Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence.[1] They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding.[2] The level of dependence on the marine 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, whereas seals feed in the ocean, but breed on land.[2] Marine mammals can be subdivided into four recognised groups; cetaceans (whales, dolphins and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semi-aquatic; 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 ocean living.[2] While the number of marine mammals is small compared to those found on land, their total biomass is large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations.[3] These two factors make them an integral component of the marine environment. This is of particular concern considering 23% of marine mammal species are currently threatened.[4]
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Mammals have returned to the water in at least nine separate evolutionary lineages (Cetacea, Sirenia, Desmostylia, Pinnipedia, Ursus maritimus (polar bear), Kolponomos (marine bear), Thalassocnus (aquatic sloth), Enhydra lutris (sea otter) and Lontra feline (marine otter)). Three of these lineages are extinct (Desmostylia; Kolponomos; Thalassocnus).[1] Despite the diversity in morphology seen between groups, improving foraging efficiency has been the main driver in the evolution in these lineages.[5] Today, marine mammals belong to one of three orders; Cetacea, Sirenia, or Carnivora. Within the Order Carnivora are the pinnipeds (sealions, walruses and seals), the polar bear (Ursus maritimus), and two otters (Endydra lutris and Lontra feline.[2]
Marine mammals are widely distributed throughout the globe, but their distribution is patchy and coincides with the productivity of the oceans.[6] Species richness peaks at around 40°of 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.[3]
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.[6] 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.[6] Most marine mammals are hypoosmotic and as a result they are constantly losing water to the surrounding environment. They have evolved a number of mechanisms to overcome this, but most retain their water by using highly efficient kidneys, that can concentrate urine.[6] 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.[6] Sound travels differently through water therefore marine mammals have developed a number of ways to ensure effective communication, prey capture, and predator detection.[7] The most notable adaptation is the development of echolocation in whales and dolphins.[6] Lastly, 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, Mysticetes have evolved baleen plates to filter feed plankton and small fish from the water.[6]
Marine mammals were hunted by coastal aboriginal humans historically for food and other resources. The effects of this were only localised as hunting efforts were on a relatively small scale.[6] Later, commercial hunting was developed and marine mammals were heavily exploited. This led to the extinction of the Steller's Sea Cow and the Caribbean monk seal.[6] Today, populations of species that were historically hunted, such as blue whales Balaenoptera musculus and B. m. brevicauda), and the North Pacific right whale (Eubalaena japonica), are much lower compared to their pre-exploited levels.[8] 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.[7] Despite the fact commercial whaling is generally a thing of the past since the passage of the International Whaling Commission’s (IWC) moratorium on commercial whaling, a number of marine mammals are still subject to direct hunting. The only remaining commercial hunting of whales is by Norway where several hundred northeastern North Atlantic minke whales are harvested each year. Japan also harvests several hundred Antarctic and North Pacific minke whales each year under the guise of scientific research.[8] However, the illegal trade of whale and dolphin meat is a significant market in some countries.[9] Seals and sealions are also still hunted in some areas such as Canada.
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 both trap and pot lines are also common, long lines, and mid-water trawls.[10] Tuna seines are particularly problematic for entanglement by dolphins.[11] 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.[8] 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.[8] Abandoned or lost nets and lines cause mortality through ingestion or entanglement.[12] Marine mammals also get entangled in aquaculture nets, however, these are rare events and not prevalent enough to impact populations[13]
Vessel strikes cause death for a number of marine mammals, especially whales.[8] 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 northern right whale is particularly affected by vessel strikes. Tourism boats designed for whale and dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior[14]
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.[7] For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.[13]
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,[15] starvation of young, and reduced recruitment into the population.[16] 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.[17]
Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish harvest.[13] 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.[13]
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.[7] Contaminants that are found in the tissues of marine mammals include heavy metals such as mercury and lead; organochlorides; and polycyclic aromatic hydrocarbons.[7] For example, These can cause disruptive effects on endocrine systems;[12] impair the reproductive system, and lower the immune system of individuals, leading to a higher number of deaths.[7] Other pollutants such as oil, plastic debris and sewage threaten the livelihood of marine mammals.[18] 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.[19] 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.[8] 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.[13]
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.[7] An increase in ultraviolet radiation has the capacity to decrease phytoplankton abundance, which forms the basis of the food chain in the ocean.[20] 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.[21]
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