The health of any animal is affected by age, behavior, and environment. Like terrestrial species, marine mammals are subject to infection, injury, and metabolic disturbances. Our understanding of marine mammal health has been impeded not only by the difficulties inherent in studying these species in the wild, but also by their unique biology. In recent decades, the challenge has been compounded by human impacts on the health of marine mammals and their environment.
I. Adaptations to Life at Sea
Cetaceans, sirenians, pinnipeds, and sea otters, although taxonomically distant, have evolved similar biological mechanisms to cope with a marine existence. These functional adaptations include strategies for controlling body temperature, diving, maintaining salt and water balance, and promoting reproductive success—adaptations vital to health and survival.
A. Thermal Balance
Except for some areas in the tropics, the sea is almost always cooler than mammalian body temperature. Even a few degrees difference is enough to drain a mammal’s thermal reserves, as water steals body heat about 20 times faster than air. To cope with this drain, marine mammals have generally evolved mechanisms to conserve body heat. Of these, blubber has arguably been the key to evolutionary success. This coat of fat provides cetaceans and certain pinnipeds with mechanical protection, warmth, buoyancy, nutrients when food is scarce, and fresh water in reserve. Otariid pinnipeds have thinner blubber and less body fat than phocids or the walrus (Odobe-nus rosmarus) and are thus less tolerant of cold and depend to a certain extent on their pelage for insulation. This is especially true for otariid pups, which mav not acquire an adult coat or adequate fat until tliev are about 3 months old and, in the meantime, are prone to hypothermia when they become wet. Species with less blubber rely on other strategies. The sea otter (Enhtjdra lutris) depends entirely on a high metabolic rate (and high caloric intake) to generate heat and on its dense, well-groomed fur to prevent heat loss. The living sirenians, with low metabolic rates and little ability to control surface heat loss, are functionally restricted to tropical and subtropical waters.
Environmental temperature has more than a subtle bearing on health. To survive in a cold climate, a marine mammal must be robust, appropriately insulated, and have all surface heat control mechanisms operating. If not, the only recourse is to increase the metabolic rate and either eat more or borrow fat from vital fat reserves. Ironically, as a last measure to conserve heat, pinnipeds and sea otters may haul out on land where the prospect of feeding is hopeless.
Overheating is rarely a problem for a marine mammal in water. In a warming environment, a whale may eat less and metabolize blubber, which effectively reduces insulation, and shed excess heat by increasing blood flow to the skin, particularly of the extremities. Losing heat on land is not as easy. A wet seal or sea otter may get some comfort from evaporative cooling, but once it is dry, it depends mostly on circulatory and behavioral adaptations (e.g., seeking shade, sleeping, mo\ing to the surf zone) to avoid hyperthermia. These strategies work to a point. A sea otter out of water can become distressed at air temperatures as low as 10°C and die within hours at 21°C. A cetacean stranded on a sunny beach can literally cook inside its own blubber.
B. Breathing and Diving
Marine mammals forage at all depths. While sea otters and sirenians may have little need to dive deeply or for more than just a few minutes, some species of phocids and odontocete cetaceans make prolonged dives to 2000 to 3000 feet or more. Such deep diving is made possible by a suite of adaptations for coping with pressure and potentially deadly nitrogen, and for storing and utilizing oxygen efficiently. During a prolonged dive, circulation to the skin and viscera may almost cease, allowing oxygen to be channeled to organs that need it most, such as the heart and brain.
Deep dives may require a shift to anaerobic metabolism. Such dives may be useful for escape and exploration but are costly in terms of time and energy. For most animals, survival depends on obtaining sufficient prey within the depth and time limits imposed by aerobic diving capacity, which in turn depends on the species and the size, age, and health of the individual. Because of their relatively greater capacity to store oxygen, large animals tend to be better divers. It is not surprising that juveniles may find it difficult to reach prey that is easily accessible to adults.
C. Salt and Water Balance
The osmotic concentration of the sea is nearly four times greater than that of mammalian body fluids. Chemical equilibrium thus favors loss of body fluids into the sea and encroachment of salts into the animal. Marine mammals have evolved a number of strategies to prevent this from happening: (1) external surfaces are impermeable to seawater; (2) body water is highly conserved—sweat glands are either reduced or absent and the kidneys efficiently concentrate urine; (3) they drink little seawater and acquire most of their fresh water from food (water makes up about 70% of a fish. 80% of a squid, and over 90% of aquatic plants, and each gram of dietary fat or metabolized blubber yields close to its weight in fresh water). In pinnipeds and cetaceans, the physiological response to stress is also designed to conserve water. Secretion of aldosterone, a hormone produced by the adrenal cortex, promotes the resorption of sodium from the kidney, thereby drawing water back into the body. Maintaining electrolyte balance thus depends on adequate blubber, well-functioning kidneys, proper hormonal balance, and a healthy, intact epidermis.
D. Strategies for Rearing Young
The social, physical, and biological conditions that together create a healthy environment are especially critical during the period of an animal’s life when it depends totally on its mother. Vulnerabilities can often be predicted on the basis of species, location, patterns of maternal care, and environmental conditions. For example, inanv pinnipeds have evolved strategies to ensure breeding opportunities for animals that may be dispersed for much of the year, with the result that pupping and mating occur at predictable times and locations. While an effective strategy for the population, consequent crowding on rookeries can increase the risk of injury, infection, and disease transmission for individuals.
II. What Can Go Wrong?
Body systems work together, each complementing the others. Impairment of one system can disturb the entire equilibrium, leading to secondary problems, which then become a threat. For example, blubber is a source of energy, insulation, water reserves, and buovaney. If it becomes depleted because food is unavailable, the animal may eventually be unable to rest at the surface, maintain body heat, forage, escape predators, or keep up with a group. The ensuing stress may open the door to infection, further reducing the chance of survival.
Injuries and illnesses are not always apparent and are often detected only after analyzing blood or tissue samples from a living animal or dissecting a dead one. Even careful study might not reveal serious biochemical and physiological conditions. Stress is poorly understood and difficult to quantify. What little is known about the process in marine mammals shows that it can disrupt thyroid and adrenal cortical function, water and electrolyte balance, and metabolism and reproduction, and can decrease circulating levels of certain blood cells, perhaps reducing resistance to parasitic infection and compromising immune responses.
A. Reproductive Failure and Death of the Newborn
An orderly, coordinated progression of biological and behavioral factors is required for an animal to reproduce successfully. Weakness or disruption at any point can result in reproductive failure, evident as abortion, stillbirth, premature birth, or death of the newborn. The causes of reproductive failure are often obscure, particularly in species that cannot be studied from shore.
In some species, the risk of abortion or stillbirth appears to be greater for first-time mothers. Young mothers are also often smaller and give birth to smaller offspring, which are more vulnerable to hypothermia and injury. The health and nutritional condition of any mother, regardless of age or size, affects the fetus. Decreased prey abundance, such as associated with El Nino events, may be associated with decreased fertility, increased abortions, and reduced pup production. Infectious disease [e.g., morbillivirus in harbor seals (Phoca vitulina) in Europe] may also lead to increased premature births and abortions. In fact, a rise beyond the expected level of reproductive failure may be one of the first signs of an environmental disruption, such as a viral epidemic, reduced prey stocks, or high levels of certain anthropogenic contaminants.
A successful birth is no guarantee of prolonged survival. Neonates that are weak at birth or suffer from serious congenital defects soon die. Healthy neonates may, for one reason or another, be abandoned by their mothers or face an early death if the mothers fail to provide proper care, whether due to illness, injury, disturbance, or simply to inexperience.
B. Starvation
Marine mammals spend much of their time searching for food of the appropriate type, size, and quality to satisfy needs that may vary seasonally and with age. Some animals, e.g., dependent young, the sick, and the very old, can starve even when food is plentiful. Many factors determine how long an animal can survive without food: its age, fat reserves, metabolic rate, energy demands, and general health. Large animals with low metabolic rates survive longer than those with high energy demands, such as small species, newborn, and growing pups. Baleen whales may feed very little for 6 to 8 months of the year, but a sea otter without food for even 2 days can die from gastroenteritis and shock. Starvation is a major cause of death in pinniped and sea otter pups.
Throughout the period of dependency, a young animal’s survival hinges on the health of its mother. Before giving birth, a phocid seal or baleen whale must develop ample fat reserves to carry it through a period of fasting or reduced feeding during lactation. The pup or calf born of a malnourished mother is at risk from the moment of birth and its longevity is compromised early in its development. The young of species in which females feed continuously during lactation face a different threat. A bottlenose dolphin (Tursiops truncatus) calf depends on the state of its mothers nourishment throughout what may be a year or more of nursing. An otariid pup risks starvation if a shift in prey abundance forces its mother to spend longer periods away from the rookery.
Weaning frees a young animal from dependence to face the challenge of providing for itself. Manatees (Trichechus spp.) and some cetaceans and otariids remain with their mothers long enough to learn foraging skills. Not so for all sea otters. Newly independent juveniles, handicapped by high metabolic demands and inexperience, and at a time when erupting teeth create problems with chewing, often starve. Females may be especially vulnerable because they tend to remain in established areas of the range even when prey become depleted.
Depletion of food stocks, whether from overexploitation, overfishing, or climatic or oceanographic fluctuation, can affect entire populations. Food scarcity in one area may cause some animals to move elsewhere. When food abundance changed during the El Nino of 1982-1983, California sea lions (Zalo-phus californianus) moved northward, and many northern fur seals (Callorhimis ursinus) may have emigrated from San Miguel Island to rookeries in the Bering Sea. Some animals are unable or unwilling to make such excursions, e.g., females with pups, territorial males, or populations in remote ranges. When fish disappeared from surface waters around the Galapagos Islands during the 1982-1983 El Nino, widespread starvation among the islands’ fur seal population (Arctocephalus galapa-goensis) soon followed.
Starving animals eventually die—some quickly, as would a pup deprived of milk or a sea otter overcome with hypothermia and exhaustion. Others die after a period of illness triggered by malnutrition and mediated by factors such as hypothermia, dehydration and electrolyte imbalance, hormonal disturbances, and infection by parasites and opportunistic pathogens. Some starving seal pups may ingest whatever is nearby—gravel, stones, or grass—and consequently die of an impacted stomach.
While a sudden shortage of prey may cause outright starvation of large numbers of animals, the more subtle effects of nutritional stress, including low productivity and decreased juvenile survival, may prove equally damaging to a population.
C. Direct Environmental Effects
Extreme weather conditions can take a toll on all age classes. Among Florida manatees (Trichechus manatus latirostris), intense or prolonged cold weather can cause mortality equivalent to 1.5-2% of the estimated population, with the greatest impact on juveniles. Storms hitting a crowded pinniped rookery at the peak of breeding season can be disastrous: pups become hypothermic, are battered on rocks or drowned, are separated from their mothers and starve, or become victims of adult aggression. Unusual ice conditions can be hazardous for cold-water species. Sea otters trapped out of water by heavy ice soon die of starvation, stress, and shock. An untimely freeze in polar waters can trap cetaceans in ice, where they may ultimately suffocate or starve. Ice-breeding seals can drown or be crushed in large numbers if their ice floes are broken up by storms or unseasonably warm temperatures.
D. Trauma
For most marine mammals, the risk of injury is continual, whether from natural sources, such as storms, predators, and aggressive encounters, or human activities, such as fishery operations and recreational boating. For example, injuries are common on pinniped rookeries, where pups are often trampled accidentally or attacked by adults, fall into gullies or crevices, or wash off unprotected beaches into pounding surf. Adults can be victims as well, as bulls compete for territories and females, and females compete for space.
Historically, commercial hunting had serious impacts on certain species or stocks of marine mammals. Today, more animals die in accidents; interaction with fishers is a leading cause of death and injury. Pelagic odontocetes die in purse seines and drift nets, coastal cetaceans and pinnipeds in gill net and trawl fisheries, and some river dolphins by fishing methods that employ electricity and explosives. Marine mammals thought to compete with commercial operations may be killed deliberately.
Entanglement in discarded net fragments, ropes, packing bands, monofilament line, and other debris is a risk for many species. The effects on populations vary; some suffer no appreciable impact, whereas others may be seriously threatened. For the individual, the problem is always serious. An animal that does not drown immediately may escape with fractures and internal injuries or may carry net fragments, ropes, or bands that increase drag, impede swimming ability, or become snagged. A seal pup growing into its packing-band “collar” will eventually die, either from suffocation or from deep cuts and infection.
Coastal dwellers are vulnerable to injury from a variety of human activities. For example, many dugongs (Diigong dugon) in Queensland (Australia) waters have died in shark nets set to protect public beaches. Right whales (Eubalaena glacialis) in the northwest Atlantic and manatees in Florida are injured or killed by collisions with vessels at rates that jeopardize these populations.
E. Predation
There are times in a marine mammal’s life when it draws the attention of predators. Probably the easiest meal is a small, inexperienced animal that can be found in a particular place on schedule—criteria often met by young pinnipeds, whether on land or ice or at sea. As some examples, arctic foxes and polar bears break into ringed seal (Pusa hispida) birth lairs to take pups and. sometimes, their mothers. Steller sea lions (Eume-topias jtibata) on the Pribilof Islands eat young northern fur seals that venture into the water, whereas southern sea lions (Otariaflavescens) raid South American fur seal (Arctocephalua nustralis) rookeries, driving away the adults and killing pups. Leopard seals (Hydmrga leptonyx) consume large numbers of crabeater seals (Lobodort carcinophaga) from the time the weaned pups leave the safety of the ice until they are several months old and large enough to escape attack. Killer whales (Orcinus orca) patrol some pinniped rookeries and may work vigorously to wash a seal into the water or even chase one onto the beach; other pods may attack baleen or sperm (Phijseter macrocephalus) whales. Sharks pose a danger to many species or populations, including sea otters along California and Hawaiian monk seals (Monachits schauinslnndi).
The impact of a predator can extend beyond its effect on the individual prey. Killing a pregnant mother with a dependent young removes not one, but three animals from the population. A female northern elephant seal (Mirounga angustiroatris) may recover rapidly from a shark attack, as many seem to do, but may be less able to nurse her pup and is unlikely to mate in the compressed breeding season. In this case, a single attack, while only injuring the mother, may have cost the population two pups.
F. Parasites
Almost all marine mammals are infected by parasites by the time they are weaned or shortly afterward. Most of these parasites have evolved with their hosts and, under normal circumstances, cause little damage to otherwise healthy animals. Among these are the amphipods and copepods that eat bits of whale skin, seal lice that normally occur in small numbers and consume insignificant amounts of blood, and gastrointestinal helminths (“stomach worms “). Others are harmful enough to affect the well-being of individuals and even segments of a population. For pinnipeds, these include heartworms, some lungworms, and the hookworm Uncinaria lucasi; and in cetaceans, the nematodes Crassicauda spp. (in the mammary glands, cranial sinuses, and kidneys) and the trematodes Na-sitrema spp. (in the cranial sinuses) and Campula spp. (in the liver and pancreas). However, any parasite can become destructive when the mechanisms that maintain the host-parasite balance break down, as they do when an animal is ill or starving. Prolonged stress, by retarding wound healing and destroying protective blood cells, can set the stage for a parasite to do the most harm. Debilitated animals that come ashore often suffer from serious parasitic conditions.
An animal’s parasite burden can offer clues to its overall health and to changes in its environment, such as alterations in prey abundance. Seal lice are transmitted and proliferate on the animal only on land. A heavy infestation requires that the animal be on shore a long time, one sign that it may be ill. A fast-swimming odontocete offers barnacles little opportunity to attach; the presence of species such as Lepas sp. or Xenobal-anus sp. on a dolphin’s flukes or dorsal fin suggests that the animal has been moving unusually slowly, a common sign of illness. Differences in parasite fauna can indicate differences in feeding habits. For example, walruses feeding on benthic invertebrates have few if any nematodes in their stomachs, whereas those that eat fish have more. The relationship between diet and parasitism is predictable enough that variations in parasite burden may be used to distinguish populations and help identify segregated social groups.
G. Microorganisms
Microorganisms of all kinds—bacteria, fungi, and viruses among them — abound in the sea. Some are of the types found on land and in land dwellers; others, including certain Vibrio bacteria, thrive only in aquatic habitats. Like terrestrial mammals, marine mammals harbor many organisms that may be regarded as normal. Few of these are necessarily pathogenic, meaning they do not always cause infectious disease, but some are more threatening than others. The fine line between infection and infectious disease depends on both the virulence of the organism and the susceptibility of the host, which is determined by the history of previous contact with the organism and the health of the animal’s immune system. Age is also a factor. A very young animal may be protected by maternal antibodies, which protect it against organisms with which the mother has earlier come into contact. The pup or calf then develops its own active immune capability, which affords increasing protection until its declining years, when immune function once again weakens. For these reasons the very young and the very old are more likely to acquire infections. Of course, natural and human-related stresses may compromise immune function in animals of all ages.
1. Bacteria The nature and severity of bacterial infections can be influenced by the animal’s behavior and age, and environmental conditions. Habitat also plays a determining role. A phocid pup born on clean sand is less likely to acquire a serious navel infection as it drags its unhealed umbilicus across the rookery than a pup born in areas fouled by ieces, stagnant water, or decaying vegetation. For pups in fouled environments, bite wounds provide another route for infection by bacteria such as Streptococcus sp, and Confnebacterium sp.
Infections are sometimes predictable. During molt, seals slough skin and hair. In northern elephant seals the process is exaggerated and large sheets of epidermis are lost: many animals, particularly yearlings, come ashore with skin infections during this time. Weddell seals (Leptomjchotes wcddellii), which use their teeth to maintain breathing holes in ice, and sea otters that feed on hard-shelled prey grind down their teeth to such an extent that they develop abscesses and bone infection.
A few bacteria are inherently pathogenic. Leptospirosis, caused by the spirochete Leptospira sp., occurs in domestic and wild animals worldwide. Infection in California sea lions has caused kidney disease in juvenile and subadult males and abortion in females. Mycobacteria of the complex associated with tuberculosis (M. bovis, M. tuberculosis) are of growing concern. An outbreak in a captive colony of New Zealand fur seals (Arctocephalus forsteri) and Australian sea lions (Ncophoca cinerea) was the first indication that this disease, subsequently found in free-ranging pinnipeds from Australia, New Zealand, and South America, may be endemic in certain wild populations. Bacteria representing an apparently new strain or species of Brucella have been found in a number of marine mammal species, primarily from the North Atlantic and Artie oceans. Although implications for marine mammals are uncertain, infection in terrestrial mammals commonlv leads to abortion.
The impact of an infection on animal health depends on the organ involved. An isolated abscess in a muscle may have little apparent effect, while a similar infection in the lung can be seriously debilitating. Bacterial pneumonia, often associated with lungworms, can be serious enough to cause death or stranding. Infections that increase metabolic stress or disturb water and electrolyte balance, such as gastroenteritis with vomiting and diarrhea, can be rapidly fatal.
2. Mycotic Infections Fungal organisms rank low on the list of primary pathogens of marine mammals. They tend to infect animals that are weakened, perhaps by other chronic debilitating disease. Infections are usually acquired from soil-, dust-, or water-borne fungi and enter the body through the skin or by inhalation. A wide variety of organisms have been isolated from marine mammals, including Candida, Aspergillus, Coc-cidioides, Blastomyces, Histoplasma, Fusarium, Nocardia, and Loboa.
Lobomycosis, a skin infection caused by the yeast Loboa loboi, has an unusual range. The disease occurs in free-ranging and captive bottlenose dolphins from Florida waters and in tu-cuxi (Sotalia fluviatilis) in South America. Curiously, other than in cetaceans, Lobo’s disease occurs only in people inhabiting low-lying wetlands of Central and South America.
Coccidioidomycosis, generally a rather innocuous fungal disease of domestic animals, was until recently considered rare in marine mammals. What might be described as outbreaks of infection in California sea lions and sea otters between 1986 and 1994 coincided with a rise in human infections, attributed to environmental conditions that favored the growth of Coc-cidioides imitis.
3. Viruses First recognized in the late 1960s, viral infections in marine mammals have emerged as the greatest cause of large-scale mortality. To spread rapidly, a virus requires a naive host population of a minimum density, which can arise either through population growth or changes in social behavior. Once infected, a migrating or wandering animal may carry the virus to new habitats.
More than 450 harbor seals (Phoca vitulina) died in a disease outbreak in New England during the winter of 1979-1980. The cause was found to be an influenza virus of avian origin that had infected the seals, probably as they hauled out on the rookeries of Cape Cod. Seals oi all ages developed pneumonia, which forced many out of the water and onto crowded beaches where the virus could spread easily from seal to seal by aerosol transmission. This was the first marine mammal die-off of demonstrated viral origin.
More devastating were the outbreaks of morbillivirus infection that swept through populations of pinnipeds and cetaceans in Europe in the late 1980s and early 1990s. The series of epidemics began with the outbreak of canine distemper vims infection that killed thousands of Baikal seals (Pusa sibirica) in 1987-1988. A related morbillivirus (phocine distemper virus) killed about 17,000 harbor seals and a few hundred gray seals (Halichoerus giypus) in Europe in 1988-1989. Between 1990 and 1992, another morbillivirus (dolphin morbillivirus) killed thousands of striped dolphins (Stenella coeruleoalba) in the Mediterranean Sea. Infected animals developed pneumonia, fever, and neurological disorders associated with encephalitis. The immunosuppressive effect of these viruses led to the development of secondary, often overwhelming, infections by bacteria, fungi, and other viruses.
Studies since 1988 indicate that morbillivirus infection, often without recognized illness, is common in many marine mammal species and may have occurred in many North Atlantic marine mammal populations prior to the European epidemics. The outbreaks in European seals may have been the result of viruses, perhaps introduced by infected migrating harp seals (Pagophilus groenlandicus) entering naive populations that were dense enough to support transmission. The epidemic in striped dolphins showed that the brief periods during which dolphins surface in unison, and perhaps some behaviors underwater, may be enough for a viral infection to spread rapidly from one cetacean to another.
The morbillivirus outbreaks offered clues to past events, such as the unexplained die-off of crabeater seals along the Antarctic Peninsula in 1955. Serological studies have tentatively linked that event to morbillivirus, perhaps transmitted from sled dogs. The 1990-1992 epizootic in striped dolphins suggested that morbillivirus infection, which was observed in some bottlenose dolphin carcasses examined during a die-off along the U.S. mid-Atlantic coast in 1987-1988, may have played an important role in that event. Indeed, retrospective studies present strong evidence that morbillivirus outbreaks have occurred sporadically in coastal bottlenose dolphin populations along the southeast United States since the early 1980s.
A number of viruses are associated with less serious health conditions. Poxviruses, for example, commonly cause skin lesions in pinnipeds and cetaceans; pox disease often appears in conjunction with other illnesses or stress. Herpesviruses are also common in cetaceans and pinnipeds and, although not usually serious, have been associated with fatal pneumonia and hepatitis in harbor seal pups and encephalitis in one stranded harbor porpoise (Phocoena phocoena). Calicivirus infection is common among many marine mammals in the North Pacific; clinical disease, which in California sea lions appears as vesicular lesions on the skin of the flippers and mouth, may accompany stress, debilitation, or other infectious conditions, particularly leptospirosis.
Numerous other viruses have been found in marine mammals, many without recognized effect. The list will undoubtedly grow, as will our understanding of their significance, as investigators become more alert to the presence and effects of viruses, and as techniques to isolate and identify them continue to improve.
H. Metabolic Disorders
Metabolic processes sometimes break down. Environmental and biological factors that control hormonal regulation may fail to become synchronized, demands on the system may be overtaxing, and organ function, under the influence of a genetic clock, deteriorates with age and illness. The animal becomes incapacitated, but the underlying reason may be evident only at the molecular level and therefore is difficult to detect. Not surprisingly, little is known about metabolic diseases in aquatic species.
In marine mammals, salt and water balance is regulated in part by the adrenal gland. Aldosterone, secreted from the adrenal cortex, normally acts on the kidney tubules to conserve sodium and thereby maintain salt and water balance. In pinnipeds, conditions that lead to prolonged stress, including molt, malnutrition, and disease, can exhaust the gland of aldosterone, resulting in loss of sodium from the body, a condition known as hyponatremia. Affected animals lose their appetite, become weak and disoriented, and eventually die. Aldosterone features in the stress response of cetaceans as well, only it does not become depleted and the animals do not develop hyponatremia.
Quite the contrary, in severe stress following a stranding, a cetacean may eventually begin to drink seawater and develop a condition of salt overload, or fatal hypernatremia, that dehydrates tissues, including the brain.
I. Tumors
Marine mammals develop all kinds of tumors, from benign lipomas that are little more than fatty lumps in the great whales to highly malignant lymphomas in young seals. As studies on marine mammals have increased, so have the numbers and variety of tumors reported.
In other mammals, tumors have been associated with a variety of factors, including hormones, viruses, congenital and hereditary defects, and physical and chemical agents. Establishing these links has generally required years of investigation on large populations and a systematic consideration and elimination of other possible contributors. These requirements are difficult to meet in marine mammal studies. Hence it may never be possible to prove the assumption that environmental contaminants are responsible for the unusually high incidence of tumors in beluga whales (Delphinapterus leucas) in the St. Lawrence River, however tempting the link. One study may be more fruitful. Recent investigations suggest that a virus may be responsible for the high incidence of urogenital tract carcinomas observed in stranded California sea lions.
J. Biotoxins
Certain species of dinoflagellates and algae produce toxins that accumulate in some fishes and invertebrates, eventually poisoning animals further up the food chain. Prior to the late 1980s, such biotoxins had been suspected, but not proven, to play a role in several events involving marine mammals. For example, ciguatoxin, a dinoflagellate neurotoxin, was implicated in the illness of about 50 Hawaiian monk seals on Laysan Island in 1978; the weak, lethargic seals eventually became emaciated, suffered from severe parasitic infections, and died. Fourteen humpback whales (Megaptera novaeangliae) died in Cape Cod Bay (Massachusetts) in the winter of 1987 after eating mackerel containing saxitoxin, a neurotoxin that even in minute quantities causes respiratory paralysis.
In 1988, brevetoxin, a neurotoxin produced by the dinoflagellate Gymnodinium brevis, the organism responsible for “red tides,” was implicated in a die-off of several hundred Atlantic bottlenose dolphins along the U.S. mid-Atlantic coast. Although the extent of the role of brevetoxin in that event remains unclear, this toxin has since been linked to the mortality of bottlenose dolphins and Florida manatees in the Gulf of Mexico, where red tides are a recognized threat to the manatee population. Red tide outbreaks in southwest Florida in 1983 and 1996 killed about 37 and 155 manatees, respectively; these animals died from the acute and chronic effects of ingestion of toxins and inhalation of toxic brevetoxin aerosols.
In 1998, California sea lions along central California were poisoned by domoic acid, a neurotoxin produced by the diatom Pseudo-nitzschia sp. It caused convulsions, loss of coordination, and vomiting. While more than half of the stranded sea lions died, others were brought to rehabilitation centers and recovered. The similarity of this event to previous episodes in California sea lions and northern fur seals in the same area suggests that blooms of this diatom could have an effect on discrete populations.
Marine mammals may be particularly susceptible to the neurological action of biotoxins for several reasons: (1) during a dive, blood is channeled to the heart and brain, effectively concentrating toxin there, and away from the liver and kidney where it is normally metabolized and excreted; (2) a short period of disorientation may be enough to impede an animal’s ability to reach the surface for a vital breath of air; and (3) animals that remain in the area of a bloom may be subject to the cumulative effects of toxins ingested or toxic aerosols inhaled over a period of days or weeks.
K. Standings
Stranding is defined as having run aground. The term here describes any marine mammal that falters ashore ill, weak, or simply lost. Most animals die at sea and only a fraction reach the shore. Those that do generally reflect the age, sex, and density of the animals in the area. Any change in the expected profile may be a signal that something unusual is happening, such as a toxic event, a disease outbreak, intensive local fisheries operations, or a change in prey abundance.
Pinnipeds and, to a lesser extent, sea otters normally spend time ashore, but only those unwilling or unable to return to sea are considered stranded. These would include pups that become separated from their mothers prematurely or fail to make a successful transition to independence. Most strand in the vicinity of the rookery, although some may stray far from their normal range. Other than in spring, when pups come ashore in large numbers, and in the absence of unusual events such as disease outbreaks, pinnipeds normally strand alone.
Many cetaceans that strand singly are debilitated in some way. Some offshore species strand with characteristic illnesses. Short-beaked common dolphins (Delphinus delphis) along California, for example, develop parasite-related brain damage, and dwarf (Kogia sima) and pygmy (K breviceps) sperm whales along the U.S. Atlantic and Gulf coasts come ashore with impacted stomachs after ingesting plastic bags and other debris.
A mass stranding can be defined as two or more cetaceans, excluding mother-calf pairs, that come ashore alive at the same time and place. Highly social species of odontocetes [e.g., sperm whales, pilot whales (Globicephala sp.), false killer whales (Pseudorca crassidens), and Atlantic white-sided dolphins (Lagenorhynchus acutus)] are the most probable victims. Many explanations have been proposed, but the only common link seems to be the strong social nature of these species. Once one or more animals strand, for whatever reason, the compulsion to stay together brings others ashore.
A stranded animal’s chances of surviving diminish by the hour. Sea otters and pinnipeds risk hyperthermia, injury from terrestrial predators, and starvation. A cetacean has difficulty shedding heat even in cold weather, and a larger one may develop respiratory fatigue and distress as the chest cavity is compressed under its own weight. Within a few hours of stranding, some cetaceans begin to show evidence of shock or vascular collapse, which leads to poor circulation and impaired organ function. The onset of shock further impairs the whales health and may prevent its recovery, even if it is returned to sea in what appears to be good condition.
L. Habitat Alteration and Disturbance
Marine mammals have adapted over millions of years to the often harsh conditions of the marine environment. In the past few decades, environmental change has proceeded at a rate far exceeding the slow pace of evolution. How well can marine mammals cope with urban and industrial wastes, coastal dredging, undersea construction, vessel traffic, and noise? As with other influences on health, the effects—if they can be determined with any degree of certainty—will vary depending on species, sex, age, individual tolerance and behavior, and a host of other factors.
1. Contaminants As long-lived predators at the top of the food chain, marine mammals accumulate contaminants in their tissues. The concentrations and distribution within tissues depend on the type of contaminant and the animal’s age and sex. Because most compounds accumulate over time, older animals generally have more. Fat-soluble substances, such as the persistent DDT, PCBs, and related organochlorines, reside in fatty tissues like blubber, liver, and brain; heavy metals are found in liver, but also distribute in muscle, kidney, and other organs. Pregnant and lactating females produce milk using stored fat and the chlorinated hydrocarbons that came with it. While the suckling offspring loads up with contaminated milk, the female depletes her stores and, over time, has proportionally less and less than a male of equivalent size and age. What concentrations are eventually harmful to the male, to the female as she loads and unloads the compounds with each reproductive cycle, or to the pup that may be even more sensitive? What happens to an animal of any age that becomes ill, stops eating, and uses stored fat, which releases these potentially toxic compounds into the bloodstream where, in increasingly higher concentrations, they are carried to other tissues?
As yet, no clear picture emerges, and broad differences in effects among species continue to invite speculation. In Baltic seals, organochlorine levels seem to be associated with low pregnancy rates and uterine pathology, as well as a disease complex characterized by metabolic disorders, hormonal imbalance, cranial bone lesions, and reduced immune function. The nature of marine mammals and the environment they live in pose serious challenges to conducting investigations that require tight controls and sophisticated technology. Meanwhile we rely on empirical observations and preliminary studies that offer clues. Experimental studies are, nevertheless, yielding data supporting the link between exposure to certain chlorinated hydrocarbons and impaired immune function in at least some species. A better understanding of the influence of contaminants on susceptibility to infectious disease will likely emerge from continued laboratory investigations.
2. Oil Spills Oil spills are visible and unsightly, and sea otters show us how quickly fatal one can be. The 1989 Exxon Valdez incident in Prince William Sound, Alaska, was dramatic and beyond the proportion of other spills that have affected marine mammals. Until that event, relatively few marine mammals were known to have been killed by oil.
The impact of spilled oil depends on its composition, environmental conditions, and the species involved. During the first few hours or days after a spill, low molecular weight fractions are the most acutely toxic. They irritate and harm tissues, especially the sensitive membranes of the eyes and mouth; they can be ingested during feeding or when a fouled animal is grooming; or their vapors can be inhaled and damage the lungs. Light fractions are absorbed into the blood where they can attack the liver, nervous system, and blood-forming tissues. Sea otters caught in the Exxon Valdez spill showed signs of lethargy, respiratory distress, and diarrhea, and evidence of liver damage, kidney failure, and endocrine imbalance. Between 3500 and 5500 otters were estimated to have died. Three hundred harbor seals also died; many had brain lesions, probably resulting from inhalation of vapors from fresh oil.
Evaporation of the low molecular weight fractions leaves heavy residues and thick, foamy emulsions called mousse. By sticking tenaciously to vital insulating hairs of sea otters, polar bears, and some species of pinnipeds (e.g., fur seals), these substances can destroy the animals’ ability to maintain thermal balance. The sea otter is especially vulnerable because its entire existence depends on a well-groomed hair coat.
Except for the sea otter, there is no real evidence that marine mammals ingest much oil. They may be able to deal with small quantities of fresh oil or that premetabolized by their prey because they, like other mammals, have the liver enzymes required to metabolize and excrete such compounds.
3. Ingesting Debris Some marine mammals become entangled in fishing nets and debris. Others are as likely to ingest various types of discarded items and trash that enter the oceans— mostly from land sources—at a rate of over 6 million metric tons each year. Florida manatees, for example, face increasing risks of ingesting fishing line and hooks, wire, plastic bags, and other rubbish trapped in floating mats of vegetation. Some cetaceans, including pygmy sperm whales and some beaked whales, share a tendency to ingest plastics. Some items are small and inconsequential; others may block or perforate the gastrointestinal tract, leading to slow starvation or sudden death.
4. Other Disturbing Influences Habitat degradation can take many other forms: prey depletion, nutrient enrichment that leads to toxic algal blooms, underwater drilling noise, heavy vessel traffic, and disturbance of pupping or calving areas, to name a few. The potential range of effects is immense. A boat traveling through one of Florida’s canals might collide with a manatee and kill it or raise the turbidity and inhibit the growth of water plants that are vital to its diet. Individuals might respond to food shortage or disturbance by moving to marginally suitable environments, e.g., northward to colder waters, where risks of cold stress are increased. A harp seal wandering far from its normal range following a collapse of prey stocks might introduce a pathogenic virus into a susceptible population. A sudden, unusual noise near a crowded pinniped rookery might cause animals to panic and stampede, trampling or abandoning their young.
Other reactions to disturbances may be more subtle. In terrestrial mammals, intense noise alone can cause disorders ranging from long-term hearing loss to physiological stress, hypertension, hormonal imbalance, and lowered resistance to disease. Such effects are nearly impossible to document in marine mammals. It can be assumed that animals are generally unlikely to become habituated physiologically to any disturbances that are associated with threatening situations.
III. The Future
We have a growing understanding of the range of pathogens in the sea and the mechanisms marine mammals have evolved to counter their effects. Except for the inevitable rise of new disease agents and the discovery of old ones, the elements of this endless tug-of-war are unlikely to change. Here, humans are only observers. However, the expression of illness, whether in an individual or a population, is governed by dynamic environmental conditions, and some of these are within our ability to control. Responsible stewardship of the oceans and coastal waters may, by that reckoning, be the key to marine mammal health in the future.
