted to California until the southern sea otter was likewise nearly extinct.[69] The British and American maritime fur traders took their furs to the Chinese port of Guangzhou (Canton), where they worked within the established Canton System. Furs from Russian America were mostly sold to China via the Mongolian trading town of Kyakhta, which had been opened to Russian trade by the 1727 Treaty of Kyakhta.[70]

Commercial sealing was historically just as important as the whaling industry. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal.[71] The scale of seal harvesting decreased substantially after the 1960s,[72] after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.[73] Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its historic range, but has a population bottleneck.[71] Conversely, the Mediterranean monk seal was extirpated from much of its former range, which stretched from the Mediterranean to the Black Sea and northwest Africa, and only remains in the northeastern Mediterranean and some parts of northwest Africa.[74]

Ocean traffic and fisheries[edit]

Further information: Cetacean bycatch
The remains of a North Atlantic right whale after it collided with a ship propeller.

By-catch is the incidental capture of non-target species in fisheries. Fixed and drift gill nets cause the highest mortality levels for both cetaceans and pinnipeds, however, entanglements in long lines, mid-water trawls, and both trap and pot lines are also common.[75] Tuna seines are particularly problematic for entanglement by dolphins.[76] By-catch affects all cetaceans, both small and big, in all habitat types. However, smaller cetaceans and pinnipeds are most vulnerable as their size means that escape once they are entangled is highly unlikely and they frequently drown.[65] While larger cetaceans are capable of dragging nets with them, the nets sometimes remain tightly attached to the individual and can impede the animal from feeding sometimes leading to starvation.[65] Abandoned or lost nets and lines cause mortality through ingestion or entanglement.[77] Marine mammals also get entangled in aquaculture nets, however, these are rare events and not prevalent enough to impact populations.[78]

Vessel strikes cause death for a number of marine mammals, especially whales.[65] In particular, fast commercial vessels such as container ships can cause major injuries or death when they collide with marine mammals. Collisions occur both with large commercial vessels and recreational boats and cause injury to whales or smaller cetaceans. The critically endangered North Atlantic right whale is particularly affected by vessel strikes.[79] Tourism boats designed for whale and dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior.[80]

The fishery industry not only threatens marine mammals through by-catch, but also through competition for food. Large scale fisheries have led to the depletion of fish stocks that are important prey species for marine mammals. Pinnipeds have been especially affected by the direct loss of food supplies and in some cases the harvesting of fish has led to food shortages or dietary deficiencies,[81] starvation of young, and reduced recruitment into the population.[82] As the fish stocks have been depleted, the competition between marine mammals and fisheries has sometimes led to conflict. Large-scale culling of populations of marine mammals by commercial fishers has been initiated in a number of areas in order to protect fish stocks for human consumption.[83]

Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish harvest.[78] On the other hand, marine mammals regularly take finfish from farms, which creates significant problems for marine farmers. While there are usually legal mechanisms designed to deter marine mammals, such as anti-predator nets or harassment devices, individuals are often illegally shot.[78]

Habitat loss and degradation[edit]

Map from the U.S. Geological Survey shows projected changes in polar bear habitat from 2005 to 2095. Red areas indicate loss of optimal polar bear habitat; blue areas indicate gain.

Habitat degradation is caused by a number of human activities. Marine mammals that live in coastal environments are most likely to be affected by habitat degradation and loss. Developments such as sewage marine outfalls, moorings, dredging, blasting, dumping, port construction, hydroelectric projects, and aquaculture both degrade the environment and take up valuable habitat.[24] For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.[78]

Contaminants that are discharged into the marine environment accumulate in the bodies of marine mammals when they are stored unintentionally in their blubber along with energy.[24] Contaminants that are found in the tissues of marine mammals include heavy metals, such as mercury and lead, but also organochlorides and polycyclic aromatic hydrocarbons.[24] For example, these can cause disruptive effects on endocrine systems;[77] impair the reproductive system, and lower the immune system of individuals, leading to a higher number of deaths.[24] Other pollutants such as oil, plastic debris and sewage threaten the livelihood of marine mammals.[84]

Noise pollution from anthropogenic activities is another major concern for marine mammals. This is a problem because underwater noise pollution interferes with the abilities of some marine mammals to communicate, and locate both predators and prey.[85] Underwater explosions are used for a variety of purposes including military activities, construction and oceanographic or geophysical research. They can cause injuries such as hemorrhaging of the lungs, and contusion and ulceration of the gastrointestinal tract.[65] Underwater noise is generated from shipping, the oil and gas industry, research, and military use of sonar and oceanographic acoustic experimentation. Acoustic harassment devices and acoustic deterrent devices used by aquaculture facilities to scare away marine mammals emit loud and noxious underwater sounds.[78]

Two changes to the global atmosphere due to anthropogenic activity threaten marine mammals. The first is increases in ultraviolet radiation due to ozone depletion, and this mainly affects the Antarctic and other areas of the southern hemisphere.[24] An increase in ultraviolet radiation has the capacity to decrease phytoplankton abundance, which forms the basis of the food chain in the ocean.[86] The second effect of global climate change is global warming due to increased carbon dioxide levels in the atmosphere. Raised sea levels, sea temperature and changed currents are expected to affect marine mammals by altering the distribution of important prey species, and changing the suitability of breeding sites and migratory routes.[87] The Arctic food chain would be disrupted by the near extinction or migration of polar bears. Arctic sea ice is the polar bear’s habitat. It has been declining at a rate of 13% per decade because the temperature is rising at twice the rate of the rest of the world.[58][88] By the year 2050, up to two-thirds of the world's polar bears may vanish if the sea ice continues to melt at its current rate.[89]

As food[edit]

Pilot whale meat (bottom), blubber (middle) and dried fish (left) with potatoes, Faroe Islands

For thousands of years, indigenous peoples of the Arctic have depended on whale meat. The meat is harvested from legal, non-commercial hunts that occur twice a year in the spring and autumn. The meat is stored and eaten throughout the winter.[90] The skin and blubber (muktuk) taken from the bowhead, beluga, or narwhal is also valued, and is eaten raw or cooked. Whaling has also been practiced in the Faroe Islands in the North Atlantic since about the time of the first Norse settlements on the islands. Around 1000 Long-finned pilot whales are still killed annually, mainly during the summer.[91][92] Today, dolphin meat is consumed in a small number of countries world-wide, which include Japan[93] and Peru (where it is referred to as chancho marino, or "sea pork").[94] In some parts of the world, such as Taiji, Japan and the Faroe Islands, dolphins are traditionally considered food, and are killed in harpoon or drive hunts.[93] Dolphin meat is consumed in a small number of countries world-wide, which include Japan[95] and Peru (where it is referred to as chancho marino, or "sea pork").[94]

There have been human health concerns associated with the consumption of dolphin meat in Japan after tests showed that dolphin meat contained high levels of methylmercury.[95][96] There are no known cases of mercury poisoning as a result of consuming dolphin meat, though the government continues to monitor people in areas where dolphin meat consumption is high. The Japanese government recommends that children and pregnant women avoid eating dolphin meat on a regular basis.[97] Similar concerns exist with the consumption of dolphin meat in the Faroe Islands, where prenatal exposure to methylmercury and PCBs primarily from the consumption of pilot whale meat has resulted in neuropsychological deficits amongst children.[96]

The Faroe Islands population was exposed to methylmercury largely from contaminated pilot whale meat, which contained very high levels of about 2 mg methylmercury/kg. However, the Faroe Islands populations also eat significant amounts of fish. The study of about 900 Faroese children showed that prenatal exposure to methylmercury resulted in neuropsychological deficits at 7 years of age

Ringed seals were once the main food staple for the Inuit. They are still an important food source for the people of Nunavut[98] and are also hunted and eaten in Alaska. Seal meat is an important source of food for residents of small coastal communities.[99] The seal blubber is used to make seal oil, which is marketed as a fish oil supplement. In 2001, two percent of Canada's raw seal oil was processed and sold in Canadian health stores.[100]

In captivity[edit]

Main article: Marine mammal park

Cetaceans[edit]

Performing killer whale at SeaWorld San Diego, 2009

Various species of dolphins are kept in captivity. These small cetaceans are more often than not kept in theme parks and dolphinariums, such as SeaWorld. Bottlenose Dolphins are the most common species of dolphin kept in dolphinariums as they are relatively easy to train, have a long lifespan in captivity and have a friendly appearance. Hundreds if not thousands of Bottlenose Dolphins live in captivity across the world, though exact numbers are hard to determine. Other species kept in captivity are Spotted Dolphins, False Killer Whales and Common Dolphins, Commerson's Dolphins, as well as Rough-toothed Dolphins, but all in much lower numbers than the Bottlenose Dolphin. There are also fewer than ten Pilot Whales, Amazon River Dolphins, Risso's Dolphins, Spinner Dolphins, or Tucuxi in captivity.[101] An unusual and very rare hybrid dolphin, known as a Wolphin, is kept at the Sea Life Park in Hawaii, which is a cross between a Bottlenose Dolphin and a False Killer Whale.[102]

Organizations such as World Animal Protection and the Whale and Dolphin Conservation Society campaign against the practice of keeping cetaceans, particularly killer whales, in captivity. In captivity, they often develop pathologies, such as the dorsal fin collapse seen in 60–90% of male killer whales. Captives have vastly reduced life expectancies, on average only living into their 20s. In the wild, females who survive infancy live 46 years on average, and up to 70–80 years in rare cases. Wild males who survive infancy live 31 years on average, and up to 50–60 years.[103] Captivity usually bears little resemblance to wild habitat, and captive whales' social groups are foreign to those found in the wild. Captive life is also stressful due the requirement to perform circus tricks that are not part of wild killer whale behavior and restricting pool size. Wild killer whales may travel up to 160 kilometres (100 mi) in a day, and critics say the animals are too big and intelligent to be suitable for captivity.[104] Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress.[105]

Pinnipeds[edit]

A sea lion balancing a ball on its nose

Their large size and playfulness of pinnipeds make them popular attractions. Some exhibits have rocky backgrounds with artificial haul-out sites and a pool, while others have pens with small rocky, elevated shelters where the animals can dive into their pools. More elaborate exhibits contain deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most common pinniped species kept in captivity is the California sea lion as it is abundant and easy to train.[106] These animals are used to perform tricks and entertain visitors.[107] Other species popularly kept in captivity include the grey seal and harbor seal. Larger animals like walruses and Steller sea lions are much less common.[106] Some organizations, such as the Humane Society of the United States and World Animal Protection, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory, and a pool could never replace the size and biodiversity of the ocean. They also oppose using sea lions for entertainment, claiming the tricks performed are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment.[108]

Others[edit]

Sea otters can do well in captivity, and are featured in over 40 public aquariums and zoos.[109] The Seattle Aquarium became the first institution to raise sea otters from conception to adulthood with the birth of Tichuk in 1979, followed by three more pups in the early 1980s.[110] In 2007, a YouTube video of two sea otters holding paws drew 1.5 million viewers in two weeks, and had over 20 million views as of January 2015.[111] Filmed five years previously at the Vancouver Aquarium, it was YouTube's most popular animal video at the time, although it has since been surpassed.[112]

The oldest manatee in captivity is Snooty,[113] at the South Florida Museum's Parker Manatee Aquarium in Bradenton, Florida. Born at the Miami Aquarium and Tackle Company on July 21, 1948, Snooty was one of the first recorded captive manatee births. He was raised entirely in captivity, and will never be released into the wild.[114][115] Manatees can also be viewed in a number of European zoos, such as the Tierpark in Berlin,[116] the Nuremberg Zoo,[117] in ZooParc de Beauval in France,[118] and in the Aquarium of Genoa in Italy.[119] The River Safari at Singapore features seven of them.[120]

Military[edit]

A dolphin wearing a locating pinger, performing mine clearance work in the Iraq War

Bottlenose dolphins and California sea lions were used in the United States Navy Marine Mammal Program (NMMP) to detect mines, protect ships from enemy soldiers, and recover objects. The Navy has never trained attack dolphins, as they would not be able to discern allied soldiers from enemy soldiers. There were five marine mammal teams, each purposed for one of the three tasks: MK4 (dolphins), MK5 (sea lions), MK6 (dolphins and sea lions), MK7 (dolphins), and MK8 (dolphins); MK is short for mark. The dolphin teams were trained to detect and mark mines either attached to the seafloor or floating in the water column, because dolphins can use their echolocative abilities to detect mines. The sea lion team retrieved test equipment such as fake mines or bombs dropped from planes usually out of reach of divers who would have to make multiple dives. MK6 protects harbors and ships from enemy divers, and was operational in the Gulf War and Vietnam War. The dolphins would swim up behind enemy divers and attach a buoy to their air tank, so that they would float to the surface and alert nearby Navy personnel. Sea lions would hand-cuff the enemy, and try to outmaneuver their counter-attacks.[121][122]

The use of marine mammals by the Navy, even in accordance with the Navy's policy, continues to meet opposition. The Navy's policy says that only positive reinforcement is to be used while training the military dolphins, and that they be cared for in accordance with accepted standards in animal care. The inevitable stresses involved in training are topics of controversy, which is unlike the animals' natural lifestyle, especially towards their confined spaces when not training. There is also controversy over the use of muzzles and other inhibitors, which prevent the dolphins from foraging for food while working. The Navy states that this is to prevent them from ingesting harmful objects, but conservation activists say this is done to reinforce the trainers' control over the dolphins, who hand out food rewards. The means of transportation is also an issue for conservation activists, since they are hauled in dry carriers, and switching tanks and introducing the dolphin to new dolphins is potentially dangerous as they are territorial.[121][122]

See also[edit]

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