Pacific White-sided Dolphin (낫돌고래)

 

낫돌고래(Lagenorhynchus obliquidens)

영명 : Pacific White-sided Dolphin

 

몸길이 : 2~2.1m, 몸무게 : 87~93kg. 2색의 등지느러미는 높고 뒤쪽으로 많이 굽어 있다. 북태평양 한랭역, 오호츠크해, 베링해에 서식하고 있다. 주로 근해에 분포하나 대륙붕과 연안으로도 왕래한다.

등지느러미는 크고 낫 모양이며, 뒤쪽 면이 흰색으로 예리한 낫의 날과 같은 모양이다. 부리는 입술처럼 작다. 등은 흑색이며 배는 흰색이다. 배, 이마, 눈, 가슴 부분, 꼬리지느러미 기저로부터 등지느러미 사이의 몸체 옆면이 흰색 무늬이다. 아래위턱 좌우에 30∼35개의 이빨이 있다.

 

무리성이 강하여 가끔 수백 혹은 수천 마리가 군을 이루는 경우가 있으며 고추돌고래와 큰머리 돌고래 등 다른 종류와 섞여 유영하는 것이 자주 관찰된다. 협력에 의한 집단적인 공격으로 어류, 오징어류, 갑각류, DSL(심해 산란층)에 밀집한 플랑크톤류 등을 주로 먹는다.

 

◈ 북태평양 : 988,000 마리. 

 

 

 

 

 

 

 

 

 

 

 

 

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 (Lagenorhynchus 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.

Close scrutiny of morphological and life history parameters as well as recent cytochrome c sequence analysis supports the premise that Lagenorhynchus obscurus and Lagenorhynchus obliquidens are sister species which diverged 1.9~3 million years ago.

Among the North Pacific populations, the animals off Baja California have consistently larger crania than the ones from northern California northward, with intergrading populations occupying the intervening area off southern and central California. 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. Lux et al. found that population- by-population mtDNA comparisons of four geographic strata from the eastern Pacific indicated that all could be considered isolated, but likely incompletely, from one another.

Distribution

Lagenorhynchus 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 south-western 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.

This species tends to remain south of colder waters influenced by arctic currents, and stays north of the tropics. Vagrant to Bahia de La Paz in the south-western Golfo de California and infrequently, in the southern Bering Sea.

Population size

Buckland et at. 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. However, precision is low for both studies, and vessel attraction probably resulted in overestimation of population size.
In the eastern North Pacific, separate abundance estimates different regions and seasons exist. Off Oregon and Washington, a peak abundance of 23,400 animals was estimated in May 1992. 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. This contrasts with a ship-based estimate of only 5,900 in August - November 1991 for the same study area, a discrepancy which may be explained by seasonal migrations. Off San Clemente Island, California, Pacific white-sided dolphins were the most abundant of the cold-water species in 1998~99, with a count of 1,649. In the coastal waters of British Columbia, Canada, the Pacific white-sided dolphin is probably the most abundant cetacean.

 

Habitat

Lagenorhynchus 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. 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. Ferrero investigated habitat segregation between various species of small cetaceans in the central North Pacific Ocean. Sea surface temperature was the most influential habitat parameter examined, with L. borealis occupying the warmest waters, P. dalli the coolest, and Lagenorhynchus obliquidens in between, but with greater preference overlap with P. dalli. Their findings suggest that habitat preference patterns for these three species may be specific to reproductively active females, while coincident habitat use among other species constituents is common.

 

Behaviour

Lagenorhynchus obliquidens is very inquisitive and may even approach stationary boats. It is highly acrobatic and playful, commonly bowriding, and often leaping, flipping, or somersaulting.

 

Reproduction

Calving apparently occurs during a protracted summer breeding season, which extends into autumn.

 

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 Rissos dolphins as well as other cetaceans. The interspecific relationship with the northern right whale dolphin 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. Large schools of Pacific white-sided dolphins may split into smaller groups when feeding, but re-assemble when resting or travelling.

 

Food

Pacific white-sided dolphins consume a wide variety of fishes 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, 1997). In the North Pacific they feed primarily on epipelagic fishes 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 Lagenorhynchus obliquidens feeds heavily on mesopelagic fish and cephalopods and in coastal waters of northern Japan on both mesopelagic and epipelagic fishes and cephalopods.

Migration

Some seasonal shifts occur : while more common in coastal waters during fall and winter, Lagenorhynchus obliquidens move offshore during spring and summer, in rough synchrony with the movements of anchovies and other prey.

Recent 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 of Pacific white-sided dolphins in the eastern North Pacific. Aurioles et al. 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. Brownell et al. suggest that the occurrence of the southern form of Lagenorhynchus obliquidens off Southern California appears to be variable, possibly relating to changes in oceanographic conditions on seasonal or inter-annual time scales.

In Alaskan waters, published sighting records are sparse, but the occurrence of Pacific white-sided dolphins may be related to periods of warmer waters. OffJapan, 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.

Threats

◈ Direct catch

According to Jefferson et al. Japanese drive and harpoon fisheries take hundreds or even thousands of Pacific white-sided dolphins in most years but Brownell et al. report that only "small numbers" are taken annually.

 

◈ Incidental catch

In the eastern Pacific they are occasionally captured in fishing nets, and small numbers are taken in a fishery for live animals. 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 ; however, no overall mortality estimates are available for these fisheries. Pacific white-sided dolphins are 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.
In the western Pacific, Pacific white-sided dolphins are one of the most commonly caught cetaceans in the Japanese and Korean high seas squid drift net fisheries. The 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. 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. In January 1993 a United Nations moratorium on these high seas drift net fisheries went into effect. Smaller catches 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.