Dall's Porpoise (돌곱등어)

 

돌곱등어(Phocoenoides dalli)

영명 : Dall's Porpoise

 

몸길이 : 1.8~2.0m, 몸무게 : 87~123kg. 몸의 측면에서 복부에 걸쳐 있는 흰 반점의 크기로 '돌곱등어형'과 '트루돌고래형'으로 나뉜다. 큰점까치돌고래와 작은점까치돌고래 2종이 있다. 작은점까치돌고래는 흰 반점이 항문에서 등지느러미 기저전방 사이에 있으나, 큰점까치돌고래는 항문으로부터 가슴지느러미 기저까지 확장되어 있다.

 

진흑색의 몸의 바탕에 옆면과 복부에 큰 흰색 반점이 있다(까치무늬). 등지느러미를 정점으로 몸체가 삼각형이며 머리 부분와 꼬리자루는 뾰족한 편이다. 아래위턱 좌우에 20쌍의 이빨이 있다.

보통 느리게 움직이고 수면에 뒹굴며 놀지만, 빨리 유영할 때는 소형고래류 중 순간 유영속도가 가장 빠른 것으로 알려져 있다. 점프하지 않고 수면을 닭볏 모양으로 가르면서 유영하며 일정한 방향 없이 신출귀몰한다. 표층 및 중층의 어류, 오징어, 특히 앨퉁이류 등 소형 발광어류를 선호하는 기회적인 잡식성이다.

 

북태평양, 오호츠크해, 베링해, 아한대, 한대, 북서태평양의 고유종으로 연안성이며 냉수역에 서식하고 있다.

 

◈ 북태평양 (태평양) : 1,186,000 마리.

◈ 북태평양 (오호츠크해) : 554,000 마리.

 

                                     

 

 

 

 

 

 

Description

Like the other members of the phocoenid family, Dall's porpoise has a stocky body with a short, wide-based, triangular dorsal fin. The beak is an very short and poorly defined . The flippers and fluke 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. 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.

There is minor geographical variation in the colour pattern of Dalli-phase animals, with the most distinctive individuals in the Sea of Japan. Skull size also varies geographically, averaging smaller in animals from the open ocean than in animals from the Sea of Japan, the Sea of Okhotsk, the Bering Sea, and the coast of California. The results of McMillan and Bermingham, who studied mtDNA variation in 101 Dall's porpoises from the Bering Sea and western North Pacific, support the demographic distinctiveness of Bering Sea and western North Pacific stocks. More recently, Escorza-Treviño and Dizon suggested from DNA-analyses that there are nine distinct dalli-type populations that should be treated as separate units for management purposes.

Hayano et al. investigated the genetic diversity and phylogenetic relationships among three morphologically distinct populations of Dall's porpoise in Japanese waters by analyzing mitochondrial DNA variation. These populations, the Sea of Japan-Okhotsk dalli-type population, the truei-type.
population and the standard dalli-type population in the northwestern North Pacific, are clearly discriminated from each other by differences in the size of their white flank patch. A total of 479 bp of the mitochondrial control region and flanking tRNA genes was sequenced for 103 individuals. Pairwise comparisons indicated a low but significant difference between the Sea of Japan-Okhotsk and the other two populations, whereas there was no significant difference between the latter. These results suggest that there is a close evolutionary relationship among these populations despite their consistent differences in coloration.

 

Distribution

The distribution of Dall's porpoise is confined to the North Pacific Ocean and adjacent seas. Geographical variation in the colour-phase ratio is sufficient to permit the recognition of two subspecies

 

◈ Phocoenoides dalli dalli - These populations consist of >99% Dalli-phase and <1% Truei-phase animals. They range in subarctic waters from the south-eastern Sea of Okhotsk, the southern Bering Sea, and the northern Gulf of Alaska, south to the Sea of Japan, the Subarctic Boundary at about 420N across the North Pacific, and in the California Current to about 320N off Baja California Norte, except in the area occupied by the next subspecies. Although mainly an offshore deepwater inhabitant, Phocoenoides dalli dalli 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.

 

◈ Phocoenoides dalli truei - This population consists of <5% Dalli-phase and >95% Truei-phase animals and ranges in a limited area of the western North Pacific immediately east of the southern Ostrova Kuril'skiye, Hokkaido, and the Sanriku coast of Honshu.

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.

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 currently recognised by the International Whaling Commission.
The most recent estimate for the North Pacific and Bering Sea is 1,186,000.
Recently, several estimates have also been produced for smaller portions of the species' range. These report 141,800 in the western North Pacific, 104,000 off Japan, (>50% truei-type), 554,000 in the Sea of Okhotsk (all three stocks), 2,150 along the west coast of the US, and 78,400 off California.

Habitat

Dall's porpoise is found in diverse habitats, including sounds, nearshore waters (near deep water canyons) as well as offshore waters more than 1000 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 500m or more. It is not found in the southern extremes of its range during the summer or warm water months (Houck and Jefferson, 1999). Ferrero 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 55km/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".

 

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. They often associate with Pacific White-sided Dolphins (from 50°N southwards) and Long-finned Pilot Whales (from 40°N southwards). Bowriding behaviour has been observed with gray (Eschrichtius robustus), fin (Balaenoptera physalus), blue (Balaenoptera musculus) and humpback whales (Megaptera novaeangliae).

 

Food

The diet of Dall's porpoise is probably determined by size and abundance and consists primarily of small fish and squid (< 25cm). In Monterey Bay, Pacific hake (Merluccius), juvenile rockfish (Sebastes), and market squid (Loligo) made up 85% of the food items over the year. Off Japan, squid accounted for 11% of the food items ; lanternfish were over 70% of the remainder. In the north-western North Pacific and Bering Sea mostly squid, as well as epi- and mesopelagic fish were consumed, including myctophids (lanternfish), with Protomyctophum accounting for 78% of all the fish. The high percentage of deepwater and vertically migrating fish in the diet of Dall's porpoises from many areas indicates that feeding occurred either at night or at great depths, most likely both.
Amano and Kuramochi 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. 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 the both seas declined. In the Sea of Japan, the diet of Dall's porpoises switched to Theragra chalcogramma (walleye pollock), and in the Sea of Okhotsk, their diet switched to Engraulis japonicus (Japanese anchovy) and Berryteuthis magister (magistrate armhook squid).

 

Reproduction

Most Dall's porpoise calves are born in spring and summer. 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 United States Exclusive Economic Zone (EEZ). The percentage of mature males in this area is low, and most mature males are found south of the United States EEZ.

Migration

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. According to Forney and Barlow 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. Carretta et al. also found that Dall's porpoises were present off San Clemete Island, California, only during the cold-water months of November-April.

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. 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.

 

Threats

◈ 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 10,534 was taken in 1986, 13,406 in 1987, and about 39,000 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 concern. In recent years, the catch has been reduced somewhat, but still remains high, with 16,000 harpooned in 1994. The Small Cetaceans Subcommittee of IWC has attempted a review of the status of stocks of Phocoenoides dalli exploited by Japan. Because Japan refused to cooperate, on the grounds that small cetaceans were outside the remit of IWC, the subcommittee was unable to complete a full assessment of the status of the stocks.

 

◈ Incidental catch

In addition to the direct catch, Dall's porpoises are captured incidentally to other fisheries. The most important is that in the Japanese mothership salmon gillnet fishery which has operated in the north-western North Pacific and Bering Sea since 1952. Because the area for the fishery is located mainly within the United States EEZ, restrictions were devised to reduce the incidental catch. The estimated annual incidental take within the United States EEZ for the period 1981~1985 ranged from 1,850 to 4,187. Outside the United States EEZ the take ranged from 479 to 1,716. A Japanese squid gillnet fishery was reported to take nearly 2,500 Dall's porpoises every year between 1982 and 1984. Two other squid fisheries, Korean and Taiwanese, operate in the area, but the number of porpoises taken incidentally remains unknown, although it may be high. A global moratorium on pelagic drift net fishing went into effect at the end of 1992, and should reduce or eliminate these kills.
Dall's porpoises were also taken in a Japanese land-based salmon gillnet fishery in the western North Pacific. Estimated annual take in this fishery for 1981 and 1982 were 2,966 and 6,099 respectively. In the eastern North Pacific a few animals were taken incidentally to other fishing operations, and in drift nets for tuna and billfish in the central Pacific. Dall's porpoises are taken occasionally in other types of fishing gear as well, such as in trawl nets along the west coast of the United States.