Distribution (marine mammals)


The majority of marine mammals are found in marine environments; some are found in rivers and estuaries and a few in freshwater lakes. Amphibious marine mammals (the pinnipeds, sea otters, and polar bears) also occur on land and ice. Distribution is the part of ecology that deals with how they use different geographic ranges in space and time. The complexity of distribution patterns of individuals or aggregations of individuals depends on how the different factors affect the species’ habitats, biological requirements, interactions with other organisms, and the patchiness of the environment.

I. Distribution Patterns and Preferences

Marine mammals are found in almost all the different marine environments, and their distribution varies according to the physical, chemical, and biological characteristics of the water masses. The effects of oceanographic phenomena, wind-induced movements (e.g., water currents, local divergence, and upwelling areas and water fronts) and the topography, can be used to characterize distribution. In the case of pinnipeds, the breeding and molting habitats on land or ice also characterize their distribution. In the polar bear, breeding and cub-rearing habitats are also relevant.

In freshwater environments, marine mammals are found in rivers and lakes. Examples of riverine species are the river dolphins (Platanistidae, Iniidae, Lipotidae, and Pontoporiidae) and the manatees. A few Phocidae live in freshwater inland lakes: the Saimaa seal (Pusa hispida saimensis) in Finland; the Caspian seal (Pusa caspica) in the Caspian Sea; the Baikal seal (Pusa sibirica) in lake Baikal; and the Ungava (common) seal (Phoca vitulina mellonae) in freshwater lakes of the Hudson Strait.

In marine environments, distribution can be generally described as coastal (in estuarine or near shore waters), neritic (in waters on the continental shelf), or oceanic (in waters beyond the continental slope, in the open seas or oceans). Examples of marine mammals that reside primarily in coastal waters are populations of bottlenose dolphins (Tursiops spp.), sea otters (.Enhydra lutris), and dugongs {Dugong dugon). Primarily neritic species include gray whales (Eschrichtius robustus), harbor porpoise (Phocoena phocoena), and California sea lions (Zalophus californianus). Primarily oceanic species include the sperm (Physeter macrocephalus) and beaked whales (family Ziphiidae). These generalizations should be used with caution, as many species occur in multiple habitats. Some species shift from one habitat to another seasonally, such as the switch from neritic feeding grounds to coastal migratory routes and breeding grounds by gray whales. Some species have populations that reside in a variety of habitats, such as the bottlenose dolphin, which occurs in coastal, neritic, oceanic, and, occasionally, riverine habitats.

Marine mammal distribution can also be classified according to general geographic areas. These are characterized by latitudinal bands and average water temperatures. Thus, marine mammals have tropical and/or subtropical, temperate, Antarctic, or Arctic distributions. Some species can be strictly included in just one of these categories, such as exclusively Arctic species: bowhead whales (Balaena mysticetus), polar bear (Ursus maritimus), narwhal (Monodon monoceros), and beluga (.Delphinapterus leucas), but, again, other species often h^ve multiple classifications. A clear example are the baleen whales that migrate from cold high latitudes to tropical low latitudes. Some species, such as the killer whale (Orcinus orca), are found in all the marine waters of the world, from the Equator to the Arctic and Antarctic. Finally, similar and closely related species may occupy different latitudinal (hemispheres, ocean basins) or longitudinal (different oceans and seas) ranges. Examples of pairs of similar species that occur in different hemispheres are the northern (Hyperoodon rostratus) and southern (Hyperoodon planifrons) bottlenose whales and the northern (Mirounga angoustirostris) and southern (Mirounga leonina) elephant seals. An example of very similar cetacean species with different distribution preferences within the same ocean basin are the long- and short-finned pilot whales (Globicephala melas and G. macrorhynchus).

Detailed data on distribution are provided in the species account of this topic and therefore only overall patterns by taxa are given in this section to avoid redundancy. Additional detailed description of marine mammal distribution can be found in Gaskin (1982) for cetaceans, in Riedman and Estes (1988) for the sea otter (Enhydra lutris), in Riedman (1990) for pinnipeds, in Reynolds and Odell (1991) for sirenians, arid; in Wiig et al. (1995) for the polar bear.

A. Cetaceans

Cetaceans live permanently in aquatic environments. They can be found in all the oceans and most of the seas of the world, and distribution patterns vary between and within families. The Balaenidae, the Balaenopteridae, the gray whale, the sperm whale, and the killer whale are found in polar, temperate, and tropical waters. They are found in the Northern and Southern Hemispheres, except gray and bowhead whales, which are only found in the Northern Hemisphere. As noted earlier, other strictly northern and also Arctic species are the narwhal and beluga. The pygmy right whale (Caperea mar-ginata) is only found in the Southern Hemisphere. Most delphinids live in tropical and temperate waters of both hemispheres. More tropical Delphinidae are Stenella attenuata, S. longirostris, S. frontalis, Steno bredanensis, Sotalia fluvi-atilis, Globicephala macrorhynchus, Pseudorca crassidens, Peponocephala electra, and Feresa attenuata. Other tropical odontocetes are the pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales, Irrawaddy dolphin (Orcaella brevirostris), and many Ziphiidae. Most Phocoenidae live in temperate or subtropical waters with some species exclusively in the Northern Hemisphere (P. phocoena, P. sinus, and Phocoenoides dalli) and some exclusively in the Southern Hemisphere (P. spinipinnis and P. dioptrica). All Delphinidae of the genus Cephalorhynchus live in temperate waters of the Southern Hemisphere. Of the river dolphins, the Amazon River dolphin, Inia geof-frensis, lives in the large lakes and tributaries of the Amazon and Orinoco basins. The franciscana (Pontoporia blainvillei) lives in the coastal central Atlantic waters of South America, but is commonly found in the mouth of the rivers and ocean waters surrounding estuaries. Similarly, the tucuxi is distributed in both fresh and marine waters. The two subspecies of the family Platanistidae (Platanista gangetica gangetica and P. g. minor) five in the major rivers of India and Pakistan, the Indus and Ganges, and the baiji (Lipotes vexillifer) lives in the Yangtze and formerly lived in some of the lakes along this extremely large inland river system.

B. Pinnipeds

Pinnipeds are amphibious mammals and spend most of their life in aquatic environments. However, they must return to land or ice for breeding (giving birth and rearing their offspring) and molt after breeding. Other possible reasons for hauling out are resting, thermoregulation, and escape from predators. Some common characteristics of nonaquatic habitats are space availability, isolation from predators, and proximity to food supply. Pinnipeds with tropical and temperate distributions find these conditions in isolated rookeries or beaches of remote places, which often are in islands. Ice characteristics condition the distribution and activity patterns of pinnipeds; pack ice offers a more constant substrate than fast ice, which varies highly seasonally in extension. Some pinnipeds reproduce in fast ice, such as the hooded seal (Cystophora cristata), and the duration of lactation and rearing of their young strongly depend on ice conditions. In general, seasonal changes in oceanographic conditions and ice cover condition the distribution of pinnipeds in the pack ice.

Among the Phocidae, geographical or latitudinal distributions include the Arctic, sub-Arctic, and temperate areas, subtropical and tropical areas, and sub-Antarctic and Antarctic areas. Antarctic seals are the Weddell (Leptonychotes weddellii), crabeater (Lobodon carcinophaga), leopard (Hydrurga leptonyx), and Ross (Ommatophoea rossii) seals. A sub-Antarctic and Antarctic seal is the southern elephant seal. In the Northern Hemisphere, tropical and subtropical species are the Hawaiian (Monachus schauinslandi), Mediterranean (Monachus monachus), and the extinct Caribbean (Monachus tropicalis) monk seals. Sub-Arctic and temperate-water seals are the gray (Halichoerus gryptis), harbor (Phoca vitulina), and northern elephant seals. Arctic and sub-Arctic seals are the harp (Pagophilus groenlandicus), hooded, bearded (Erignatus barbatus), ringed ribbon (Histriophoca fasciata), spotted (Phoca largha), Baikal (Pusa sibirica), and Caspian seal. Among phocids, harp, hooded, bearded, ribbon, spotted, Ross, and leopard seals breed in the pack ice; the crabeater and ringed seals breed in pack and fast ice; the southern elephant seal breeds on land and fast ice; the Baikal and Caspian seals on fast ice; the harbor and gray seals on land and ice; and the north-em elephant and the monk seals breed on land. Phocids with coastal and continental shelf distribution are the harp, harbor, gray, bearded, ringed, ribbon, spotted, Weddell, crabeater, leopard, and Mediterranean monk seals. Continental slope and oceanic seals are the Hawaiian monk, northern and southern elephant, Ross, and hooded seals. The walrus (Odobenus ros-marus, family Odobenidae) breeds on the pack ice and occurs in waters of the continental shelf. All the Otariidae breed and rear their offspring on land. Most of them disperse after breeding and therefore have neritic and oceanic distributions depending on season and reproductive status. Many Otariidae have subtropical or tropical distributions, such as the California (Zalophus californianus) and Galapagos (Z. ivollebaeki) sea lions and the Guadalupe (Arctocephalus townsendi) and Galapagos (A. galapagoensis) fur seals. The Steller sea lion (Eumctopias jubatus) is found from Arctic to temperate waters of the eastern North Pacific. The other sea lions are distributed in tropical and sub-Antarctic waters in the Southern Hemisphere: the Australian (Neophoca cinerea), New Zealand (Pho-carctos hookeri), and South American (Otaria flavescens) sea lions. In the Southern Hemisphere, all the fur seals are found in temperate or sub-Antarctic waters: the New Zealand (Arctocephalus forsteri), South African (A. pusillus pusillus), sub-antarctic (A. tropicalis), Australian (A. pusillus doriferus), Juan Fernandez (A. philippi), and South American (A. australis) fur seals. Only the Antarctic fur seal (A. gazella) can be strictly considered sub-Antarctic or Antarctic. In the Northern Hemisphere, the northern fur seal (Callorhinus ursinus) is found in the sub-Arctic and temperate North Pacific.

C. Sirenians

All the Sirenia are found in tropical or subtropical waters. The manatees have restricted ranges in different oceans and river systems. The west Indian manatee (Trichechus manatus) is found from southern North America and Caribbean to northern South America, in the western Atlantic, and the Amazon manatee (T. inunguis) in the Amazon drainage. In the eastern Atlantic, the African manatee (T. senegalensis) is found in western Africa, from Senegal to Angola. Manatees are coastal, although they may be found in continental shelf waters, transiting between islands, in the Caribbean. The dugong (Dugong dugon) is the most widely distributed sirenian, in the Indian and the western Pacific oceans, with a preference for shallow coastal bays.

D. Polar Bear and Sea Otter

The polar bear has a circumpolar distribution, mostly above the Arctic circle. It uses coastal, neritic waters, and breeds and rears its offspring on ice. Ice is also important for polar bears as a platform to travel, especially in the ice floes, between foraging areas and areas where they give birth and rear their young and as a substrate to hunt seals. The sea otter is found in the Pacific coasts of North America and Russia, essentially in temperate and sub-Arctic waters. It lives near shore and comes ashore on Aleutian Islands. Its distribution is conditioned by predators (e.g, killer whales) and food availability, such as the prey they usually use in kelp forests (e.g., sea urchins and abalone).

II. Factors Affecting Marine Mammal Distribution

Marine mammal distributions are affected by demographic, evolutionary, ecological, habitat-related, and anthropogenic factors. Demographic factors include the abundance, age, and sex structure of the marine mammal populations and the reproductive status and life cycle of individuals. Evolutionary factors include morphological, physiological, and behavioral aspects of the species’ adaptations. Ecological factors include biological production and use of prey, distribution of prey and predators, and competitors. Habitat includes factors such as water temperature, salinity, density, thermocline depth, and the type of substrate and the bathymetry. Anthropogenic factors are the human effects that alter the natural distribution of marine mammals, including pollutants, human-induced sounds, and incidental and direct kills. Distribution is the product of factors that act in a parallel or interactive way over different scales of space and time on each species, and sometimes on groups of species. As an example, baleen whale distribution depends on their ability to exploit planktonic organisms (evolutionary), the oceanographic characteristics of the water masses where they feed (habitat), and the trophic level they exploit (ecological).

A. Demographic Factors

The dynamics of marine mammal populations can determine distribution changes and patterns. The number of individuals that live in particular areas depends on the capacity of those areas to sustain their biological requirements. In general, the most critical requirements are prey availability and energy. The depletion of food resources by marine mammal populations influences the movement or dispersal to other areas. The age and sex structure of marine mammal aggregations also affect the distribution patterns. Habitat requirements for breeding females or females with offspring are not the same as those of adult males. In the case of odontocetes, females with calves may require coastal areas with locally abundant food resources and protection from predators. Adult males, not having to care for their offspring, are less limited in terms of movements and distribution range. In offshore dolphins, large cohesive aggregations may be required by breeding females for protection in the open ocean and foraging distances will be greater due to patchiness of their prey. In the case of pinnipeds, distributional differences according to age and sex classes and reproductive status are related to the seasonality of their life cycles, their adaptation to aquatic feeding, and their need to periodically return to land to breed. In breeding colonies, individuals will gather seasonally to mate, give birth, and nurse their pups over variable periods of time according to species. After the breeding season, pinnipeds often display age-related differences in habitat use and foraging areas. Dispersal according to age and sex classes is often associated with these characteristics.

B. Evolutionary Factors

All factors related to the secondary aquatic adaptation of marine mammals influence their distribution to some extent. Diving capacities in terms of duration and maximum depths allow particular species to exploit different habitats. In the case of sperm whales or elephant seals, deep diving allows access to prey unavailable to the shallower diving dolphins or porpoises. Hence, their distribution is associated with deep canyons and other deep ocean areas. In sperm whales, this ability also requires complex social systems that ensure the protection of newborns or youngsters, particularly while mothers spend long times underwater in search of prey. Another notable physiological adaptation is thermoregulation, which allows marine mammals to extend their distribution ranges from the warm equatorial waters to the coldest high latitudes. Efficient insulation and body temperature regulation systems allow the polar bear and the sea otter to spend a substantial part of their life at sea and survive in cold waters. The relative inability to regulate body temperature adequately in colder water of neonates is a hypothetical factor that leads baleen whales to migrate from the cold feeding grounds to the warmer calving grounds. Morphological adaptations, such as the feeding apparatus of baleen whales, also influence their distribution. As active filter feeders, they can capture planktonic (e.g., copepods, krill) or schooling (e.g., sand lance, capelin, herring) prey, which are abundant in the particular areas where whales distribute. Finally, the cohesiveness of large dolphin schools and the sensorial integration of individuals allow them to range in offshore areas, find food actively and efficiently, and obtain protection from predators.

C. Ecological Factors

Marine mammal distribution is in great measure related to prey distribution. The ability to exploit different trophic levels and resources classifies different marine mammals from top predators, such as the killer whales, to low-trophic level feeders, such as northern right whales or manatees. Marine mammals can be considered as either specialists or generalists, and these two aspects imply differentiated distribution patterns. Manatees, being specialist feeders, have restricted distributions where sea grass meadows provide continued food. Despite often being categorized as specialists, odontocetes or phocids tend to use a wide range of prey items. Thus, they can be distributed over wider ranges and change their distribution seasonally according to the availability of their prey. The killer whale, as a species, has a broad diet, yet different populations have more specialized diets: transient killer whales feed mainly on pinnipeds and other marine mammals but must range widely to maintain this diet, whereas resident killer whales feed on large fishes such as salmon. In both instances, the distributions of the whales are synchronized to the life cycles of their prey. The transients concentrate seasonally near pinniped rookeries, whereas residents live near the mouth of salmon-spawning rivers. In other cases, marine mammals tend to use the same home range, such as coastal bottlenose dolphins, feeding on different prey species that change their distribution seasonally. In this case, distribution patterns must be studied and interpreted at a finer scale. Interspecific competition is an additional ecological factor determinant of variable distribution. Violent attacks on harbor porpoises by bottlenose dolphins have been reported in their common range in the North Sea. Finally, predation plays an important role in the selection of habitats and distribution areas by marine mammals, especially those of smaller size, such as ringed seals. This species appears to select the fast ice to avoid predation by polar bears.

D. Habitat-Related Factors

Marine mammals are usually found in waters with high densities of principal prey species. These waters are characterized by the physical conditions that facilitate the accumulation of these prey. Relevant oceanographic variates characterizing marine mammal habitats are water temperature, salinity, density, chlorophyl concentration, and thennocline depth. These characteristics are related to upwelling fronts, often related to differences in species distribution. As an example, spinner and spotted dolphins range in the same areas of the eastern tropical Pacific, often traveling in the same schools. They occur in the same overall ocean area as common and striped dolphins, but appear to have preferences for water masses of different oceanographic characteristics (Reilly, 1990). Ocean topography and bathymetry are often related to local oceanographic phenomena that influence marine mammal distribution. Underwater canyons, marine ridges, and irregular topographies concentrate prey for deep divers such as sperm whales or elephant seals. In contrast, mysticetes often have preferences for shallow waters with high topographic variation. In these waters their prey accumulates at frontal interfaces between mixed and stratified waters. The ice is also a critical habitat element for marine mammals; the seasonal and highly dynamic changes of ice cover determine their patterns of change in distribution. It provides shelter during reproduction for pinnipeds, access to seasonally abundant food, and also delimits the distribution ranges of some cetaceans, such as the bowhead whale.

E. Anthropogenic Factors

Human alteration of habitats can change marine mammal distributions significantly. Marine mammals that haul out on land are particularly affected by habitat encroachment by human development. The three species of monk seals have suffered substantial changes in their original distributions, and one of them, the Caribbean monk seal, became extinct because of this. In the case of the Mediterranean monk seal, a major change in habitat preferences occurred as a result of human development, but also of deliberate kills for human uses. The seals changed their haul outs from open beaches to difficult-to-access caves, often with underwater entrance. This has created severe habitat fragmentation. Commercial exploitation has also affected marine mammal distributions greatlv. Whale stocks were reduced to the point that many original distribution areas are not used anymore. Overfishing of prev items has led to changes in marine mammal distributions. Pollution of coastal areas has degraded many original marine mammal habitats, thus affecting their original distributions. Human-induced changes in local water temperature have changed the seasonal distribution of the Florida manatee (Reynolds and Odell, 1991). This population previously migrated to warmer waters in winter but now uses the thermal vents in waters close to power plants and has changed migration patterns substantially. Expanding sources of sound in the ecosystem (e.g., large ship traffic, naval experiments) and pollution may also affect marine mammal distribution.

III. Movements and Seasonality

The distribution of marine mammals changes seasonally as their biological and ecological requirements change. Marine mammals respond to changes in the environment, such as in temperature, ice coverage, and prey availability. Daily requirements in terms of energy or protection against predators depend on the reproductive status and the season; these are clearly not the same for females with nursing offspring as for solitary males. Movements are a response to changes in the environment and the biological requirements of a species. In tropical areas, movements are expected to vary according to the patchiness ol the environment. Distances covered in short periods of time may vary depending on the conditions, but a very marked seasonality is not commonly found. In high latitudes, changes during the cold winter affect the distribution of marine mammals, their tolerance to physical conditions, and their life history requirements. Thus, seasonality is more marked.

Movements can be classified as migration, dispersal, and daily travel. Migration is the seasonal change between two geographic locations that is related to species reproductive cycle, changes in temperature, and prey availability. Dispersal is the movement from the place of birth to other areas in which individuals reach a feeding area, join a breeding population, or find another group of individuals with which to spend the next stage of its life. The classification of movements may be somewhat arbitrary because marine mammals do not always follow strict periodic patterns. They instead respond to the limitations of the environment in providing constant food or other requirements. Short-scale movements are difficult to detect and must be put in the context of the species life cycle before being classified. A typical example of migration is the one of baleen whales; humpback whales undertake long-distance travels, often thousands of kilometers, between the tropical calving grounds in winter and the high-latitude feeding grounds in summer. In contrast, most Otariidae have dispersal movements from their birth colonies toward different feeding areas or other breeding colonies when they reach sexual maturation. In any case, movement patterns vary among individuals, according to their age, sex, and reproductive condition. A prerepro-ductive young whale may delay its departure from the high-latitude feeding grounds to extend the feeding season, whereas a pregnant female must leave for the low-latitude calving grounds to give birth to its offspring.

A. Cetaceans

Cetaceans spend their entire life in aquatic habitats and are in constant movement. Understanding their seasonal distribution is more difficult than in pinnipeds for technical and logistical reasons; the manipulation and tagging of animals is less efficient and more expensive. Thus, classifying movements as dispersal or migration is even more confusing, except in some well-studied populations of baleen whales. Their life patterns and cycles make the concept of dispersal a little ambiguous, however, because seeming residency, site fidelity, or habitat discreteness may be just apparent, short-term attributes of their distribution. Only migration in large whales is known from long-term studies. Studies on migration range from the examination ol catch statistics of whaling operations to the use of modern telemetry technology. Contrary to classic accounts of whale migration, the most recent studies show how movements van’ across whale populations and species. Mysticetes appear to have periodical migrations with relatively consistent patterns over the years. Seasonal movements in odontocetes are far less consistent over time, including those of the sperm whale, which has been classified as a migratory species with marked seasonal patterns. In general, as in other marine mammals, factors inducing migration are the biological cycle, greatly determined by reproductive needs, and factors in the environment (e.g., prey availability, changes in water temperature). These factors may trigger the start of seasonal movements, although not all individuals will respond in the same wav.

Annual migrations are best known for species with more coastal ranges, such as the gray, right, or humpback whales. However, virtually all mysticete species are known to migrate. No data are available for the pygmy right whale. Most mysticetes have latitudinal migrations, from tropical breeding grounds to high(er) latitude feeding grounds. In breeding grounds, mating and calving take place. Migratory species are right, blue (Balaenoptera musculus), fin (B. physalus), sei (B. borealis), humpback, and gray whales. Bowhead whales also migrate, but their longitudinal movements are equal to or greater than their latitudinal changes, and thev never leave Arctic waters. Bryde’s (Balaenoptera edeni), common minke (B. acutorostrata) and Antarctic minke (B. bonacrensis) whales, however, have less clear movement patterns. Biyde’s whales spend most of the year in warm tropical waters and calving does not have the same marked seasonality seen in other balaenopterids. This indicates a possibly different reproductive cycle, in which both whales feed and mate year round. In this case, whale movements are more similar to those of many odontocete species, in constant search for food, with variable utilization of prey, and different prey types through the year. Among the best known migrations are those of the gray and humpback whales. Gray whales migrate annually from feeding grounds in the Arctic to their calving areas in the lagoons of Baja California in Mexico. Interannual changes in the timing and numbers reaching the different migratory destinations have been observed. The migration of humpback whales is also very well studied, and sperm whales are the best example of long-range migration in odontocetes.

Movements in odontocetes have different scales depending on geographical areas, family, and species. It is generally accepted that most movements are in response to prey availability, and the largest movements, often called migration, occur in oceanic odontocetes. In the eastern tropical Pacific, movements are reported to be wide. Several species of Stenella had daily movements of 53 km/day and hundreds of kilometers over months, and these reflected seasonal changes in distribution. Dolphins moved inshore, toward the American continent, in fall and winter and offshore in spring and summer. Other methods, such as line transect surveys in California, have shown how several dolphin species have different patterns of abundance and distribution depending on the season. Pacific white-sided dolphins (Lagenorhynchus oblicjuidens), Risso’s dolphins (Grampus griseus), common dolphins (Delphi-tin s spp.), and northern right whale dolphins (Lissodelphis borealis) were less abundant in summer than in winter, and significant north/south shifts in distribution were reported for Dall’s porpoises and common and Pacific white-sided dolphins. Significant inshore-offshore differences were found for the northern right whale dolphin. Some dolphin species show variable distribution patterns, such as bottlenose dolphins or killer whales. Difference in patterns has been attributed to different varieties or ecotypes of the same species. In the case of bottlenose dolphins, a well-studied coastal population (Scott et al., 1990) showed a year-round residency with slight seasonal changes within the population home range. It has been argued that dolphins in Florida follow the mullet migrations into the Gulf of Mexico during the fall. Short-term movements have also been observed in a resident bottlenose population in east Scotland. In contrast, Atlantic offshore bottlenose dolphins, described as a possible different form, have wider movements and a broader distribution (Wells et al, 1999).

B. Pinnipeds

Migration is not uncommon in pinnipeds, and the advent of new telemetry has helped describe the migratory movements of several species. Dispersal is very common in pinnipeds and depends on the abundance of prey, its energy content, and the seasonality of prey distribution. In addition, their reproductive cycle mandates that individuals return to land or ice to give birth, nurse, and rear their offspring and molt. Pinnipeds also haul out for resting, thermoregulation, and to escape predators, among other reasons. If the environment provides constant food resources, such as in some tropical areas, there will not be a clear need to disperse. In contrast, pinnipeds living in high latitudes will be more dependent on ice cover, availability of seasonally changing prey, reproduction, and population size. These will create density-dependent effects, such as dispersal and distributional changes. Thus, dispersal can vary with latitude, based on the stability of prey resources.

Phocids appear to migrate more than otariids, as they generally live in higher latitudes, where the environment (e.g., the ice cover) is more variable (Bowen and Siniff, 1999). For otariids that live in tropical areas with a more constant environment, habitat regulates the growth of the standing colonies and conditions of dispersal. Otariids also have longer lactations and rearing periods than seals. Their breeding behavior and requirements in terms of habitat are also different, allowing them to stay longer in the breeding colonies. Periodic events that lead to drastic changes in food availability or other environmental limitations, such as the El Nino southern oscillation (Trillmich and Ono, 1981), also favor dispersal. Among otariids, only the northern fur seal has a well-studied and distinctive migration.

Both elephant seal species and the hooded and the harp seals are good examples of migratory seals. Northeni and southern elephant seals spend between 8 and 10 months at sea each year, with long-distance migrations between breeding and molting sites. Both species have two long migration trips between postbreeding and postmolting areas. The northern elephant seal migration, of between 18,000 and 21,000 km, is the longest reported for any mammal.

Harp and hooded seals have interannually variable distribution patterns, dependent on the time of the year, the geographic location, and the density of individuals in the breeding colonies. Harp seals live in colonies in the subarctic pack ice, where breeding takes place. The largest population of this species is in Newfoundland. Individuals from this population start their southward migration in late September along the coast of the Baffin Islands and go eastward through the Hudson Straits, reaching Labrador between October and December. There are variations in migration timing and patterns between age classes.

Gray seals from numerous colonies in the British Isles and gray seal pups disperse widely during the first year. Adult seals show high variability in their movements along the coasts of Scotland, especially in postbreeding periods and after the molt, from March to May. Although long-distance travel by adults occurs, short travel at close range from particular haul outs is more common. Juveniles tend to spend longer periods at sea. (Hammond et al, 1993).

C. Sirenians

The best-studied movements by sirenians are of manatees in waters of Florida. Water temperature is a major determinant of seasonal movements of Florida manatees, and dispersal is higher in warmer months. In winter, manatees tend to aggregate in areas of warmer waters, such as natural freshwater springs or the outfalls of power plants (Reynolds and Odell, 1991).

D. Polar Bear

Seasonal movements in polar bears have been reported in all their distribution range. Long-range movements also occur and are mostly related to the ice cover and extent. Predation on seal pups also influences movements, and bears disperse more during pinniped pupping seasons. In summer, when ice melts in many areas, bears move to land, where they remain for a few months, before leaving in November-December. Pregnant females stay longer on land.

IV. Study of Marine Mammal Distribution

The study of distribution depends on each species’ habitat and its abundance, so that scale is a significant factor. Distribution changes have to be interpreted in space and time, and different methods are to be used according to species range and density. The distribution of a species occupying an extensive ocean area is best studied by air- or shipboard surveys following systematically placed transects. In surveys, visual and/or acoustic data on species are collected according to predetermined protocols. Oceanographic variates and data on position of individuals can be incorporated in spatial modeling (Hedley et al, 1999). Results of this modeling on repeated surveys can be compared to study seasonal patterns and changes over time (Forney and Barlow, 1998). In species that live in fragmented habitats and that are not abundant, knowledge of the location of animal aggregations is essential. In these cases, the best possible information is obtained from telemetry studies using high-frequency radio tags, satellite-linked radio tags, and geoloca-tion time-depth recorders (Bowen and Siniff, 1999). The use of telemetry devices is also essential in understanding seasonal movements and patterns. The life of the batteries and permanence of the tags in the animals are critical to the duration of the studies. The study of habitat, an integral part of distribution, changes with the species life time because of the above-mentioned factors. It is difficult to monitor a cohort of animals, ideally tagged since birth, because of the long average life span of marine mammals. In practice, the general distribution patterns of a marine mammal population are the sum of the individual specific movements over space and time. Monitoring just a few animals over a restricted time duration (e.g.. that of a telemetry device battery) produces partial information on the overall patterns and may show a high variability between individuals. Therefore, inferences at the population level must be made cautiously to avoid biased perceptions of the species distribution.

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