Killer Whale (marine mammals)

 

With its striking black and white markings and cosmopolitan range, the killer whale (Orcinus area), or orca, is one of the most easily recognized and widely distributed of all cetaceans. Although the species has long been held in high regard by many aboriginal maritime cultures, other societies feared the killer whale as a reputedly ruthless and dangerous predator, and the animals were commonly vilified and persecuted. Attitudes toward killer whales have fortunately improved over the past few decades. The species has been an admired display species in aquaria for over 30 years and has been featured in numerous movies, documentaries, and other forms of popular media. It has become the focus of commercial whale-watching operations in several regions. Long-term field research using photo-identification of individuals from natural markings has resulted in certain populations of killer whales being among the best studied of any cetacean species (Fig. 1).

I. Characters and Taxonomic Relationships

The killer whale is one of the most distinctive of the odontocete cetaceans and is unlikely to be confused with any other species. It is a large dolphin, attaining maximum body lengths of 9.0 m in males and 7.7 m in females. Maximum measured weights are 3810 kg for a 6.7-m female and 5568 kg for a 6.75-m male (Dahlheiin and Heyning, 1999). In addition to sexual dimorphism in size, mature males develop disproportionately larger appendages than females (Fig. 2). This includes the pectoral flippers, tail flukes (the tips of which curl downward in males), and dorsal fin, which is erect in shape and may attain a height of 1.8 m in males. At birth, neonate killer whales are approximately 2-2.5 m long and weigh approximately 200 kg.

The most distinguishing feature of the killer whale is its striking coloration. Killer whales are generally black dorsally and white ventrally. Above and behind the eye on each lateral side of the whale’s head is a conspicuous, elliptically shaped white patch, referred to as the postocular patch. On the posterior lateral sides of the whale, the ventral white region continues dorsoposteriorly to form flank patches that extend almost half-way to the dorsal ridge. At the posterior base of the dorsal fin is a gray-pigniented area of variable shape termed the “saddle patch.” In neonates, the normally white pigmented areas on the body have an orange hue. and the saddle patch is indistinct or absent for the first year of life. Considerable variation exists among killer whale populations and individuals in the size and shape of white and gray patches. In some populations, particularly in the Southern Hemisphere, killer whales have a faint gray pigmentation over much of their body and a black dorsal “cape” anterior to the dorsal fin.

Left side view of male killer whale showing representative dorsal fin nick and saddle-patch scars that are used in photographic identification of individuals.

Figure 1 Left side view of male killer whale showing representative dorsal fin nick and saddle-patch scars that are used in photographic identification of individuals.

Adult male killer whale breaching near San Juan Island, Washington. Note large pectoral flippers and tall dorsal fin typical of mature males.

Figure 2 Adult male killer whale breaching near San Juan Island, Washington. Note large pectoral flippers and tall dorsal fin typical of mature males.

The skull of the killer whale can be distinguished from other odontocetes by its shape, size, dental formula, and large teeth. Typically, 10-12 (up to 14) teeth are found per row, with teeth usually up to 10 cm in length. Upper and lower teeth interlock when the jaws are closed, which may result in considerable wear along their anterior and posterior facets. In some populations, extreme wear of the tooth crowns has been observed, even in young individuals, which may relate to diet.

Taxonomically. the killer whale is the largest species of the family Delphinidae. The genus Orcinus is considered mono-typic. although two species, O. nanus and O. glacialis, were independently proposed for a population of purportedly small individuals in the Antarctic. These new species designations have not received general acceptance. The morphological and ecological features described in Antarctic killer whales likely represent the kinds of population-specific variations that may be typical of the species over much of its range (Hoelzel et al, 1998; Barrett-Lennard, 2000).

II. Distribution and Abundance

The killer whale is second only to humans as the most widely distributed mammal in the world. They are found in all oceans and most seas, but are generally most commonly observed in coastal, temperate waters, especially in areas of high productivity. Notable concentrations occur in waters along the northwestern coast of North America, around Iceland, and along the coast of northern Norway. In the Antarctic, killer whales are commonly found up to the pack ice edge in many areas and may extend well into ice-covered waters. In the Canadian Arctic, killer whales are rarely seen in the vicinity of pack ice, but do visit the region during the open water season in later summer. Information on the species’ distribution in most tropical and offshore waters is limited, but numerous scattered records attest to its widespread, if rare, occurrence.

Because of its general scarcity and sporadic occurrence in most regions, the killer whale is a difficult species to census. Photo-identification studies in nearshore waters of the northeastern Pacific Ocean, from the eastern Aleutian Islands to California, have yielded a total population count of approximately 1500 whales (Ford et al, 2000). Similar studies off northern Noway have identified 445 whales (Simila, 1997). No reliable global population estimate is available for the species, although some rough estimates have been given for some ocean regions. Intensive vessel sighting surveys for cetaceans in the eastern tropical Pacific have resulted in an estimate of about 8500 killer whales in an area of 19 million km2. Ship-board cetacean surveys in the Antarctic have yielded a rough estimate of 70,000 killer whales (Dahlheim and Heyning. 1999).

III. Ecology

The diet of killer whales comprises an extremely diverse array of prey species, which can vary widely both within and among regions. It is the only cetacean that routinely preys upon marine mammals, with attacks or kills documented for over 35 different species (Jefferson et al, 1991). Mammalian taxa that have been recorded as prey of killer whales include other cetaceans—both mysticetes and odontocetes—pinnipeds, sirenians, mustelids, and, on rare occasions, ungulates. A variety of fish species are also important prey of killer whales, notably salmon (On-corhynchus spp.), herring (Clupea harengus), cod (Gadus spp.), tuna (Thunnus spp.), and various sharks and other elasmobranchs (Dahlheim and Heyning. 1999; Saulitis et al, 2000; Ford et al, 1998; Visser, 1999). Other animals noted as killer whale prey include squid, octopus, sea turtles, and sea birds.

In the northeastern Pacific, there is remarkable dietary specialization in different sympatric populations of killer whales, and a growing body of evidence suggests that similar degrees of specialization may also exist in other regions. Long-term photo-identification studies in British Columbia, Washington, and Alaska have shown that two different populations inhabit the same coastal waters, yet maintain social isolation from each other (Bigg et al, 1990). These populations differ in genetic structure, morphology, behavior, distribution patterns, and ecology. One population, referred to as residents, are fish specialists, whereas the other, termed transients, are primarily mammal hunters.

Residents show strong seasonal movements associated with the coastal migrations of salmon. Observational studies of residents and analyses of stomach contents from beach-cast carcasses have determined that salmon is their predominant prey for at least half the year and that preference is shown for the largest or fattiest available species [chinook (Oncorhi/nchus tshawytscha) in British Columbia and coho (O. kisutch) in Prince William Sound, Alaska; Ford et al, 1998; Saulitis et al. 2000] (Fig, 3). Squid (Gonatopsis borealis) and a variety of nonsalmonid fish species are also eaten by residents (Ford et al., 1998). There is no evidence that marine mammals are consumed, although porpoises and seals are harassed and killed by residents on rare occasions. Foraging groups of residents typically ignore marine mammals in their vicinity and seldom elicit avoidance responses from those species (Jefferson et al, 1991; Saulitis et al, 2000). The distribution and diet of residents in winter and early spring are poorly known.

Transients show relatively little seasonal change in abundance and distribution, most likely because their preferred prey species are present year-round in coastal waters. In British Columbia, harbor seals (Phoca vitulina) are the primary prey of transients, although Steller sea lions (Eumetopias jubatus), California sea lions (Zahphtis califomianus), harbor porpoises (Phocoena phocoena), Dall’s porpoises (Phocoenoides dalli), and Pacific white-sided dolphins (Lagenorhynchus obliquidens) are also important (Ford et al, 1998) (Fig. 4). Larger cetaceans are seldom attacked and killed by transients in coastal waters of British Columbia, although this is common off California. Transients have been observed to harass and occasionally eat a variety of species of seabirds in British Columbia and southeastern Alaska. In Prince William Sound, Alaska, transients leed mostly on harbor seals and Dall’s porpoises, but seabird predation has not been recorded (Saulitis et al, 2000). Transients have not been observed to eat any species of fish, and no fish remains have been found in the stomachs of stranded transients.

Such extreme dietary specialization in sympatric populations is without precedent in mammals. These specializations likely evolved gradually over a long period by means of increasingly refined and successful foraging strategies that were learned by individuals and passed across generations. Effective foraging for the very different types of prey of residents and transients may require such divergent skills and tactics that lifestyles dependent on one or the other prey type are mutually exclusive. Foraging specializations may have played a role in the historical separation of ancestral resident and transient groups, and over time the two populations became socially and eventually repro-ductively isolated. Residents and transients are highly distinct in both mitochondrial and nuclear DNA composition (Hoelzel et al, 1998; Barrett-Lennard, 2000). A third sympatric population has been documented in coastal waters off British Columbia south to California (Ford et al, 2000). Provisionally termed “off-shores,” these whales are seldom encountered in protected inshore waters and have not been observed mixing with either the resident or the transient population. They form a genetically distinct group, although they are more closely related to residents than to transients (Barrett-Lennard, 2000). The dietary habits of this population are so far unknown.

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Figure 3 Resident killer whale with freshly killed salmon in Haro Strait, Washington.

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Figure 4 Transient killer whale ramming a Steller sea lion dtuing an attack in Blackfish Sound, British Columbia.

Populations of killer whales in other regions may also be highly specialized in feeding habits. In the Antarctic, two sympatric populations have been reported that differ morphologically and ecologically, with one population preying primarily on marine mammals and the other on fish. Off the northern coast of Norway, a population of killer whales moves seasonally in relation to the migration pattern of its principal prey, herring (Simila, 1997). It is likely that populations with dietary specializations exist wherever sufficiently abundant and reliable prey resources are available to sustain them year-round. In other regions, more opportunistic foraging strategies may be expected. For example, in the sub-Antarctic Crozet Islands, killer whales feed seasonally on southern elephant seal (Mirounga leonina) pups, but also forage for fish. Groups of killer whales have been observed to attack and kill large baleen whales and sperm whales in various locations (Jefferson et al, 1991), but it is not known whether these groups specialize on such prey items.

The killer whale has no natural predators other than humans.

IV. Life History

Most detailed information on reproduction, mortality, and other life history parameters of killer whales has been derived from long-term photo-identification studies of resident killer whales in British Columbia and Washington (Olesiuk et al, 1990). The reliability and completeness of this information are due to the extremely stable social structure of residents, where emigration from the natal group does not take place and mortalities can be documented reliably (see next section for more detail). It is not known whether these life history parameters are typical of other populations or regions.

Studies of captive whales indicate that sexually mature females have periods of polyestrous cycling interspersed with noncycling intervals of 3-16 months. The gestation period is 15-18 months. In resident killer whales, births may take place in any month, although most are in October-March. Neonate mortality is high, with an estimated 43% dying in the first 6 months (Olesiuk et al, 1990). Calves are nursed for at least a year, but may start taking solid food from the mother while still nursing. Typical age at weaning is not known, but is likely between 1 and 2 years of age.

Females give birth to their first viable calf at between 11 and 16 years of age (mean of 15 years) (Olesiuk et al, 1990). Intervals between viable calves average about 5 years (range 2-14 years). Females have an average of 5.35 viable calves over a 25-year reproductive life span, which ends at approximately 40 years of age. Females then become reproductively senescent for an average period of 10 years, but this postreproductive period may extend to more than 30 years. Mean life expectancy for females (calculated at age 0.5 years, following the period of high neonate mortality) is estimated to be approximately 50 years, and maximum longevity is 80-90 years. Males attain sexual maturity at about 15 years of age, as indicated by a rapid growth of the dorsal fin. Males continue to grow until they reach physical maturity at about 21 years of age. Mean life expectancy for males (calculated at age 0.5 years) is estimated to be about 29 years, with maximum longevity about 50-60 years. Mortality curves for both males and females are U-shaped, although die male curve is narrower.

V. Social Organization

Killer whales are usually observed traveling alone or in groups of up to about 50 individuals. Reports of larger groups likely involve temporary’ aggregations of smaller, more stable social units. Long-term photo-identification studies have provided information on the social organization of the species in several regions of the world. The most detailed of these are studies in coastal British Columbia, Washington, and Alaska, particularly for the resident population (Bigg et al, 1990; Matkin et al, 1999; Ford et al, 2000). Resident societies can be arranged into a number of groupings based on maternal genealogy, social association, and acoustical relationship. The basic social unit of residents is the matriline, which is a highly stable group of individuals linked by maternal descent. A typical matriline is composed of a female, her sons and daughters, and the offspring of her daughters. Because females may live to 80-90 years of age, and females have their first viable calf at about 15 years of age, a matriline may contain as many as four generations of maternally related individuals. Some matrilines contain only one generation, which can result if a female dies and leaves only sons or daughters that have no young of their own. The bonds among members of a matriline are extremely strong, and individuals are seldom seen apart from the group for more than a few hours. No permanent dispersal of individuals has been observed from a resident matriline.

The next level of social organization in resident killer whales is the pod, which is a group of related matrilines that shared a common maternal ancestor in the recent past. Matrilines within pods are thus more closely related to one another than to matrilines in other pods. Pods are less stable than matrilines, and member matrilines may travel apart for periods of weeks or months. However, these matrilines still tend to travel more often with others from their pod than with matrilines from other pods (Ford et al, 2000). The majority of pods are composed of one to three matrilines. Resident pods in British Columbia, Washington, and Alaska contain a mean of 18 whales (range = 2-49).

Above the pod is the clan, which is a level of social structure defined by the acoustic behavior of pods. It is composed of pods that have similar vocal dialects (see Section VII). All pods within a clan have most likely descended from a common ancestral pod through a process of growth and fragmentation along matrilines. Thus, the related dialects of clan members seem to reflect the common matrilineal heritage of the pods. Those pods with very similar dialects are probably more closely related, and have split more recently, than those with more different features in their dialects. Clans are sympatric, and pods from different clans frequently travel together. Clan membership is occasionally—but usually not—reflected in patterns of association. It is not clear how clans are related to each other, as they have no acoustical features in common, nor is the origin of clans known.

The top level of structure in a resident society is the community, which is made up of pods that regularly associate with one another. The community is thus defined solely by association patterns rather than maternal genealogy or acoustic similarity. Pods from one community have not been seen to travel with those from another, although their ranges may partly overlap. Three communities of residents have been identified in coastal waters of British Columbia, Washington, and Alaska: southern (83 whales in 3 pods, 1 clan), northern (214 whales in 16 pods, 3 clans), and southern Alaskan (237 whales in 11 pods, 2 clans) (Matkin et al, 1999; Ford et al, 2000).

Social organization in mammal-eating transient killer whales is not as well known as in residents (Ford and Ellis, 1999). Like residents, the basic social unit is the matriline, but unlike residents, offspring may disperse from matrilines for extended periods or permanently, either as juveniles or as adults. As a result, transient matrilines tend to be smaller than those of residents, and lone individuals, particularly males, are often observed. Small group sizes of transients appear to reflect the marine-mammal foraging specialization of this population (Baird and Dill, 1996; Ford and Ellis, 1999). Association patterns of transient matrilines are very dynamic, and they do not form consistent groupings of matrilines equivalent to resident pods.

All transient groups in a community can be linked together via this network of associations. All members of a transient community share a related call repertoire, as in a resident clan. However, regional differences exist in die vocal repertoire of transients.

Social organization based on matrilineal descent may be typical of killer whales globally. In other regions where long-term photo-identification studies have been undertaken, close and prolonged associations of mothers and offspring are commonly seen (e.g., Norway, Crozet Islands, Argentina). Temporal persistence of these bonds may be a primary variable determining group sizes and structure.

VI. Activity States and Behavior

The activity states of killer whale groups are of four basic types: foraging, traveling, resting, and socializing (Ford, 1989; Saulitis et al, 2000). Minor differences in definitions and classification criteria of activities by different researchers make detailed comparisons difficult, but general patterns are evident. Foraging and traveling are the predominant activity states noted in all populations, although the proportions of the activity budget dedicated to these activities vary. Mammal-eating transients in coastal waters of the northeastern Pacific spend the great majority of their time (approximately 90-95%) foraging and traveling, whereas fish-eating residents spend only about 60-70% of their time doing so, at least during summer when salmon is abundant. Residents spend considerably more time resting and socializing than transients. Fish-eating killer whales in northern Norway have activity budgets very similar to those of northeastern Pacific residents (Simila, 1997).

A. Foraging

Behavior patterns of foraging killer whales vary considerably among populations and prey types. Groups of salmon-hunting residents often disperse over large surface areas while foraging, with members moving at roughly the same speed (mean = 6.0 km/lir) and direction. Foraging episodes are typically 2-3 hr in duration, but may last up to 7 lir. Individual salmon are pursued, captured, and eaten by single animals or small subgroups, usually a mother and juvenile offspring. Norwegian killer whales feed on herring in a coordinated manner referred to as “carousel feeding” (Simila, 1997). Using percussive actions such as tail lobbing, releasing blasts of bubbles, and flashing the white ventral side of their bodies, the whales herd herring into a tight ball close to the surface. The whales then stun fish by striking the edges of the ball with their tail flukes and eat the debilitated prey.

Transient killer whales in the northeastern Pacific typically forage in smaller groups than fish-eating killer whales. Transient groups hunt harbor seals in groups averaging three to four individuals (Baird and Dill, 1996; Ford et al, 1998; Saulitis et al, 2000), usually close to shore and near seal haul-out sites. While foraging, transients remain acoustically very quiet apparently to avoid detection by potential prey and possibly to locate prey by passive listening (Barrett-Lennard et al, 1996). Harbor seals are killed and shared among group members relatively quickly compared to Steller sea lions or California sea lions, which may take over 2 hr to kill and consume. Sea lions are usually rammed or butted with the whales’ heads, and slapped repeatedly with tail flukes, until the animal is debilitated sufficiently to be taken underwater and drowned. When hunting porpoises or dolphins, transients forage in slightly larger groups averaging five members, which spread out in open water in a rough line abreast. The whales single out an individual porpoise, chase it until it tires, and then ram it or jump on it to complete the kill (Ford et al, 1998). Larger schools of Pacific white-sided dolphins are often driven by transients into confined bays where individual dolphins are trapped against the shore and killed (Ford and Ellis, 1999).

A variety of specialized tactics have been described for killer whales hunting marine mammals in other regions. In Patagonia, Argentina, killer whales hunt southern sea lion (Otaria flavescens) and elephant seal (Mirounga leonina) pups in the shallows along sloping pebble beaches, and often intentionally strand themselves temporarily in the process (Fig. 5). The whales hunt cooperatively and share their prey after capture. Killer whales in the Crozet Islands hunt elephant seal pups in a manner similar to those in Patagonia, and adults appear to teach this technique to their offspring (Guinet and Bouvier, 1995). In the Antarctic, killer whales have been observed to locate seals hauled out on ice pans by spyhopping and then dislodging them from the pan with a wave created by the whale’s fast approach. Attacks on baleen whales or sperm whales often involve groups of 10-20 killer whales, which work together in a coordinated manner to subdue the prey. Individuals will attempt to grasp the tail flukes or pectoral flippers to immobilize the larger whale, while others attack the head and blowhole area evidently to prevent the whale from breathing. Once killed, often just the tongue, lips, and blubber are consumed (Jefferson et al, 1991). Because most baleen whales sink upon death, killer whales may only be able to feed extensively on carcasses of whales killed in shallow waters (Guinet et al, 2000).

Female killer whale catching a southern sea lion pup in Patagonia, Argentina.

Figure 5 Female killer whale catching a southern sea lion pup in Patagonia, Argentina.

B. Traveling

Traveling killer whales move in a single direction at a consistent, fast pace, with no evidence of foraging or feeding. Groups often travel in a line abreast, with synchronized dives and surfacings. Resident killer whales have been documented to travel at speeds of over 20 km/hr (mean = 10.4 km/hr; Ford, 1989).

C. Resting

When resting, resident killer whales usually form a line abreast, often with individuals grouped tightly together (Fig. 6). Group diving and surfacing become closely synchronized and regular, with longer dives of 2-5 min duration separated by three or four short, shallow dives. The rate of forward progression is slow compared to foraging and traveling, and resting groups may stop altogether and rest motionless at the surface for several minutes (Ford, 1989; Simila, 1997).

D. Socializing

Socializing activity includes a wide range of physical displays and social interactions. Aerial behaviors are frequent and may include spyhops, breaches, flipper slaps, tail lobs, and head stands. Juveniles often chase each other, roll and thrash at the surface, and engage in various other forms of play behavior, including playing with objects such as kelp or jellyfish. Sexual interactions involving penile erections are commonly observed, predominantly in all-male play groups. Some individuals may rest quietly at the surface while other pod members actively socialize. Beach rubbing is a common behavior observed during socializing in some populations. Residents belonging to the “northern” community in British Columbia visit certain beaches repeatedly to rub their bodies on smooth pebbles in shallow water (Ford, 1989). Interestingly, “southern” community residents do not share this behavioral tradition and have not been seen to rub at any location.

VII. Sound Production

Like most delphinids, killer whales are highly vocal. They produce a wide variety of clicks, whistles, and pulsed calls for echolocation and social signaling. Studies of resident killer whales in British Columbia have documented vocal variations associated with activity state and group identity (Ford, 1989, 1991). Vocal exchanges among foraging resident whales are dominated by highly stereotyped, repetitive discrete calls from a repertoire averaging 12 call types (range 7-17 call types) per pod. Resting activity is usually associated with greatly reduced vocal activity, and occasional use of certain calls heard predominantly. but not exclusively, in such contexts. Socializing whales use mainly whistles and nonrepetitive. variable pulsed calls, and aberrant versions of discrete calls. Excitement or motivational levels of vocalizing individuals are reflected in minor variations in pitch and duration of discrete calls.

Resting pods of resident killer whales in Prince William Sound, Alaska.

Figure 6 Resting pods of resident killer whales in Prince William Sound, Alaska.

Call repertoires of resident killer whale pods have features that are distinct, forming systems of group-specific dialects. The entire call repertoire appears to be shared by all pod members. Some portions of a pod’s call repertoire may be shared with certain other pods, whereas other portions may be unique. Levels of similarity in these group-specific dialects appear to reflect the degree of relatedness of different pods better than do patterns of travel association. Dialects most likely are learned by young whales by mimicking their mother and siblings, and are retained in the matriline due to the lack of individual dispersal. Divergent variations in dialects among related matrilines likely accompany the gradual fission that leads to pod formation. Dialects likely provide an acoustic means of maintaining group identity and cohesion, and may serve as indicators of relatedness that help in the avoidance of inbreeding between closely related whales (Ford, 1991; Barrett-Lennard, 2000). Dialects have also been documented within a community of pods of killer whales in northern Norway, and likely exist elsewhere.

Mammal-eating transient killer whales in British Columbia and Alaska have greatly reduced vocalization rates compared to residents. Transients are generally silent when foraging. This includes the use of echolocation, which in one study was heard 27 times less often from foraging transients than foraging residents (using an index adjusted for group size; Barrett-Lennard et al., 1996). Transients are more likely than residents to use individual (or “cryptic”) clicks rather than click trains, presumably to avoid alerting potential prey to their approach. When vocal, transients off the coasts of southeastern Alaska to California produce a number of calls that are shared among all groups in the community. Certain other calls seem exclusive to transient groups in different portions of this range. Group-specific dialects as seen in resident pods are not evident, presumably due to the reduced stability of social structure in transients (Ford and Ellis, 1999).

VIII. Conservation Status

Globally, the killer whale is listed by the IUCN as “lower risk-, conservation dependent,” meaning that the species, although presently not considered at risk, could become so should existing conservation programs be discontinued and exploitation expanded. Historically, killer whales in several regions have been the target of directed fisheries, culling, and persecution. An average of 43 whales per year were taken by Japanese whalers in their coastal waters during 1946-1981, mostly for human consumption. This fishery is no longer active. Norwegian whalers took an average of 56 whales per year during 1938-1981 in a government-subsidized hunt aimed at reducing killer whale numbers to reduce competition for other fisheries. The killer whale meat from this fishery was used only for animal consumption. A small number of killer whales were taken annually by Soviet whalers in the Antarctic, with the exception of a large take of 916 animals in the 1979/1980 season. No significant directed hunt for killer whales continues today.

Killer whales have long been feared as dangerous predators or vilified as perceived or real threats to fisheries in many regions, and were often harassed or shot opportunistically. Although much reduced, some such persecution continues today. Killer whales have been shot by fishermen in Alaska and possibly other regions to prevent the whales from taking fish from long-line fishing operations (Matkin et al, 1999). There is evidence that populations in coastal waters of British Columbia and Washington state were already depressed from shootings when a live-capture fishery developed there in the mid-1960s. Killer whales became highly sought for public display in aquaria following the first successful capture and display of the species at Vancouver in 1964. During 1964-1977, 63 whales were taken in this fishery to supply aquaria in many parts ol the world (Olesiuk et al, 1990). During the late 1970s to mid-1980s, live captures shifted to the waters of Iceland, where over 50 whales were taken. The improved success of captive breeding during the past decade has reduced the need for capture from wild populations, although periodic live captures continue.

Other conservation concerns include direct effects of oil spills and other forms of toxic pollution of killer whale survival. The Exxon Valdez oil spill in Alaska was strongly correlated with the subsequent loss of 14 whales from a pod that was seen swimming through light oil slicks early in the spill, although it was not possible to directly attribute the deaths to this cause. Oil spills may also have indirect effects on killer whales bv reducing prey abundance. Being at high trophic levels in the food web, killer whales are susceptible to bioaccumulation ol organochlorine pollutants. Levels of PCBs in resident and, in particular, transient killer whales in British Columbia and Washington state have been shown to be among the highest observed in any cetacean and are higher than levels found to affect the health of European harbor seals. It is not known whether there is a direct impact of PCBs on health in these killer whales, although such effects as immunosuppression and reduced reproductive success are possible (Ross et al, 2000).

Other potential impacts of human activities on killer whale status are reduced prey availability and disturbance caused by vessel traffic. As an example, many stocks of salmon, the principal prey of residents killer whales, have declined significantly in British Columbia and Washington State as a result of overfishing, degradation of spawning grounds, and reduced ocean survival. Vessel disturbance is of particular concern in areas of intensive whale watching, although many forms of boat traffic have the potential to affect whales. The physical presence of moving boats near killer whales can disrupt their activities, particularly resting. Underwater noise from vessels has the potential to interfere with social or echolocation signals, or to mask passive acoustic cues that may be important in finding prey.

It is possible that many of these potential impacts on killer whales are, when taken alone, insufficient to negatively affect killer whale survival. However, there is a potential for more serious cumulative effects that could displace killer whales from critical habitats or result in reduced survival.

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