Phocoenoides dalli (True,
English: Dall's porpoise
Spanish: Marsopa de Dall
French: Marsouin de Dall
Phocoenoides dalli © Wurtz-Artescienza
Like the other members of the phocoenid family, Dall's porpoises
have a stocky body with a short, wide-based, triangular dorsal fin.
The beak is very short and poorly defined. The flippers and flukes
are small. The colour pattern is very characteristic, the animals
being largely dark grey to black with a large, ventrally continuous
white patch which extends up about halfway on each flank. The upper
part of the dorsal fin and the trailing edge of the flukes are light
grey. Maximum body length is 239 cm and mass 200 kg (Jefferson,
Dall's porpoises are polymorphic in their pigmentation pattern.
In dalli type animals, the flank patch extends to about the
level of the dorsal fin whereas in truei type animals the
patch extends to about the level of the flippers. Both colour morphs
were variously considered as species or subspecies in the past (e.g.
Rice, 1998), and genetic analysis confirms that they form separate
populations: Pairwise comparisons indicate a low but significant
difference between the Sea of Japan-Okhotsk dalli-type population
on the one hand and the truei-type population and the standard
dalli-type population in the northwestern North Pacific on
the other hand (Escorza-Treviño and Dizon, 2000; Hayano et
al. 2003; Amano and Hayano, 2007). Furthermore, there seems to be
a demographic distinctiveness between Bering Sea and western North
Pacific stocks (McMillan and Bermingham, 1996).
The distribution of Dall's porpoise is confined to the North Pacific
Ocean and adjacent seas. They range in subarctic waters from the
Sea of Okhotsk, Bering Sea, and the northern Gulf of Alaska, south
to the Sea of Japan, the Subarctic Boundary at about 63°N across
the North Pacific, and in the California Current to about 32°N
off Baja California Norte. Although mainly an offshore deepwater
inhabitant, Dall's porpoise also occurs in narrow channels and fjords
where the water is clear and relatively deep, such as those in Prince
William Sound and around the Alexander Archipelago in Alaska (Jefferson,
Distribution of Phocoenoides dalli: North Pacific
(Hammond et al. 2008;
© IUCN; Enlarge
There are records of the species as far south as 28°N, off
the coast of Baja California (Mexico) although reported only during
periods of exceptionally cold waters. At the northern end of the
range, sightings are infrequent north of 62°N in the Bering
Sea, but there have been occasional sightings in the Chukchi Sea
(Reyes, 1991, and refs. therein).
3. Population size
Several stocks have been recognised, based largely on geographic
variation in morphology and colour patterns, parasite loads, densities
of mother/calf pairs, and genetic differences. Eight stocks (seven
dalli-type and one truei-type) are recognised by the International
Whaling Commission (Houck and Jefferson, 1999 and refs. therein).
Nevertheless, most abundance estimates are geographically justified:
For Alaskan US EEZ waters, Angliss and Outlaw (2005) estimate population
size from 1987-1991 data at 83,400 after correcting for vessel attraction
In the inshore coastal waters of the Inside Passage, between British
Columbia (BC)-Washington and the BC-Alaska borders surveys conducted
in 2004 and 2005 yield an abundance estimate of 4,910 (CI = 2,700-8,940)
(Williams and Thomas, 2007).
The most recent estimate of Dall's porpoise abundance in the eastern
Pacific US EEZ is the geometric mean of estimates from 2001 (Barlow
and Forney 2007) and 2005 (Forney 2007) summer/autumn vessel-based
line transect surveys of California, Oregon, and Washington waters,
or 48,376 (CV = 0.24) animals.
In 2007 new abundance estimates for Dall's porpoises were made
available for Japanese waters, based on 2003 survey data. The new
population estimates are 173,638 dalli-type porpoises and 178,157
truei porpoises (IWC, 2008), lower than the estimates of 1991 of
226,000 and 217,000 (IWC, 1998) respectively.
4. Biology and Behaviour
Habitat: Dall's porpoise is found in diverse habitats, including
sounds, nearshore waters (near deep water canyons) as well as offshore
waters more than 1,000 km from shore. Waters colder than 18°C
are preferred, and the peak abundance is in waters colder than 13°C
(Reyes, 1991 and refs. therein). It may routinely forage at depths
of 500 m or more (Carwardine, 1995). It is not found in the southern
extremes of its range during the summer or warm water months (Houck
and Jefferson, 1999). Ferrero (1998) confirms, that sea surface
temperature was the most important habitat parameter examined.
Behaviour: Almost hyperactive. Darts and zig-zags around
at great speed, and may disappear suddenly. Swimming-speeds can
reach 55 km/h. This is the only porpoise that will rush to a boat
to bow-ride, but soon loses interest in anything that travels slower
than 20 km/h. They do not porpoise like other small cetaceans, but
produce a "rooster tail" (Carwardine, 1995).
Schooling: Dall's porpoises are found mostly in small groups
of 2 to 12, although aggregations of up to several thousand have
been reported. Groups appear to be fluid, often forming and breaking
up for feeding and playing (Jefferson et al. 1993). They often associate
with Pacific white-sided dolphins (Lagenorhynchus
obliquidens; from 50°N southwards) and pilot whales
macrorhynchus; from 40°N southwards) (Carwardine, 1995).
Bowriding behaviour has been observed with gray (Eschrichtius
robustus), fin (Balaenoptera physalus), blue (Balaenoptera
musculus) and humpback whales (Megaptera novaeangliae)
(Houck and Jefferson, 1999, and refs. therein).
Food: Stomach samples from Dall's porpoises collected in
pelagic waters spanning most of their range in the North Pacific
and the Bering Sea revealed a diet of myctophid fish in the subarctic
North Pacific and on gonatid squids as well as myctophid fish in
the Bering Sea, with little prey selectivity. Most of the prey items
were mesopelagic species that migrate vertically to shallower waters
at night. Stomach content was greater during twilight hours, suggesting
the porpoises foraged actively on myctophids at night in shallower
waters. According to Ohizumi et al. (2003), the annual consumption
by Dall's porpoises was estimated to be 2.0-2.8 million tons, or
4.7-6.5% of the biomass of mesopelagic fish in the subarctic North
Pacific. Comparison of stomach contents and trawl samples shows
crude consistency (Ohizumi and Watanabe, 2004).
Amano and Kuramochi (1998) suggest from their findings, that Dall's
porpoises feed opportunistically, changing prey items and feeding
times based on supply. The most common prey items in the Sea of
Okhotsk were the Japanese pilchard (Sardinops sagax) and
the squid (Berryteuthis magister) (Walker, 1996). Around
Hokkaido in the Sea of Okhotsk and the Sea of Japan, the dominant
prey species switched from the late 1980s to the early 1990s as
the Sardinops melanostictus (Japanese pilchard) populations
in both seas declined. In the Sea of Japan, Dall's porpoises switched
to Theragra chalcogramma (walleye pollock), and in the Sea
of Okhotsk, to Engraulis japonicus (Japanese anchovy) and
Berryteuthis magister (magistrate armhook squid) (Ohizumi
et al. 2000).
Reproduction: Most Dall's porpoise calves are born in spring
and summer (Jefferson et al. 1993). Segregation of age and sex classes
was determined in the western North Pacific population. Mother-calf
pairs are sighted only north of 46°N. Data obtained from gillnet
fishery confirm that pregnant and lactating females dominate in
the northern Pacific area and that newborn calves are also present.
These observations probably indicate a calving and breeding area
for the population north of the USA Exclusive Economic Zone (EEZ).
The percentage of mature males in this area is low, and most mature
males are found south of the USA EEZ (Reyes, 1991 and refs. therein).
Besides the truiei and dalli types, there seems to
be a frequent hybridization between free-ranging Dall's and harbour
phocoena. All crosses examined had Dall's porpoise as the
maternal parent, a directionality reflecting the indiscriminate
pursuit of female porpoises by male harbour porpoises (Willis et
Although the species as such is present all year round in Prince
William Sound, Alaska, a decrease in abundance of Dall's porpoises
was observed from fall to winter, indicating a movement of a portion
of that population out of the area. These seasonal migrations may
also occur in the Gulf of Alaska and the Bering Sea (Reyes, 1991
and refs. therein). According to Forney and Barlow (1998) Dall's
porpoises seem to shift their distribution southward during cooler
water periods on both interannual and seasonal time scales. In southern
California waters, Dall's porpoises were found only in the winter,
generally when the water temperature was less than 15°C (Houck
and Jefferson, 1999). Carretta et al. (2000) also found that Dall's
porpoises were present off San Clemente Island, California, only
during the cold-water months of November-April.
Houck and Jefferson (1999), suggest that this species is present
year-round in central California, northern California, Puget Sound,
Washington, and British Columbia. In these areas, waters remain
cool (about 9-15°C) throughout the year. Inshore/offshore movements
off southern California and British Columbia have also been postulated.
Although movements in the eastern Pacific also have a north/south
component, there appear to be more distinct north/south movements
in the western Pacific. These movements may be temperature-related
or food-dependent. Truei-type porpoises and mixed schools are generally
found in warmer waters, while dalli-types are found in both warmer
and colder waters (Houck and Jefferson, 1999 and refs. therein).
Porpoises of the truei-type winter off the Pacific coast of Japan,
moving in summer towards the north, reaching the southern Kuril
Islands. Migration of truei-type animals into the Okhotsk Sea was
recently confirmed, and it has been suggested that this occurs through
the Kuril Islands. The presence of a higher percentage of mother-calf
pairs in the southern part of that sea suggests that the area represents
a breeding ground for the truei-type. Up to 15,000 animals of the
dalli-type are reported to migrate through the Tsugaru Strait to
the Pacific coast of Japan (Reyes, 1991 and refs. therein)..
Direct catch: A fishery for Dall's porpoises operates only
in Japanese waters and dates back to early in the 20th century.
While this fishery was developed primarily during winter months,
it has spread to other seasons and areas, resulting in an increase
in the annual catch and the inclusion of the dall's-type in the
captures. A total of 40,000 were taken in 1988 from a population
of about 105,000 porpoises migrating to the fishing grounds. The
stock composition of the catches is not known. The effect of hunting
at such a level on the populations is a matter of serious concern
(Reyes, 1991 and refs. therein). In recent years, the catch has
been reduced somewhat, but still remains too high, with 11,000 harpooned
in 1998 (Houck and Jefferson, 1999 and refs. therein). The latest
figure is 11,357 captured in 2007, of which 4,070 are of the dalli
and 7,287 of the truei type.
The Japanese hunt of Dall's porpoise has been highlighted by the
IWC SC several times as unlikely to be sustainable. However the
Japanese government has ignored IWC SC recommendations to reduce
quotas, claiming that IWC does not have competence over small cetaceans
Incidental catch: In addition to the direct catch, Dall's
porpoises are captured incidentally, mostly in drift net fisheries.
In the Being Sea and Gulf of Alaska as well as around the Alaska
Peninsula and Aleutian Island salmon drift gillnet fishery results
in an estimated annual incidental kill rate in observed fisheries
of 33.9 porpoise per year from this stock (Angliss and Outlaw, 2005).
Dall's porpoises are also by-caught in salmon gillnet fisheries
in British Columbia, Canada waters. However, best estimates of bycatch
mortality in 2004 and 2005 exceeds only the most precautionary limits
for porpoise species (Williams et al. 2008).
Estimates of incidental marine mammal, sea turtle, and seabird
mortality in the California drift gillnet fishery for broadbill
swordfish, Xiphias gladius, and common thresher shark, Alopias
vulpinus, for the 7-year period, 1996 to 2002 amount to 44 Dall's
porpoises (Carretta et al. 2004). In the eastern Pacific US EEZ,
current mean annual takes for all fisheries for which mortality
data are available are 1.4 animals per year (Caretta et al. 2009).
Large numbers of Dall's porpoises die in driftnets within national
waters of Japan and Russia, where the UN ban on driftnets does not
apply. The estimated bycatch in the Japanese salmon driftnet fishery
operating in the Russian EEZ totaled close to 12,000 for the period
1993 to 1999, ranging from 643-3149 on an annual basis (IWC 2002).
At its 60th annual meeting, the IWC in 2008 reiterated its concern
for the stocks of Dall's porpoise off Japan and repeated its previous
recommendation that catches should be reduced to sustainable levels,
that the bycatch levels be quantified and that a full assessment
of each of the affected populations be conducted as soon as possible.
Pollution: Organochlorine compounds (OCs) such as polychlorinated
biphenyls (PCBs), dichlorodiphenyltrichloroethane and its metabolites
(DDTs), chlordane related compounds, hexachlorocyclohexane isomers
(HCHs), hexachlorobenzene (HCB) and tris-chlorophenyl methane (TCPMe)
were found in the blubber samples of Dall's porpoises collected
from Japanese coastal waters in 1998/1999. Concentrations and compositions
of DDTs and HCHs showed significant differences between the truei-type
population off the Pacific coast of northern Japan and dalli-type
from the Sea of Japan/Okhotsk. OCs levels detected in truei-type
porpoises collected in 1998/1999 were lower than those collected
in 1984, except TCPMe. On the other hand, except DDTs, the residue
levels of other organochlorines in dalli-type porpoises showed no
significant decrease since 1984 (Kajiwara et al. 2002). High concentrations
of organochlorines (especially DDT) were also reported in Dall's
porpoises from southern California (Reyes, 1991 and refs. therein).
Since females may transfer organochlorines to their offspring during
gestation and especially through lactation, and testosterone levels
in males may be reduced by high levels of PCBs and DDE, this may
have detrimental effects on production and calf survival (Houck
and Jefferson, 1999 and refs. therein; Jarman et al. 1996).
Concentrations and body burdens of 14 trace elements (Hg, Cr, Mn,
Co, Cu, Zn, Sr, Ag, Cd, V, Se, Pb, Mo, and Fe) and butyltins (BTs)
(tributyltin TBT, dibutyltin DBT, and monobutyltin MBT) were determined
in various tissues of a porpoise collected off the Sanriku coast
of Japan. Selective accumulation was observed for Hg, Mn, Cu, Ag,
Mo, Fe, and total BTs (TBT, DBT, and MBT) in the liver, Cd in the
kidney, Zn, Sr, V, Pb, and Co in the bone, and Se in the skin (Yang
et al. 2006). A mother-fetus pair collected off the Sanriku coast,
Japan was contaminated by phenyltin compounds (Yang et al. 2007).
Overfishing: It is unlikely that the fishery for salmon
could directly affect the food supply of Dall's porpoises, since
salmon is not their regular prey. However, other fisheries operating
in the North Pacific take a variety of fish species that could include
potential prey species. The development of the squid fishery in
the region could eventually represent a potential threat by reducing
food availability (Reyes, 1991 and refs. therein).
Range states (Hammond et al. 2008) :
Canada; Japan; Korea, Democratic People's Republic of; Korea, Republic
of; Mexico; Russian Federation; United States of America.
Phocoenoides dalli is included in CITES Appendix II . The
species is considered as "Least Concern" by the IUCN (Hammond
et al. 2008). It is included in Appendix II of CMS.
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.