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Orcinus orca Linnaeus,
1758
English: Killer whale
German: Schwertwal
Spanish: Orca
French: Orque
Family: Delphinidae
Orcinus orca ©Wurtz-Artescienza (see links)
1. Description
The killer whale is the largest member of the dolphin family. Maximum
body lengths are 9 m in males and 7.7 m in females. Males reach
6,600 kg, whereas female maximum weight is 4,700 kg (Ford, 2009).
Killer whales are recognized by their distinctive black, white and
grey coloration and a white eye patch, or spot, located just above
and behind the eye. Just behind the dorsal fin there is a grey saddle
patch. The whale's belly, lower jaw and the underside of the tail
flukes are white. The rest of the body is black. The wide, tall
dorsal fin is curved backwards in females and juveniles and upright
and triangular in adult males. The head is rounded, with a barely
distinguishable beak. The pectoral flippers are paddle-shaped. In
addition to sexual size dimorphism, male appendages, especially
the dorsal fin, are disproportionately larger than in females.
According to Black et al. (1997) and Ford (2009) there are at least
three recognizable ecotypes of killer whales ("Residents,"
"Transients," and "Offshores") in the eastern
North Pacific that do not associate with members of the other groups.
The ecotypes exhibit different home ranges, vocalizations, dietary
preferences, foraging patterns, morphological features, and genotypes.
Residents prey mostly on fish, Transients prefer marine mammals
and Offshores seem to feed on both types of prey but may specialize
in sharks.
In the Residents, each local "community" of pods is largely
endogamous, with mating between pods within the community. The ecotypes
differ in both morphology and genetics, as well as in traditions
such as migratory behaviour and prey choice. The communities within
the resident ecotype can differ in dialects (Rice, 1998 and refs.
therein).
Recently, three visually distinct forms of killer whales were described
from Antarctic waters and designated as types A, B and C (Pitman
and Ensor, 2003). These broadly sympatric but at least seasonally
microallopatric forms show consistent differences in prey selection
and habitat preferences, morphological divergence and apparent lack
of interbreeding, which is also confirmed by genetic studies. However,
a relatively low level of sequence divergence indicates that these
evolutionary changes occurred relatively rapidly and recently (LeDuc
et al. 2008). Pitman et al. (2007) with photogrammetery confirmed
that the small ice-dwelling fish-eating form (type C) has a modal
length of about 5-5.5 m, much smaller than more offshore whales.
However, further studies are needed to ascertain whether these small
whales deserve recognition as a separate species or subspecies (Ford,
2009).
Renner and Bell (2008) observed a white adult male killer whale
off Adak Island, Aleutians. An open saddle and a rounded dorsal
fin tip suggest that this whale belongs to the fish-eating ("resident")
ecotype. A circular scar matching a cookie-cutter shark (Isistius
sp.) bite mark suggested that the animal originated in warmer waters.
2. Distribution
This is probably the most cosmopolitan of all cetaceans and can
be seen in literally any marine region. O. orca occurs throughout
all oceans and contiguous seas, from equatorial regions to the polar
pack-ice zones, and may even ascend rivers. However, it is most
numerous in coastal waters and cooler regions where productivity
is high (Jefferson et al. 1993; Dahlheim and Heining, 1999 and refs.
therein).
Distribution of Orcinus orca: this species
is found in all regions of the world (map mod.
from Taylor et al. 2008; © IUCN; enlarge
map).
In the Atlantic it ranges north to Hudson Strait,
Lancaster Sound, Baffin Bay, Iceland, Svalbard, Zemlya Frantsa Iosifa,
and Novaya Zemlya; its range includes the Mediterranean Sea. In
the Pacific it ranges north to Ostrov Vrangelya, the Chukchi Sea,
and the Beaufort Sea. In the Southern Ocean, the range extends south
to the shores of Australia and the Philippines, South Africa, South
America and Antarctica, including the Ross Sea at 78°S (Rice,
1998).
Data from the central Pacific are scarce. Killer whales have been
reported off Hawaiibut do not appear to be abundant in these waters
(Barlow, 2003).
3. Population size
Although the available data are far from complete, abundance estimates
for the areas that have been sampled provide a minimum worldwide
abundance estimate of about 50,000 killer whales (Taylor et al.
2008).There are several recent population estimates for parts of
the species range. In the eastern North Pacific, these are, for
the different sub-populations:
Resident stocks:
In coastal waters of the western Gulf of Alaska and the Aleutian
Islands, abundance estimates of resident killer whales are 991 (95%
CI = 379-2,585) and 1,587 (95% CI = 608-4,140), respectively (Zerbini
et al. 2007).
The eastern North Pacific Northern Resident stock numbered 216 in
1998 (Ford et al. 2000). Sightings surveys conducted in the inshore
coastal waters of the Inside Passage, between the British Columbia
(BC)-Washington and the BC-Alaska borders, yielded an abundance
estimate of 161 (95% CI = 45-574) northern resident killer whales
(Williams and Thomas, 2007) in these waters.
The eastern North Pacific Southern Resident stock numbered 86 whales
in 2007, 79 in 2001 and 99 in 1995 (Carretta et al. 2009). The population
has fluctuated considerably over the past 35 years, due to a variety
of reasons (Krahn et al. 2004).
Offshores:
Surveys conducted in 2001 (Barlow and Forney 2007) and 2005 (Forney
2007), estimated the total number of killer whales within 300 nmi
of the coasts of California, Oregon and Washington to be 1,014 (CV=
0.29).
Transients:
Estimated transient killer whale abundance for the Gulf of Alaska
and the Aleutian Islands, were 200 (95% CI = 81-488) and 251 (95%
CI = 97-644) whales, respectively (Zerbini et al. 2007).
The West Coast Transient stock is a trans-boundary stock, including
killer whales from British Columbia. In British Columbia and south-eastern
Alaska, 219 whales were catalogued (Ford and Ellis 1999). Off the
coast of California, 105 'transients' were identified (Black et
al. 1997). These are the most recent estimates (Angliss and Outlaw,
2004).
A 2002 shipboard line-transect survey of the entire Hawaiian Islands
EEZ resulted in an abundance estimate of 430 (CV=0.72) killer whales
(Barlow 2003).
On the Atlantic coast of the USA, the estimate of abundance for
killer whales in oceanic waters of the northern Gulf of Mexico (2003
- 2004 data), was 49 (CV=0.77) (Mullin 2007).
In the North Atlantic, questionnaire surveys yielded 483-1,507
killer whales for Norwegian coastal waters (Dahlheim and Heining,
1999 and refs. therein). Sightings in the eastern North Atlantic
gave rough estimates of around 3,100 killer whales for the area
comprising the Norwegian and Barents Seas and Norwegian coastal
waters and some 6,600 whales for Icelandic and Faroese waters (Reyes,
1991 and refs. therein).
Off the Japanese coast the estimate is 1,200 individuals north of
35°N and 700 south of 35°N (Dahlheim and Heyning, 1999 and
refs. therein). Around Antarctica, the most recent estimate is 25,000
killer whales south of 60°S (Branch and Butterworth, 2001).
Locally, Poncelet et al. (2002) reported a strong decline of O.
orca in the coastal waters of Possession Island in the Southern
Indian Ocean between 1988 and 2000. Williams et al. (2009) considered
the conservation status of fish-eating killer whales in southern
African waters to be 'vulnerable', because the populations are very
small and are subject to both short- and long-term impacts from
longline fisheries.
4. Biology and Behaviour
Habitat: Sightings range from the surf zone to the open
sea, though usually within 800km of the shoreline. Large concentrations
are sometimes found over the continental shelf. Generally, killer
whales prefer deep water, but they can also be found in shallow
bays, inland seas, and estuaries (but rarely in rivers). They readily
enter areas of floe ice in search of prey (Carwardine, 1995). Resident
killer whales in Pacific Northwest waters use regions of high relief
topography along salmon migration routes, whereas transient whales
forage for pinnipeds in shallow protected waters (Dahlheim and Heyning,
1999 and refs. therein). In the southwestern Atlantic Ocean, the
majority of sightings per unit of effort occurred between 35°
and 37° S, over depths of 200-3000 m. The presence of killer
whales there coincides mainly with surface temperature fronts (Passadore
et al. 2007).
Whalewatching in Tysfjord, Norway, 2005 @ Boris
Culik
Reproduction: In the Pacific Northwest, calving occurs in
non-summer months, from October to March. Similarly, in the Northeast
Atlantic, it occurs from late autumn to mid-winter (Jefferson et
al. 1993). Gestation lasts 15 to 18 months and is first observed
in females 12-14 years old. Intervals between calves average 5 years,
and the reproductive life span is around 25 years long. Mean life
expectancy is 50 years and longevity up to 90 years (Ford, 2009).
Schooling: Pods of resident killer whales in British Columbia
and Washington represent one of the most stable societies known
among non-human mammals; individuals stay in their natal pod throughout
life. Differences in dialects among sympatric communities appear
to help maintain community discreteness. Most pods contain 1 up
to 55 whales; resident pods tend to be larger than those of transients
(Jefferson et al. 1993). Social organization can be classified into
communities, pods, subpods, and matrilineal groups: a community
is composed of individuals that share a common range and are associated
with one another; a pod is a group of individuals within a community
that travel together the majority of time; a subpod is a group of
individuals that temporarily fragments from its pod to travel separately;
and a matrilineal group consists of individuals within a subpod
that travel in very close proximity. Matrilineal groups are the
basic unit of social organization, and consist of whales from 2-3
generations. Membership at each group level is typically stable
for resident whales, except for births and deaths (Dahlheim and
Heining, 1999 and refs. therein).
Social organisation of mammal-eating transients is less well understood.
Although the basic socal unit is the matriline, offspring often
disperse for extended periods or permanently, and the transient
matrilines are smaller than those of residents. Transient group
size is often only one, reflecting the hunting specialisation of
these killer whales (Ford, 2009). Baird and Dill (1996) summarize
that the typical size of transient killer whale groups is consistent
with the maximisation-of-energy-intake hypothesis. Larger groups
may form for the occasional hunting of prey other than harbour seals,
for which the optimal foraging group size is probably larger than
three, and for the protection of calves and other social functions.
Food: Killer whales are generalist predators on a global
scale (Ford, 2009). However, local populations can exhibit remarkable
specialisations with respect to their food preferences. The best
studied example is that of resident, salmon-eating killer whales
off the North American west coast, which show seasonal movements
synchronised to their main prey, e.g. the fattiest salmon species
Oncorhynchus tshawytscha (Washington and British Columbia)
and O. kisutch (off Alaska).
Mammal-eating transient killer whales live in the same area, without
undergoing seasonal migrations because their main prey, harbour
seals (Phoca vitulina), harbour porpoises (Phocoena
phocoena) and Dall's porpoises (Phocoenoides
dalli) are present year round. These transient whales have
never been observed to eat any species of fish (Ford et al. 1998).
In the eastern Aleutian Islands, Alaska, the diet of transient killer
whales in spring was primarily gray whales (Eschrichtius robustus)
and in summer primarily northern fur seals (Callorhinus ursinus).
Steller sea lions (Eumetopias jubatus) did not appear to
be a preferred prey or major prey item during spring and summer
(Matkin et al. 2007).In the Gulf of Alaska, transient killer whales
feed on Steller sea lions, but because takes are lower than previously
assumed, this seems to have only a minor effect on the recovery
of populations (Maniscalco et al. 2007). Chemical tracer analysis
also shows that these transients feed on other prey species as well
(Krahn et al. 2007).
A third ecotype, genetically distinct from the other two (Barrett-Lennard,
2000) are the so-called Offshores. They are seldom encountered in
the inshore waters of Washington and British Columbia and seem to
prey on fish, including halibut and sharks (Ford, 2009). Chemical
tracers show that offshores consume prey species that are distinctly
different from those of sympatric resident and transient killer
whales. These offshores forage as far south as California (Krahn
et al. 2007).
In northern Norway, killer whales feed on herring (Clupea harengus).
They co-operatively herd herring into tight schools close to the
surface. During herding and feeding, the killer whales swim around
and under a school of herring, periodically lunging at it and stunning
the herring by slapping them with the underside of their flukes
while completely submerged (Domenici et al. 2000). The whales prefer
to search out small patches of herring in the early morning, in
shallow waters and near underwater seamounts, which aids in herding
their prey. Killer whales are not capable of catching these fish
unless they have stunned them first with tail slaps. However, the
whales have also learned to follow the fishing fleet and feed on
herring that fall from the nets when the catch is being pumped in
(Similae, 2005).
Icelandic killer whales have developed another strategy. They can
emit a 3-s, 680-Hz call that ends 1 s before the tail slap. The
frequency of the call falls within the herring audiogram, but outside
that of killer whales. This call seems suited for herding the herring
into tighter groups, making it possible to debilitate more fish.
However, the herring are not defenceless. The school can produce
a flatulent bubble net that could hinder detection by killer whale
biosonar (Miller et al. 2006).
In the waters between northern Scotland and Norway, killer whales
are frequently observed in the vicinity of the Scottish pelagic
fleet targeting mackerel (Scomber scombrus) and herring.
They approach the vessels during retrieval of the net, and remain
there until this is completed. There is no evidence that killer
whales ever become entangled in the nets (Luque et al. 2006). Killer
whales are known to follow fish-processing vessels for many miles,
feeding of discarded fish. In the Bering Sea, the same pod of whales
was reported to follow a vessel for 31 days for approximately 1,600
km (Dahlheim and Heyning, 1999 and refs. therein).
In the Strait of Gibraltar, killer whales prey on migrating bluefin
tuna Thunnus thynnus. Their strategy is to chase their prey
for up to 30 min at a relatively high sustained speed (3.7 m/s)
prior to capture, pushing medium-sized tuna (< 1,5 m long) beyond
their aerobic limits until exhaustion. Larger tuna may be inaccessible
to killer whales unless they use cooperative hunting techniques
or benefit through depredation of fish caught on long lines, drop
lines or trap nets (Guinet et al. 2007). Off southern Brazil (Secchi
and Vaske, 1998; Rosa and Secchi, 2007) and in may other areas world
wide, killer whales have learned to prey on fish hooked to longlines.
In the Southern Ocean e.g., longline fisheries for Patagonian toothfish
(Dissostichus eleginoides) suffer catch rate decreases of
more than 50% when killer whales occurr close to longline vessels
(Kock et al. 2006).
Off the coast of Chubut, Argentina, killer whales attack sevengill
sharks (Notorhynchus cepedianus). The same animals were observed
to feed on pinnipeds in Peninsula Valdes. Therefore some killer
whale groups in the southwestern Atlantic may regularly feed on
both fish and marine mammals (Reyes et al. 2004). In New Zealand
waters, killer whales were found to capture and eat thresher (Alopias
vulpinus) and smooth-hammer-head (Sphyrna zygaena) sharks;
ten species of elasmobranchs are now recorded as prey for this population
(Visser 2005).
In Antarctic type "C" killer whale populations profiles
of individual chemical tracers are consistent with a fish diet (Krahn
et al. 2008). While type "B" killer whales feed on pinnipeds
in loose pack-ice, the larger type "A" killer whales are
open water marine mammal hunters specialising on minke whales (Balaenoptera
bonaerensis) (Pitman and Ensor, 2003).
Mehta et al. (2007) suggested that most killer whale attacks on
baleen whales target young animals, probably calves on their first
migration from low-latitude breeding and calving areas to high-latitude
feeding grounds. Their results imply that adult baleen whales are
not an important prey source for killer whales in high latitudes.
5. Migration
Based on photo-identification studies, year-round and seasonal
occurrences are recorded for the waterways of British Columbia and
Washington State, where pods are known to range approximately 370
nautical miles (Reyes, 1991). However, numerous individual whales
and/or pods have also been documented to move between Puget Sound
(Washington)/British Columbia and southeastern Alaska; between southeastern
Alaska and Prince William Sound; and between Prince William Sound
and Kodiak Island. On an international level, whale movements from
Alaska (USA) and British Columbia (Canada) to California (USA),
from California to Mexico, and from Mexico to Peru have been documented.
In most geographical regions, killer whale movements may be related
to movements of their prey. Orcas may travel 125-200 km per day
while foraging (Dahlheim and Heyning, 1999 and refs. therein; Guerrero-Ruiz
et al. 1998).
In the Beaufort, Chukchi and northern Bering Seas, killer whales
move south with the advancing pack ice, performing long-range movements.
Similar movements are reported for the western North Atlantic. Killer
whales approach the Chukotka coasts in June and leave the area in
November or even as late as December (Reyes, 1991 and refs. therein).
Killer whales present in off-shore Norwegian waters appear to arrive
there from Icelandic waters, following the migration of herring.
Similae and Christensen (1992) photoidentified killer whales around
the Lofoten and Vesteralen Islands northern Norway during fall-winter
(October-February) and summer (June-August) in 1990 and 1991. Based
on a capture- recapture estimate, they determined that about 500
killer whales are present in these overwintering areas of the herring.
Most of the whales leave the study area in January when herring
migrate to the spawning grounds 700 km farther south (Similae et
al. 2002). Based on the seasonal distribution, killer whale groups
can be divided into three different types; whales present in fall-winter
(25 groups), whales present both in fall and summer (12 groups)
and whales present in summer (six groups).
In Northern Patagonia the seasonal distribution of killer whales
is correlated with the distribution of South American sea lions
(Otaria byronia) and southern elephant seals (Mirounga
leonina). Most encounters with the whales at Punta Norte occurred
in December and March-May, during the sea lions' breeding cycle.
Whales depart the area in May when pinnipeds migrate to winter rookeries.
One pod, Patagonia Norte B (PNB) was photographed in Golfo San José
on 9 January 1986 and in Punta Norte 1 day later, some 60 km distant
(Iniguez, 2001).
Evidence of seasonality is also observed in the southern part of
the northeastern Atlantic. In the southern hemisphere, killer whales
are found in warm waters in winter and migrate into high latitudes
in the summer. This migration appears to be related to the migration
of prey species, in particular the Antarctic minke whale (Reyes,
1991 and refs. therein). However, Gill and Thiele (1997) reported
sighting killer whales in Antarctic sea ice in August, i.e. in late
winter, indicating that some individuals may be resident year-round.
Transient whales appear to cover a more extensive range than residents.
A distance traversed of over 2,600 km (California to Alaska) was
reported for a transient group of three individuals photographed
in Monterey Bay, California, that had previously been identified
off Alaska. (Forney and Barlow 1998). However, Californian killer
whales may also move to the southern hemisphere. Guerrero et al
(2005) reported photo-identifying a male killer whale in Magdalena
Bay, Baja California, Mexico in 1988, in La Paz Bay in 1994 and
finally ca. 148 km off Pucusana, Peru in 2001. The minimum distance
between these sites is 5,500 km, extending the known maximum range
that killer whales are able to travel, and raising questions in
relation to population structure and interactions.
6. Threats
Direct catch: Killer whales have been exploited at low levels
in several regions world-wide (Jefferson et al. 1993). Norwegian
whalers in the eastern North Atlantic took an average of 56 whales
per year from 1938 to 1981. The Japanese took an average of 43 whales
per year along their coastal waters from 1946 to 1981. The Soviets,
whaling primarily in the Antarctic, took an average of 26 animals
annually from 1935 to 1979, but took 916 animals in the 1979/80
Antarctic season (Dahlheim and Heyning, 1999 and refs. therein;
Reyes, 1991).
After 1976, Iceland was involved in live-captures of killer whales
for export. During the period 1976-1988, 59 whales were collected,
of which 8 were released, 3 died and 48 (an average 3.7 per year)
were exported (Reyes, 1991 and ref. therein). In 1991, the lcelandic
Government announced that after expiry of existing permits for live
capture, no new ones would be issued (Jefferson et al. 1993). Live-captures
of killer whales have also taken place in Japanese waters (Reyes,
1991 and ref. therein). Because individuals play various roles in
maintaining social integrity of mammalian populations, not all individuals
are equal, and historic live-captures are likely to have broken
matriline networks into isolated groups (Williams and Lusseau, 2006).
Killer whales are still taken in small numbers in coastal fisheries
off Japan, Greenand, Indonesia and the Caribbean Islands (Reeves
et al. 2003).
Incidental catch: Incidental takes during fishing operations
occur but are considered rare (Dahlheim and Heyning, 1999 and refs.
therein). Baker et al. (2006) reported on the results of molecular
monitoring of 'whalemeat' markets in the Republic of (South) Korea
based on nine systematic surveys from February 2003 to February
2005, which revealed meat from killer whales. In southern Brazil,
killer whales prey on longline- caught tuna (Thunnus spp.)
and swordfish (Xiphias gladius), and cetacean by-cacth is
an issue in these fisheries (Rosa and Secchi, 2007).
Killing: Fishermen in many areas see killer whales as competitors,
and shooting of whales is known to occur. This problem has been
especially serious in Alaska, where conflicts with longline fisheries
occur (Jefferson et al. 1993). Although much reduced, some such
persecution continues today in Alaska and in the Strait of Gibraltar
(Ford, 2009).
Pollution: High levels of PCBs and DDT (250 ppm and 640 ppm,
respectively) were reported in the blubber of an adult male transient
killer whale in Washington State and 38ppm PCB and 59ppm DDE wet
weight levels in a resident male (Dahlheim and Heyning, 1999 and
refs. therein). Ross et al. (2000) reported that total PCB concentrations
were surprisingly high in three killer whales communities (2 resident
and 1 transient population) frequenting the coastal waters of British
Columbia, Canada. Transient killer whales were particularly contaminated.
Toxic equivalents in most killer whales surpassed adverse effects
levels established for harbour seals, suggesting that the majority
of free-ranging killer whales in this region are at risk from toxic
effects. The southern resident and transient killer whales of British
Columbia can be considered among the most contaminated cetaceans
in the world (Ross et al. 2000). Estimated concentrations in both
the northern and the more contaminated southern resident populations
have declined gradually in recent years. Projections suggest that
the northern resident population could largely fall below health
effects threshold concentrations by 2030 while the endangered southern
residents may not do so until at least 2063 (Hickie et al. 2007).
Recent studies from other parts of the world have produced similar
results. Killer whales in northern Norway are among the most polluted
arctic animals. Average total polychlorinated biphenyl (PCB) and
pesticide levels were similar, approximately 25 µg/g lipid,
and polybrominated diphenyl ethers (PBDEs) were approximately 0.5
µg/g, exceeding the already very high levels in polar bears.
The levels in Norwegian killer whales are more than 20 times higher
than those found in beluga whales (Delphinapterus
leucas)(Wolkers et al. 2007).
In eastern Hokkaido, Japan DDTs were the predominant contaminants,
with concentrations ranging from 28 to 220 µg/g on a lipid-weight
basis, followed by PCBs and other organochlorine pesticides. Japanese
killer whales also had high hepatic residue levels of butyltins
(from 13 to 770 ng/g wet weight) reflecting their extensive use
as antifouling paint (Kajiwara et al. 2006).
Noise pollution: Killer whales use sound for echolocation,
social communication, and passive listening. Anthropogenic noise
including sonar, acoustic harassment devices, vessel traffic, and
construction noise has the potential to interfere with bioacoustics.
In the northwestern USA, the endangered Southern Resident killer
whales are suffering from noise pollution in their environment (Holt,
2008).
From a sound propagation and impact model, Erbe (2002) deduced that
fast boats are audible to killer whales for over 16km, mask killer
whale calls over 14 km, elicit a behavioral response over 200m,
and cause a temporary threshold shift (TTS) in hearing of 5 dB after
30-50 min within 450m. For boats cruising at slow speeds, the predicted
ranges were 1km for audibility and masking, 50m for behavioral responses,
and 20 m for TTS. Superposed noise levels of a number of boats circulating
around or following the whales were close to the critical level
assumed to cause a permanent hearing loss over prolonged exposure.
From a study on the effects of acoustic harassment devices, Morton
and Symonds (2002) deduced that whale displacement resulted from
the deliberate introduction of noise into their environment.
Williams et al. (2002) investigated whether the current guidelines
for whalewatchers are sufficient to minimise disturbance to northern
resident killer whales in Johnstone Strait, British Columbia, Canada.
Local guidelines request that boaters approach whales no closer
than 100m. Additionally, boaters are requested not to speed up when
close to whales in order to place their boat in a whale's predicted
path: a practice known as "leapfrogging". Williams et
al. (2002) found that leapfrogging is a disruptive style of whalewatching
and should be discouraged; as the experimental boat increased speed
to overtake the whale's path, the source level of engine noise increased
by 14-dB. Assuming a standard spherical transmission loss model,
the fast-moving boat would need to be 500m from the whale for the
received sound level to be the same as that received from a slow-moving
boat at 100m. Whale-watching guidelines should therefore encourage
boaters to slow down around whales and not to resume full speed
while whales are within 500m.
Habitat degradation: Habitat disturbance may be a matter
for concern in areas inhabited by killer whales and supporting whale-watching
industries (Reyes, 1991). Visser (1999) e.g. reports on propeller
scars observed on killer whales and their possible cause of mortality.
Vessel traffic may have contributed to southern resident killer
whales becoming endangered. Lusseau et al. (2008) observed a reduction
in time spent foraging, confirming an effect also previously observed
in northern resident killer whales. Each school was within 400 m
of a vessel most of the time during daylight hours from May through
September. If reduced foraging effort results in reduced prey capture,
this would result in decreased energy acquisition and biological
fitness (Lusseau et al. 2008).
After the 1989 'Exxon Valdez' oil spill in Alaska, the resident
AB Pod and the transient AT1 group suffered losses of 33 and 41%,
respectively, in the year following the spill. By 2005, AB Pod had
not recovered to pre-spill numbers. Moreover, its rate of increase
was significantly less than that of other resident pods that did
not decline at the time of the spill. The AT1 Group, which lost
9 members following the spill, continued to decline and is now listed
as depleted under the U.S. Marine Mammal Protection Act (Matkin
et al. 2008).
Overfishing: Some populations of killer whales could be
affected by reduction of their food supply. For example, coastal
Norwegian populations reportedly feed mainly upon herring, a fish
heavily exploited in the area (Reyes, 1991 and refs. therein). In
Alaska, anthropogenic effects on the ecosystem are thought by some
to be responsible for killer whale predation on sea-otters and associated
ecological implications (Estes et al. 1998). In British Columbia,
Canada, and Washington State, US, salmon stocks have significantly
declined as an effect of overfishing, habitat degradation and reduced
ocean survival. This is likely to affect fish-eating resident killer
whales in that area (Ford, 2009).
Other factors: In the southern Indian Ocean, the strong decline
reported by Poncelet et al. (2002) for the coastal waters of Possession
Island between 1988 and 2000 may be attributed to several factors:
i) a low and decreasing fecundity, possibly impacted by a density
dependence (Allee effect); ii) the decline of the main preys (large
baleen whales due to past whaling and southern elephant seals (Mirounga
leonina) from the 1970 to 1990 which remained in low numbers
up to at least 1997; iii) the possible mortality induced by recent
interactions with the Patagonian toothfish (Dissostichus eleginoides)
longline fishery; and iv) the possible dispersion of individuals
or groups from coastal waters. A preliminary toxicological study
indicates that PCB levels are considerably lower than in British
Columbia transients, however the burdens are not negligible (Ross,
pers. com.) and the effects of PCBs on health at the observed concentrations
are unknown. Poncelet et al. (2002) feared that the killer whales
of Possession Island might disappear, losing unique genetic diversity
and social culture, like AT1 transients in Alaska.
7. Remarks
Range states (Taylor et al. 2008) :
Albania; Algeria; American Samoa; Anguilla; Antarctica; Antigua
and Barbuda; Argentina; Aruba; Australia; Bahamas; Bangladesh; Barbados;
Belize; Benin; Bermuda; Brazil; British Indian Ocean Territory;
Brunei Darussalam; Cambodia; Cameroon; Canada (Newfoundland); Cape
Verde; Cayman Islands; Chile; China; Cocos (Keeling) Islands; Colombia;
Comoros; Congo; Congo, The Democratic Republic of the; Cook Islands;
Costa Rica; Côte d'Ivoire; Cuba; Denmark; Djibouti; Dominica;
Dominican Republic; Ecuador (Galápagos); El Salvador; Equatorial
Guinea; Falkland Islands (Malvinas); Faroe Islands; Fiji; France;
French Guiana; French Polynesia; French Southern Territories (the)
(Kerguelen); Gabon; Gambia; Ghana; Gibraltar; Greenland; Grenada;
Guadeloupe; Guam; Guatemala; Guinea; Guinea-Bissau; Guyana; Haiti;
Heard Island and McDonald Islands; Honduras; Iceland; India; Indonesia;
Iran, Islamic Republic of; Ireland; Israel; Italy; Jamaica; Japan;
Kenya; Kiribati; Liberia; Madagascar; Malaysia; Maldives; Marshall
Islands; Martinique; Mauritania; Mexico; Micronesia, Federated States
of; Monaco; Morocco; Mozambique; Myanmar; Namibia; Nauru; Netherlands;
Netherlands Antilles; New Caledonia; New Zealand; Nicaragua; Nigeria;
Niue; Northern Mariana Islands; Norway; Oman; Pakistan; Palau; Panama;
Papua New Guinea; Peru; Philippines; Pitcairn; Portugal; Puerto
Rico; Russian Federation; Saint Helena; Saint Kitts and Nevis; Saint
Lucia; Saint Pierre and Miquelon; Saint Vincent and the Grenadines;
Samoa; Senegal; Sierra Leone; Singapore; Solomon Islands; Somalia;
South Africa; South Georgia and the South Sandwich Islands; Spain;
Sri Lanka; Suriname; Svalbard and Jan Mayen; Taiwan, Province of
China; Tanzania, United Republic of; Thailand; Timor-Leste; Togo;
Tonga; Trinidad and Tobago; Tunisia; United Arab Emirates; United
Kingdom; USA (including Aleutian Is., Hawaiian Is.); USA Minor Outlying
Islands; Uruguay; Vanuatu; Venezuela; Viet Nam; Virgin Islands,
British; Virgin Islands, U.S.; Wallis and Futuna; Western Sahara;
Yemen.
Orcinus orca is listed in Appendix II of CITES and in Appendix
II of CMS. The species is categorised as "Data Deficient"
by the IUCN. Some regional populations are small or highly specialised
and may be threatened by habitat deterioration. This is the case
in the critically threatened small Strait of Gibraltar population,
which suffers declines in numbers and prey availability; as well
as in the southern resident community of Washington and British
Columbia, which is listed as Endangered under the US Endangered
Species Act and the Canadia Species at Risk Act (Ford, 2009).
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.
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