Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions 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. Their diet varies considerably as well; some may eat zooplankton, others may eat fish, squid, shellfish, sea-grass 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, especially concerning the maintenance of marine ecosystems, through processes including the regulation of prey populations. This role in maintaining ecosystems makes them of particular concern as 23% of marine mammal species are currently threatened.
Marine mammals were first hunted by aboriginal peoples for food and other resources. Many 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 degrade arctic environments.
A humpback whale (Megaptera novaeangliae), a member of infraorder Cetacea of the order Cetartiodactyla
Taxonomy
Marine mammals of varying sizes and shapes
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.
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A common minke whale (Balaenoptera acutorostrata), a member of the order Cetartiodactyla.
Classification of extant species
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| The taxa in bold are marine.[1] |
- Order Cetartiodactyla[2]
- Suborder Whippomorpha
- Family Balaenidae (right and bowhead whales), two genera and four species
- Family Cetotheriidae (pygmy right whale), one species
- Family Balaenopteridae (rorquals), two genera and eight species
- Family Eschrichtiidae (gray whale), one species
- Family Physeteridae (sperm whale), one species
- Family Kogiidae (pygmy and dwarf sperm whales), one genus and two species
- Family Monodontidae (narwhal and beluga), two genera and two species
- Family Ziphiidae (beaked whales), six genera and 21 species
- Family Delphinidae (oceanic dolphins), 17 genera and 38 species
- Family Phocoenidae (porpoises), two genera and seven species
- Suborder Whippomorpha
- Order Sirenia (sea cows)[2]
- Family Trichechidae (manatees), two species
- Family Dugongidae (dugongs), one species
- Order Carnivora (carnivores)[2]
- Suborder Caniformia
- Family Mustelidae, three species
- Family Ursidae (bears), one species
- Suborder Pinnipedia (sealions, walruses, seals)
- Family Otariidae (eared seals), seven genera and 15 species
- Family Odobenidae (walrus), one species
- Family Phocidae (earless seals), 14 genera and 18 species
- Suborder Caniformia
- Order Chiroptera
- Family Vespertilionidae
- Myotis vivesi (fish-eating bat)[3]
- Family Vespertilionidae
Evolution
Illustration of †Prorastomus, an early sirenian (40 mya)
The cetaceans became aquatic around 50 million years ago (mya).[11] Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the Artiodactyla (even-toed ungulates).[12][13] 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.[14][15][16][17]
Sirenians, the sea cows, became aquatic around 40 million years ago. 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,[8] unlike other marine mammals which originated from the Old World (such as cetaceans[18]). The first known quadrupedal sirenian was †Pezosiren from the early Eocene.[19] The earliest known sea cows, of the families †Prorastomidae and †Protosirenidae, were both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures.[20] The first members of Dugongidae appeared by the end of the Eocene.[21] At this point, sea cows were fully aquatic.[20]
Pinnipeds split from other caniforms 50 mya during the Eocene. Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of †Puijila darwini in early Miocene deposits in Nunavut, Canada. Like a modern otter, †Puijila had a long tail, short limbs and webbed feet instead of flippers.[22] The lineages of Otariidae (eared seals) and Odobenidae (walrus) split almost 28 mya.[23] Phocids (earless seals) are known to have existed for at least 15 mya,[24] and molecular evidence supports a divergence of the Monachinae (monk seals) and Phocinae lineages 22 mya.[23]
Fossil evidence indicates the sea otter (Enhydra) lineage became isolated in the North Pacific approximately two 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.[25]
Polar bears are thought to have diverged from a population of brown bears, Ursus arctos, that became isolated during a period of glaciation in the Pleistocene[26] or from the eastern part of Siberia, (from Kamchatka and the Kolym Peninsula).[27] The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004.[28] The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear roughly 150,000 years ago.[28] Further, some clades of brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,[29] meaning that the polar bear might not be considered a species under some species concepts.[30]
In general, terrestrial amniote invasions of the sea have become more frequent in the Cenozoic than they were in the Mesozoic. Factors contributing to this trend include the increasing productivity of near-shore marine environments, and the role of endothermy in facilitating this transition.[31]
Distribution and habitat
Marine mammal species richness: A) All species (n = 115), B) Odontocetes (n = 69), C) Mysticetes (n = 14), D) Pinnipeds (n = 32), based on data from 1990–1999[32]
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.[34]: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.[35] 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.[36] Sea otters live in protected areas, such as rocky shores, kelp forests, and barrier reefs,[37] although they may reside among drift ice or in sandy, muddy, or silty areas.[38]
Many marine mammals seasonally migrate. Annual ice contains areas of water that appear and disappear throughout the year as the weather changes, and seals migrate in response to these changes. In turn, polar bears must follow their prey. In Hudson Bay, James Bay, and some other areas, the ice melts completely each summer (an event often referred to as "ice-floe breakup"), forcing polar bears to go onto land and wait through the months until the next freeze-up. In the Chukchi and Beaufort seas, polar bears retreat each summer to the ice further north that remains frozen year-round.[39] Seals may also migrate to other environmental changes, such as El Niño, and traveling seals may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water.[34]:256–257 Baleen whales famously migrate very long distances into tropical waters to give birth and raise young,[40] possibly to prevent predation by killer whales.[41] The gray whale has the longest recorded migration of any mammal, with one traveling 14,000 miles (23,000 km) from the Sea of Okhotsk to the Baja Peninsula.[42] During the winter, manatees living at the northern end of their range migrate to warmer waters.[43]
Adaptations
The anatomy of a dolphin showing its skeleton, major organs, and body shape
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. They are also capable of bradycardia (reduced heart rate), and vasoconstriction (shunting most of the oxygen to vital organs such as the brain and heart) to allow extended diving times and cope with oxygen deprivation.[33] 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.[44][45][46]
Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection.[47] The most notable adaptation is the development of echolocation in whales and dolphins.[33] Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips.[48]:p. 112 These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities.[48]:121[49]
Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast, baleen whales have evolved baleen plates to filter feed plankton and small fish from the water.[33]
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. Wading and bottom-feeding animals (such as manatees) 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. Some marine mammals, such as polar bears and otters, have retained four weight-bearing limbs and can walk on land like fully terrestrial animals.[50]
Ecology
Diet
Killer whale hunting a Weddel seal
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.[57][58] They ram a baitball at high speeds in order to feed, but this is only energy-effective when used against a large baitball.[59] 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.[60]
Sea otters have dexterous hands which they use to smash sea urchins off rocks.
Pinnipeds mostly feed on fish and cephalopods, followed by crustaceans and bivalves, and then zooplankton and warm-blooded prey (like sea birds).[34]:145 Most species are generalist feeders, but a few are specialists.[67] They typically hunt non-schooling fish, slow-moving or immobile invertebrates or endothermic prey when 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.[34]:168
The polar bear is the most carnivorous species of bear, and its diet primarily consists of ringed (Pusa hispida) and bearded (Erignathus barbatus) seals.[68] Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.[69] The polar bear's most common hunting method is still-hunting:[70] 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 100 yards (90 m), and then crouches. If the seal does not notice, the bear creeps to within 30 to 40 feet (9 to 10 m) of the seal and then suddenly rushes to attack.[71] A third hunting method is to raid the birth lairs that female seals create in the snow.[70] They may also feed on fish.[72]
A dugong feeding on the sea-floor
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.[76] Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems,[77] and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California[65] However, some kelp forest ecosystems in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors.[65] 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.[65].jpg)
Killer whales are apex predators throughout their global distribution, and can have a profound effect on the behavior and population of prey species. Their diet is very broad and they can feed on many vertebrates in the ocean including salmon,[82] rays, sharks (even white sharks),[83][84] large baleen whales,[85] and nearly 20 species of pinniped.[86] The predation of whale calves may be responsible for annual whale migrations to calving grounds in more tropical waters, where the population of killer whales is much lower than in polar waters. Prior to whaling, it is thought that great whales were a major food source; however, after their sharp decline, killer whales have since expanded their diet, leading to the decline of smaller marine mammals.[41] A decline in Aleutian Islands sea otter populations in the 1990s was controversially attributed by some scientists to killer whale predation, although with no direct evidence. The decline of sea otters followed a decline in harbor seal and Steller sea lion populations, the killer whale's preferred prey, which in turn may be substitutes for their original prey, now reduced by industrial whaling.[87][88][89]
Whale pump
"Whale pump" – the role played by whales in recycling ocean nutrients[90]
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.[92] Deterioration of whale carcasses happens though a series of three stages. Initially, moving organisms, such as sharks and hagfish, scavenge soft tissue at a rapid rate over a period of months to 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.[93]
Interactions with humans
Threats
Exploitation
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.[33] 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 (along with subsistence hunting) and the †Caribbean monk seal.[33] 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 than their pre-whaling levels.[94] 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.[47]
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.[95][96] Japan also harvests several hundred Antarctic and North Pacific minke whales each year, ostensibly for scientific research in accordance with the moratorium.[94] However, the illegal trade of whale and dolphin meat is a significant market in Japan and some countries.[97]
Historical and modern range of northern sea otters
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.[100] The scale of seal harvesting decreased substantially after the 1960s,[101] after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.[102] 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.[100] 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.[103]
Polar bears can be hunted for sport in Canada with a special permit and accompaniment by a local guide. This can be an important source of income for small communities, as guided hunts bring in more income than selling the polar bear hide on markets. The United States, Russia, Norway, Greenland, and Canada allow subsistence hunting, and Canada distributes hunting permits to indigenous communities. The selling of these permits is a main source of income for many of these communities. Their hides can be used for subsistence purposes, kept as hunting trophies, or can be bought in markets.[104][105]
Ocean traffic and fisheries
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.[106] Tuna seines are particularly problematic for entanglement by dolphins.[107] 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.[94] 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.[94] Abandoned or lost nets and lines cause mortality through ingestion or entanglement.[108] Marine mammals also get entangled in aquaculture nets, however, these are rare events and not prevalent enough to impact populations.[109]
Vessel strikes cause death for a number of marine mammals, especially whales.[94] 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.[110] Tourism boats designed for whale and dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior.[111]
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,[112] starvation of young, and reduced recruitment into the population.[113] 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.[114]
Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish harvest.[109] 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.[109]
Habitat loss and degradation
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.
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.[47] 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.[47] For example, these can cause disruptive effects on endocrine systems;[108] impair the reproductive system, and lower the immune system of individuals, leading to a higher number of deaths.[47] Other pollutants such as oil, plastic debris and sewage threaten the livelihood of marine mammals.[115]
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.[116] 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.[94] 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.[109]
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.[47] An increase in ultraviolet radiation has the capacity to decrease phytoplankton abundance, which forms the basis of the food chain in the ocean.[117] 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.[118] 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.[79][119] 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.[120]
Protection
Signatory countries of the International Whaling Commission (IWC)
The 1979 Convention on the Conservation of Migratory Species of Wild Animals (CMS) is the only global organization that conserves a broad range of animals, of which includes marine mammals.[123][124] Of the agreements made, three of them deal with the conservation of marine mammals: ACCOBAMS, ASCOBANS, and the Wadden Sea Agreement.[125] In 1982, the United Nations Convention on the Law of the Sea (LOSC) adopted a pollution prevention approach to conservation, which many other conventions at the time also adopted.[126]
The Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and contiguous Atlantic area (ACCOBAMS), founded in 1996, specifically protects cetaceans in the Mediterranean area, and "maintains a favorable status", a direct action against whaling.[126] There are 23 member states.[127] The Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) was adopted alongside ACCOBAMS to establish a special protection area for Europe's increasingly threatened cetaceans.[126] Other anti-whaling efforts include a ten-year moratorium in 1986 by the IWC on all whaling,[128] and an environmental agreement (a type of international law) the International Convention for the Regulation of Whaling which controlled commercial, scientific, and subsistence whaling.[129]
The Agreement on the Conservation of Seals in the Wadden Sea, enforced in 1991, prohibits the killing or harassment of seals in the Wadden Sea, specifically targeting the harbor seal population.[130]
The 1973 Agreement on the Conservation of Polar Bears between Canada, Denmark (Greenland), Norway (Svalbard), the United States, and the Soviet Union outlawed the unregulated hunting of polar bears from aircraft and icebreakers, as well as protecting migration, feeding, and hibernation sites.[131]
Various non-governmental organizations participate in marine conservation activism, wherein they draw attention to and aid in various problems in marine conservation, such as pollution, whaling, bycatch, and so forth. Notable organizations include the Greenpeace who focus on overfishing and whaling among other things, and Sea Shepherd Conservation Society who are known for taking direct-action tactics to expose illegal activity.[132]
As food
Pilot whale meat (bottom), blubber (middle) and dried fish (left) with potatoes, Faroe Islands
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.[137][139] 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.[140] 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.[139]
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 ageRinged seals were once the main food staple for the Inuit. They are still an important food source for the people of Nunavut[141] and are also hunted and eaten in Alaska. Seal meat is an important source of food for residents of small coastal communities.[142] 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.[143]
In captivity
Aquariums
- Cetaceans
Performing killer whale at SeaWorld San Diego, 2009
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.[146] 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, as well as restricting pool size. Wild killer whales may travel up to 100 miles (160 km) in a day, and critics say the animals are too big and intelligent to be suitable for captivity.[147] Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress.[148] Dolphins are often trained to do several anthropomorphic behaviors, including waving and kissing—behaviors wild dolphins would rarely do.[149]
- Pinnipeds
A sea lion trained to balance a ball on its nose
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.[153]
- Otter
- Sirenians
A leopard seal (Hydrurga leptonyx), a member of the clade Pinnipedia of the order Carnivora
Military
A dolphin wearing a locating pinger, performing mine clearance work in the Iraq War
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, as their treatment 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.[168][169]
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