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Lagenorhynchus obliquidens Gill,
1865 
English: Pacific white-sided dolphin
German: Weißstreifendelphin
Spanish: Delfín de costados blancos del Pacífico
French: Dauphin à flancs blancs du Pacifique
Family Delphinidae
Lagenorhynchus obliquidens © Würtz-Artescienza
(see "links")
1. Description
The boldly coloured Pacific white-sided dolphin is black or dark
grey on the back and posterior sides, as well as on the short snout,
the leading edge of the tall dorsal fin, and the pointed flippers.
The light grey thoracic patch is sharply delineated from the white
belly by a thin dark line, in contrast with the dusky dolphin (L.
obscurus) which lacks this line and the sharp demarcation.
Grey, linear dorsal flank blazes, often called "suspender stripes",
project forward from the grayish flank patches along the back and
disappear above the eyes. Average adult size is 2.1-2.2 m and body
mass reaches 75-90 kg (van Waerebeek, 2002).
Specimens from Korea Strait are on average larger than those from
far offshore in the western North Pacific (35°-46°N, 158°-180°E).
A tiny proportion of individuals exhibit an alternate colour phase
(Rice, 1998, and refs. therein), e.g. in Volcano Bay, Hokkaido,
Japan, individuals with anomalous white colour patterns exhibiting
various degrees of lack of pigmentation were identified (Tsutsui
et al. 2001). Furthermore, investigation of genetic diversity and
differentiation (Hayano et al. 2004) suggests that Pacific white-sided
dolphins in Japanese coastal waters and offshore North Pacific belong
to different populations between which gene flow has been severely
restricted.
Animals off Baja California have consistently larger crania than
the ones from northern California northward, with inter-grading
populations occupying the intervening area off southern and central
California (Rice, 1998, and refs. therein). However, Lux et al.
(1997) found that population-by-population mtDNA comparisons of
four geographic populations in the eastern Pacific indicated that
all could be considered isolated, but likely incompletely, from
one another. Black (2009) concluded that 6 populations can be differentiated:
coastal Japan, offshore Japan, North Pacific, British Columbia,
Oregon to California, Baja California.
Close scrutiny of morphological and life history parameters as well
as recent cytochrome c sequence analysis supports the premise that
L. obscurus and L. obliquidens are sister species
which diverged 1.9-3 million years ago (van Waerebeek, 2002).
2. Distribution
L. obliquidens is found in the cool temperate waters of
the North Pacific. It ranges in the west from the South China Sea
northward, throughout Japanese waters, and around the Kuril Islands,
extending north to the Commander Islands, and also occurs in the
Sea of Japan and in the southwestern Okhotsk Sea. In the eastern
Pacific, the species occurs primarily in shelf and slope waters
from the southern Gulf of California, Mexico along the western coast
of North America north to the Gulf of Alaska and as far west as
Amchitka in the Aleutian Islands. Across the North Pacific, the
species is generally found to have a relatively narrow distribution
between 38°N and 47°N (Brownell et al. 1999, Black, 2009).
Distribution of Lagenorhynchus obliquidens: deep
temperate waters of the northern north Pacific, predominantly off-shore
(Hammond et al. 2008; © IUCN; enlarge
map).
Vagrant to Bahia de La Paz in the south-western Golfo
de California (Rice, 1998) and infrequently, in the southern Bering
Sea (Brownell et al. 1999).
3. Population size
Buckland et at. (1993) estimated the abundance of Pacific white-sided
dolphins in the North Pacific at 931,000 animals. This is in close
agreement with the estimate of 989,000 by Miyashita (1993). However,
precision is low for both estimates, and vessel attraction probably
resulted in overestimation of population size (Buckland et al. 1993).
For the eastern North Pacific, there are separate abundance estimates
for different regions and seasons. Off Oregon and Washington, a
peak abundance of 23,400 animals was estimated in May 1992 (Forney
et al. 1995). The 2001-2005 geometric mean abundance estimate for
California, Oregon and Washington waters based on the two most recent
ship surveys was 20,719 (CV =0.22) Pacific white-sided dolphins
(Barlow and Forney 2007; Forney 2007). No long-term trends in abundance
were suggested based on historical and recent surveys (Caretta et
al. 2008).
In February-April 1991 and 1992, aerial surveys conducted along
the continental shelf and slope of California resulted in a population
estimate of 122,000 (Forney et al. 1995). This contrasts with a
ship-based estimate of only 5,900 in August-November 1991 for the
same study area (Forney and Barlow, 1998), a discrepancy which may
be explained by seasonal migrations (Brownell et al. 1999).
In the coastal waters of British Columbia, Canada, the Pacific
white-sided dolphin is probably the most abundant cetacean (Heise,
1997a): In the inshore coastal waters of the Inside Passage, between
the British Columbia (BC)-Washington and the BC-Alaska borders,
Williams and Thomas (2007) estimated the population size in 2004-2005
at 25,900 (95% CI = 12,900-52,100).
4. Biology and Behaviour
Habitat: L. obliquidens is mainly found offshore,
as far as the edge of the continental shelf, but does come closer
to shore where there is deep water, such as over submarine canyons
(Carwardine, 1995). It is known to occur close to shore in regions
such as the inshore passes of Alaska, British Columbia, and Washington,
and seasonally off southern California (Brownell et al. 1999, and
refs. therein).
Investigation of habitat segregation between various species of
small cetaceans in the central North Pacific Ocean revealed that
sea-surface temperature was the most influential habitat parameter
examined, with L.
borealis occupying the warmest waters, P.
dalli the coolest, and L. obliquidens in between
but with greater preference overlap with P. dalli. (Ferrero,1998;
Ferrero et al. 2002).
Associations between cetacean distributions, oceanographic features,
and bioacoustic backscatter were examined in the northern California
Current System (CCS) during late spring and summer 2000. Pacific
white-sided dolphin were the most numerous small cetacean in early
June but were rare during August. Up to 45% of the variation in
their occurrence pattern was described by distance to the upwelling
front and acoustic backscatter at 38 kHz (Tynan et al. 2005).
Behaviour: L. obliquidens is very inquisitive and
may even approach stationary boats (Carwardine, 1995). It is highly
acrobatic and playful, commonly bowriding, and often leaping, flipping,
or somersaulting (Jefferson et al. 1993).
Reproduction: Calving occurs from May to September. Females
become sexually mature at 8-11 years (175-186 cm length) and males
at 9-12 years (170-180 cm length). Gestation lasts 11-12 months.
Males may live to 42 years and females to 46 years (Heise 1997b).
Schooling: Often seen in large herds of hundreds or even
thousands, these highly gregarious dolphins are also commonly seen
with other species, especially northern right-whale dolphins and
Risso's dolphins (Jefferson et al. 1993) as well as other cetaceans
(Brownell et al. 1999). The interspecific relationship with the
northern right-whale dolphin (L. borealis) appears to be
a unique association in which large groups of both species are frequently
observed to form heterogeneous herds and subgroups. The reason for
this close association may be food related, particularly in the
oceanic environment, as there is considerable overlap in preferred
mesopelagic prey (Brownell et al. 1999 and refs. therein). Large
schools of Pacific white-sided dolphins may split into smaller groups
when feeding but re-assemble when resting or travelling (Carwardine,
1995).
Food: Pacific white-sided dolphins consume a wide variety
of fish and cephalopods. However, considerable differences in feeding
preference are evident between animals from coastal and offshore
regions. Off British Columbia, Canada, herring (Clupea harengus)
was the most commonly occurring prey species (59%), followed by
salmon (Oncorhynchus spp.; 30%), cod (Family Gadidae; 6%),
shrimp (Order Decapoda; 3%) and capelin (Mallotus villosus;
1%; Heise, 1997a). In the North Pacific they feed primarily on epipelagic
fish and cephalopods: northern anchovy (Engraulis mardax),
Pacific hake (Merluccius productus), Pacific saury (Cololabis
saira), juvenile rock fish (Sebastes spp., and horse
mackerel (Trachurus symmetricus). The market squid (Loligo
opalescens) is also frequently ingested. In the central North
Pacific L. obliquidens feeds heavily on mesopelagic fish
and cephalopods and in coastal waters of northern Japan on both
mesopelagic and epipelagic fish and cephalopods (Brownell et al.
1999 and refs. therein).
5. Migration
Some seasonal shifts occur; while more common in coastal waters
during fall and winter, L. obliquidens move offshore during spring
and summer, in rough synchrony with the movements of anchovies and
other prey (van Waerebeek, 2002 and refs. therein).
Seasonal abundance estimates off the entire coast of California
are an order of magnitude higher in February-April than in August-November,
while peak abundances off Oregon and Washington are observed during
May. This pattern strongly suggests seasonal north-south movements
eastern North Pacific (Forney and Barlow, 1998). Aurioles et al.
(1989) also noted that the species is found seasonally, in spring
and summer, in the southwestern Gulf of California. Off San Clemente
Island, California, Pacific white-sided dolphins were present only
during the cold-water months of November-April (Carretta et al.
2000). Brownell et al. (1999) suggested that the occurrence of the
southern form of L. obliquidens off Southern California appears
to be variable, possibly relating to changes in oceanographic conditions
on seasonal or inter-annual time scales (i.e. El Niño events).
In Alaskan waters, published sighting records are sparse, but the
occurrence of Pacific white-sided dolphins may be related to periods
of warmer waters (Dahlheim and Towell, 1994). Off Japan, Pacific
white-sided dolphins occupy the Korean Strait and waters of western
Japan in the winter and appear to move to the east from March to
July. Nothing is known about the movements of the two forms described
from Japanese coastal waters (Brownell et al. 1999 and refs. therein;
Tsutsui et al. 2001).
6. Threats
Direct catch: According to Jefferson et al. (1993), Japanese
drive and harpoon fisheries took hundreds or even thousands of Pacific
white-sided dolphins in most years, but Brownell et al. (1999) reported
that only "small numbers" are taken annually. Black (2009)
confirmed that dolphins are still harpooned in Japan today for human
consumption, whereas the drive fishery in Taiji does not generally
catch Pacific white-sided dolphins. However, the Japanese government
is currently considering a renewed direct harvest of this species
(Hammond et al. 2008). Few live captures have been reported in the
past, the most recent one in 1992 when 3 animals were take for public
display (Forney, 1994).
Incidental catch: In the eastern Pacific a total of 363
animals were estimated to have been killed in the shark and swordfish
drift net fishery in California during the period from April 1988
to December 1995. Additional mortality has been documented for trammel
and set nets in California coastal waters, for drift gill nets in
British Columbia and Alaska, and for trawl fisheries in Alaska.
Pacific white-sided dolphins were rarely taken in the tuna purse
seine fishery in the eastern tropical Pacific because most of the
fishing takes place south of the range of these dolphins (Brownell
et al. 1999 and refs. therein). The most recent average estimate
of fishery-related mortality of Pacific white-sided dolphins in
US eastern Pacific waters is very low. Including mortality from
drift gillnet, groundfish trawl, and unknown fisheries, 1.4 (CV
= 0.86) animals are removed annually. Similar low levels of mortality
have also been documented in the California/Oregon/ Washington domestic
groundfish trawl fisheries (Caretta et al. 2008). By-catch mortality
estimates for 2004-2005 in salmon gillnet fisheries off British-Columbia,
Canada, were also below precautionary limits (Williams et al. 2008).
In the western Pacific, Pacific white-sided dolphins were one of
the most commonly caught cetaceans in the Japanese and Korean high
seas squid drift net fisheries (Hobbs and Jones, 1993). They were
also taken in the Japanese large-mesh and Taiwanese squid and large-mesh
fisheries. In 1989, the estimated total by-catch for only the Japanese
squid drift net fishery was about 6,100; in 1990, the total estimate
for all drift net fisheries combined was 5,759 animals (Hobbs and
Jones, 1993). Effort for these fisheries was estimated to have increased
during the late 1970s and early 1980s and then remained relatively
stable at least until 1990 (Hobbs and Jones, 1993). In January 1993
a United Nations moratorium on these high seas drift net fisheries
went into effect. Smaller catches (e.g. at least 194 in 1987) are
reported from the Japanese land-based salmon drift net fishery.
Small numbers are taken yearly in seines, set nets, and trap nets
around Japan (Brownell et al. 1999 and refs. therein).
Molecular monitoring of 'whalemeat' markets in the Republic of
(South) Korea between 2003 and 2005 revealed that Pacific white-sided
dolphins were for sale there. As Korea has no programme of commercial
or scientific whaling and there is a closure on the hunting of dolphins
and porpoises, the only legal source of these products was assumed
to be incidental fisheries mortality (Baker et a. 2006).
Killing: Japanese government-supported "cull"
programmes to control several small cetaceans, including Pacific
white-sided dolphins, were initiated during the 1970s. Between 1976
and 1980, which were the peak years of this programme, at least
466 L. obliquidens are reported to have been killed (Brownell et
al. 1999 and ref. therein).
Pollution: The maximum concentrations of DDT and PCBs reported
in the blubber of Pacific white-sided dolphins in Japanese waters
were 99 ppm and 71 ppm wet weight, respectively. Organochlorine
levels in the blubber of two stranded animals from Californian waters
were 2.08 ppm and 99.5 ppm DDT, and 0.23 ppm and 4.88 ppm PCBs.
Overall, pollutant loads for this species appear to be variable
(Brownell et al. 1999 and refs. therein).
7. Remarks
Range states (Hammond et al. 2008) :
Canada; China; Japan; Korea, Democratic People's Republic of; Korea,
Republic of; Mexico; Russian Federation; United States of America
IUCN Status: "Least Concern" (Hammond et al. 2008). CMS
status: "not listed". However, the Pacific white-sided
dolphin is a migratory species which presumably crosses the boundaries
of several countries on the east and west coasts of the Pacific
Ocean. The species should therefore be included in Appendix II of
the CMS. The species is listed in Appendix II of CITES.
8. Sources
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VJ (1989) The white-sided dolphin Lagenorhynchus obliquidens
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· Baker CS, Lukoschek V, Lavery S, Dalebout ML, Yong-un M,
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and porpoise 'bycatch' revealed by molecular monitoring of Korean
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© Boris Culik (2010) Odontocetes.
The toothed whales: "Lagenorhynchus obliquidens".
UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza. ©
Maps by IUCN.
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