Like other animals, marine mammals may have preferred locations in which they spend the majority of time or where they engage in particular important life history activities such as giving birth, calf rearing, or feeding. The array of physical and oceanographic features that typify those locations forms the habitat of a species or local population. Often these are difficult to define. An ice-breeding seal clearly depends on pack ice upon which to give birth and that constitutes its breeding habitat, and a gray whale may seek out a sheltered tropical lagoon to calve, but for a large open-ocean baleen whale such as the blue whale, identifying its habitat requirements for breeding can be a difficult task. The same applies to feeding habitats: manatees and dugongs, for example, require specific habitats such as shallow seagrass beds for feeding, but oceanic dolphins may range the high seas in pursuit of shoaling fish, making it hard to identify whether they have specific habitat requirements. Human activities impinge upon the lives of marine mammals if they damage or destroy those habitats that may be important to them. Our knowledge of habitat pressures facing marine mammals is therefore limited to particular species and especially to locations nearshore where animals have been studied more intensively and their ecological requirements better defined.
Habitats formed by eddies, thermoclines, and fronts may shift from one locality to another during the life span of a marine mammal, leading to shifts in their geographic distributions. Habitats determined by geomorphological features, such as depth, topography, and available haul-out or den sites [in the case of pinnipeds and polar bears (Ursus niaritimus), respectively], are relatively stable over time in relation to location. Strong site fidelity may lead a population to have difficulty adjusting to changes in local food availability.
Habitat pressures on marine mammals from anthropogenic influences may be grouped into five categories: (1) physical damage to their environment: a river or seabed and its constituent communities; (2) contamination from chemical pollutants; (3) direct removal of important prey through fisheries; (4) disturbance from human activities either by the introduction of sound into the environment or through ship strikes; and (5) physical and oceanographic effects from global climate change.
I. Physical Damage
Human population pressures frequently lead to direct changes to coastal and riverine environments. Estuaries are turned into industrial harbors, wetlands are drained for agricultural purposes or for tourism, and coastal waters are modified often irreversibly by dredging of the seabed and input of a wide variety of pollutants. Some of the most obvious detrimental changes to a habitat come from the alteration of rivers inhabited by particular dolphin species. Water is often taken out of rivers for other uses, such as for drinking, flood control, or irrigation agriculture. In Pakistan, for example, most of the annual flow of the Indus River is diverted into canals, and this, along with dam construction, has resulted in the Indus river dolphin (Platanista gangetica minor) losing probably at least half of its historical range. Dams modify water flow and affect the sedimentation of rivers; they also block traditional movement patterns of marine mammals, which can lead to population fragmentation. The construction of large dams (such as the Ghezouba Dam and the Three Gorges Dam) along the Yangtze Kiang river system thus poses serious threats to the already endangered Chinese river dolphin or baiji (Lipotes vexillifer), which now numbers only a few hundred (or fewer) individuals. It may also restrict movements of more widespread species such as the Amazonian manatee in Brazil.
On land, one of the greatest habitat pressures leading to mass extinctions of fauna and flora is that of deforestation, particularly in the tropics. In the 1980s, Latin American countries are estimated to have eliminated 7.4 million hectares of tropical forests annually, with Brazil sustaining the greatest annual loss with 3.2 million hectares per year. This deforestation directly affects the freshwater habitats of the boto or Amazon river dolphin (Inia geoffrensis), as well as the Amazonian manatee (Trichechus inunguis).
After centuries of direct exploitation, pinnipeds have largely sought sites remote from human activities to give birth to their pups. They therefore are less likely to experience direct physical damage to those breeding habitats.
II. Chemical Pollution
Nearshore environments in particular are exposed to a potential wide range of pollutants as a result of industrial and agricultural activities. Those pollutants may concentrate in the food web and either degrade the habitat by removing important prey populations or cause health deficiencies in the local populations of marine mammal species. Although high levels of potentially damaging pollutants have been detected frequently in marine mammals, particularly seals and coastal small cetaceans inhabiting nearshore environments, direct causal links with health status have rarely been demonstrated. Baltic ringed (Pusa hispida) and gray (Halicharus grypus) seals during the 1970s had lesions of the reproductive system attributed to high PCB and DDT levels in their tissues. By the late 1980s-1990s, as levels in those pollutants declined, the proportion with lesions had declined substantially, along with an increase in their pregnancy rate. In an experimental study with harbor seals (Phoca vitulina). females fed with fish from the heavily polluted Dutch Wadden Sea had poorer reproductive success than those fed less contaminated fish from the North Atlantic. The effects were attributed to PCBs or their metabolites, and seals with the highest PCB intake were found to have reduced blood levels of thyroid hormones and vitamin A, both of which are known to be important in reproduction, including spermatogenesis.
Belugas (Delphinapterus leucas) in the highly polluted St. Lawrence estuary in North America had a high prevalence of tumors that had been attributed to carcinogenic compounds, such as polycyclic aromatic hydrocarbons, and other toxic compounds. such as PCBs. These were thought to account for low reproductive success in this population. However, although both sets of compounds occurred at high levels in this population, it remains difficult to demonstrate a direct link, and the population in fact appears to have increased in the 20 years since hunting ceased in 1979.
Harbor porpoises (Phocoena phocoena) inhabiting the continental shelf around the British Isles on postmortem are significantly more likely to be diseased when they have high hydrocarbon concentrations. Mass mortalities of striped dolphins (Stenella coeruleoalba) in the Mediterranean, common bottlenose dolphins (Tursiops truncatus) in the eastern United States, harbor seals in the North and Baltic Seas, and Baikal seals (Pusa sibirica) in Lake Baikal all showed significantly high concentrations of PCBs, which was thought to have reduced resistance to disease, thus making these populations more susceptible to virus infection.
Despite examples like these of apparent links between contamination and health status, the biological significance and the nature of effects remain uncertain, and it has been impossible to demonstrate conclusively that demographic changes to a population can be attributed to pollution. The only exceptions are where pollution can be shown to lead directly to mortality. After the Exxon Valdez went aground in Price William Sound, Alaska, in 1989, releasing large volumes of crude oil. several thousand sea otters (Enhydra lutris) and about 300 harbor seals died as a result of their oiled pelts losing vital insulation properties.
III. Competition with Fisheries
Habitats comprise animal and plant communities in an often complex web of interaction. When one or more members of the community are removed in large numbers this can have repercussions throughout the food web, altering predator-prey relationships and competition for resources. Following the intense exploitation of large baleen whales in the Southern Ocean during the first half of the 20th century, it was estimated that their overall biomass was reduced from 43 million tons to about 6.6 million tons, and that this made available a “surplus” of about 153 million tons of krill. These massive changes to the food web of the Southern Ocean had important effects on the remaining members, with individual whales growing faster, reaching sexual maturity at an earlier age, and exhibiting increased pregnancy rates. Similar changes in life history parameters were seen in other marine species, such as the Antarctic crabeater seal (Lobodon carcinophaga) and several seabird species.
During the 20th century, fisheries around the world intensified to such an extent that major changes in fish stocks have been observed for many species. Rarely, however, is it possible to show that prey depletion has reduced the numbers of a particular marine mammal species. Many marine mammals have catholic diets and appear to respond by switching prey. The relative ease of capture and nutritive contents of different prey species may vary, but it has scarcely ever been possible to demonstrate that these have affected reproductive or survival rates, and hence led to a decline in that population. More often than not. the species appears to respond by shifting its distribution.
On both sides of the North Atlantic, fishing activities have reduced the stocks of Atlantic mackerel and herring markedly, resulting in other fish (upon which they prey), such as sand lance, sprat, and gadoid species, becoming very abundant locally. Not only did some cetacean species such as harbor porpoise and humpback whale (Megaptera novaeangliae) switch their diet to include those prey in greater amounts, but some also showed geographic shifts in distribution. Gray seals, feeding largely on sand lance, increased in number in the North Sea at around 7% per year, whereas right whales (Eubalaena glacialis), feeding largely on plankton (the prey of sand lance) in the northwest Atlantic, showed local declines. When some local sand lance and sprat populations crashed a few years later, further changes were witnessed. In the Gulf of Maine, for example, fish-eating humpback and fin whales (Balaenoptera physalus) were replaced by plankton-eating right and sei (B. borealis) whales, harbor porpoises moved nearer to shore, and Atlantic white-sided dolphins (Lagenorhynchus acutus) became abundant and white-beaked dolphins (L. albirostris) rare.
In the Bering Sea and Gulf of Alaska, substantial declines in the numbers of Steller sea lions (Eurnetopias jubatus). harbor seals, and northern fur seals (Callorhinus ursinus), as well as several species of fish-eating birds, have occurred since the 1970s. Although other factors may also be involved, most ol these declines have been attributed to a decline in food availability resulting from the development of the walleye pollock fishery, a key prey species for many of these marine mammals following the demise of local herring stocks. Similarly, the collapse of productivity of the Barents Sea ecosystem, brought on partly from excessive fishing mortality, has had far-reaching effects on a range of species from seabirds to marine mammals.
IV. Disturbance
Sounds are introduced into marine and freshwater environments from a wide variety oi sources: motor-powered vessel traffic of various sizes; active sonar for object detection, including fish finding; seismic exploration and subsequent drilling and production for oil and gas; explosions from military exercises and ocean science studies; and marine dredging and construction. Most of the sounds produced are concentrated between 10 and 500 Hz frequency. However, speedcraft of various types generate noise mainly between 2 and 20 kHz by cavitation of the propellor, and sidescan and military sonar generate sounds between 2 and 500 kHz.
Among cetaceans, baleen whales appear to have rather different hearing sensitivities to those ol toothed whales and dolphins. The former are thought to be most sensitive at low frequencies below 5 kHz, and the latter above 10 kHz. Thus baleen whales are likely to be most vulnerable to large vessels, oil and gas activities, marine dredging, and construction, whereas toothed whales and dolphins may be more susceptible to recreational speedboats and most forms of active sonar.
Changes in behavior (e.g., movement away from the sound, increased dive times, clustering behavior) are often recorded in the vicinity of loud sounds. Few experimental studies have been conducted to test the nature and duration of negative responses. One such study in relation to low-frequency regular ATOC (acoustic thermometry’ of ocean climate project) sound pulses was conducted west of California. Aerial surveys showed no significant differences in numbers of marine mammals of any species between control and experimental surveys, but humpback and sperm whales (Phi/seter macrocephalus) were on average further from the sound source during the experimental periods. Although many other studies have reported negative reactions, there is very little information concerning the long-tenn impact of sound disturbance. In Hawaii, humpback whale mothers with their calves are thought to have shifted their distribution offshore in response to the high volume of recreational traffic. Whale and seal watching itself can impose pressures on marine mammals, disturbing seals from haul-out or breeding sites, and whales (and dolphins) from favored feeding areas.
In addition to those indirect effects where sound disturbance may interfere with or frighten marine mammals, there is some evidence that loud sounds can cause physical damage. Temporary or permanent shifts in hearing thresholds mav occur that could affect auditory acuity, and postmortem examination of humpback whales found dead in the vicinity of drilling operations has revealed ear damage.
Powered vessels pose an obvious threat to marine and freshwater mammals through direct damage. Collisions have been reported in a wide variety of species, and in some, such as the Florida manatee (Trichechus manatus latirostris) and the North Atlantic right whale, they are regarded as the major threat to their survival. With the advent of high-speed ferries in many parts of the world, ship strikes are being reported with increasing frequency, affecting especially the slower swimming species such as sperm and pilot whales.
V. Climate Change
As a result of emissions by humans of substances that deplete the ozone layer, our increasing use of hydrocarbons for energy and fuel, and large-scale deforestation and desertification, the world is experiencing climate change such that it is predicted that, in the next hundred years, temperatures will rise by 1.0-3.5°C and the overall sea level will rise by anywhere from 15 to 95 cm. Obvious consequences will be the melting of polar ice, drowning of coastal plains, and changes to shallow seas. Other less direct implications include an increase in the frequency and velocity of storms, and more extreme seasonal fluctuations in local climate (including, for example, El Nino Southern Oscillation events). Shifts in areas of primary productivity may lead to distributional changes for many marine mammal species, but some, such as the polar bear (Ursus maritimus), land-breeding pinnipeds, and coastal cetaceans and sirenians, may find it difficult to adjust to the loss of important feeding or breeding habitats. Already there is concern that less stable ice in some parts of the Arctic has reduced the availability of ringed seals (Pusa hispida) to polar bears, thus reducing the breeding success of the bears, which in those areas depend on this species for food.
During recent El Nino events, there has been reproductive failure in many seabird populations and some colonies of fur seals. During the 19S2 El Nino, for example, all Galapagos fur seal (Arctocephalus galapagoensis) females lost their pups due to starvation. However, many pelagic toothed whales and dolphins, being less tied to a particular locality, simply shifted their distributions: short-finned pilot whales (Globicephala maerorhijnchus), for example, left southern Californian waters following the departure of a species of squid, their main prey. Such changes can affect other members of the ecosystem. When the squid returned some years later, the temporarily vacant niche became occupied by another cetacean species, the Rissos dolphin (Grampus griseus).
Despite the many pressures on their habitats, marine mammals appear to be remarkably resilient, often living in highly modified coastal and riverine environments. Of course, because demographic changes may be slow and difficult to detect, we rarely know whether these are having negative effects. In the case of small local populations of endangered species such as the North Atlantic right whale, vaquita (Phocoena sinus), various river dolphins, monk seals (Monachus spp.), and manatee populations, the dangers of habitat pressures are all to obvious. However, even for other species, a precautionary approach would be prudent, and there is hope for the establishment of protective areas where human activities can be zoned.
