Short-finned Pilot Whale (길잡이고래)

길잡이고래, 들쇠고래(Globicephala macrorhynchus)

영명 : Short-finned Pilot Whale

 

북방형 - 몸길이 : 4.7~6.5m, 몸무게 : 1~1.2t. 남방형 - 몸길이 : 3.6~4.7m, 몸무게 : 0.6~1.3t.

일본에서는 북방형을 '닷파나가', 남방형을 '마곤도' 라고 부른다. 머리가 크고 사회성이 높다. 극지를 제외한 전 해역, 북반구와 남반구의 북위 50°와 남위 40°사이의 온대와 열대의 심해에 서식하고 있다. 과거에는 북서태평양에서만 분포하는 것으로 알려지기도 하였다.

 

머리는 멜론처럼 둥글다. 폭이 넓은 낫 모양의 등지느러미가 몸체의 1/3 앞쪽에 위치한다. 등은 흑색 혹은 흑갈색이며 대부분 등지느러미 후방에 자전거 안장모양의 회백색 반점이 있다. 목과 가슴지느러미 사이 닻 모양의 회백색 무늬가 있다. 아래턱과 위턱 좌우에 8∼9쌍의 이빨이 있다. 무리생활을 한다. 보통 15∼40마리, 최대 수백 마리가 무리를 이루기도 한다. 단위 무리는 10∼15마리로 구성되며 소수의 성숙 수컷과 다수의 성숙 암컷 및 새끼들로 된 모계사회를 이룬다. 사회성이 매우 높아 무리 전체가 동시에 좌초하는 경우가 허다하다. 가끔 어류를 잡아먹기도 하나 주 먹이는 오징어류이다.

 

◈ 북태평양 (서부) : 53,008 마리.

 

 

 

 

 

 

 

 

 

 

 

 

Description

The body in pilot whales is robust, with a thick tail stock. The melon is exaggerated and bulbous and the beak is barely discernible or non-existent. The dorsal fin is wide, broad based, falcate and set well forward on the body. The flippers are long, slender, and sickle-shaped. A faint grey saddle patch may be visible behind the dorsal fin. A grey midventral line extends to the front into an anchor-shaped chest patch and widens posteriorily to a genital patch. The short-finned pilot whale has a wider skull than the long-finned species.
Long- and shortfinned pilot whales (Globicephala melas and Globicephala macrorhynchus) are difficult to distinguish at sea. However, both species differ, as the name suggests, in flipper length, skull shape and number of teeth. On average, the pectoral fins of the short-finned pilot whales are 1/6 the body length. Adults reach a body length of approx.

Distribution

Globicephala macrohynchus is probably circumglobal in tropical and warm temperate waters. In the Atlantic it ranges north to New Jersey and to Charente-Maritime in France (it is not present in the Mediterranean) ; in the Pacific, its range extends north into cooler temperate waters as far as Hokkaido (50°N, 145°W), and Vancouver Island. The southern limits of the range are not fully determined due to past confusion with the Globicephala melas, but Globicephala macrohynchus is known to range south to São Paulo, Cape Province, Western Australia, Tasmania, and Cape Farewell on North Island in New Zealand. Short-finned pilot whales are found in deep offshore areas and usually do not range north of 50°N or south of 40°S. There is some overlap in range between the two species. Globycepahala macrorhynchus is vagrant to the Alaska Peninsula (57°N, 156°W). There are hypotheses that the short-finned pilot whale is in the process of expanding to fill the former range of long-finned pilot whales in the North Pacific.

Globicephala macrohynchus appears to vary geographically, but no comprehensive study has been undertaken. Off the Pacific coast of Japan, a northern and a southern population differ sharply in colour pattern and in body size and shape and also in cranial features. However, their taxonomic status remains unsettled. Water temperature seems to be the primary factor determining the relative distributions of these two populations.

Population size

Several aerial line-transect surveys of short-finned pilot whales were conducted off the coast of California during the late 70s-early 80s. The size of the population was estimated at between 200 and 4000, but it is not clear if this represents one or more than one stock. The abundance of short-finned pilot whales in the eastern tropical Pacific was estimated to be 160,000 from an extensive series of line-transect sighting surveys. Off the coast of Japan, line-transect analysis of sightings data from 1984~1985 yielded an estimate of 5,300 for the northern form and 53,000 for the southern form. Dolar estimated a total of 7,700 individuals in the eastern Sulu Sea.

 

Behaviour

Entire pods can sometimes be seen logging, allowing close approach by boats. The strong blow may be visible in calm weather.

 

Habitat

The species prefers deep water and occurs mainly at the edge of the continental shelf, and over deep submarine canyons. Davis et al. found that Globicephala macrorhynchus in the Gulf of Mexico preferred water depths between 600 and 1000m.

 

Schooling

Pods of up to several hundred short-finned pilot whales have been reported, and members of this highly social species are almost never seen alone. Strong social bonds may partially explain why pilot whales are among the species of cetaceans that most frequently mass-strand. Although detailed studies of behaviour have only begun recently, pilot whales appear to live in relatively stable female-based groups. Three types of social organisation for pilot whale pods off southern California were described : travelling/hunting groups, feeding groups, and loafing groups. The travelling/hunting groups have also been appropriately described as "chorus lines" as the animals in these are oriented in a broad rank of up to 2 miles in width, but only a few animals deep. Sexual and age-class segregation also have been observed in chorus lines. In the second type of group described, the feeding group, there was sometimes general movement of whales in a given direction, but individuals tend to remain fairly independent of one another. The third type of pod, the "loafing group", was described as an almost stationary aggregation of 12~30 or more individuals, floating at the surface, nearly or actually touching one another. A wide variety of types of behaviour, including mating, was reported to occur in loafing groups.
In the eastern tropical Pacific, approximately 15% of pilot whale sightings include other cetaceans. They are sighted with bottlenose dolphins (Tursiops truncatus) and with tuna-dolphin aggregations (Thunnus albacares and Stenella spp.) and S. coeruleoalba. The most common associate in coastal waters is the bottlenose dolphin; pilot whales have been sighted also with common dolphins (Delphinus delphis), Pacific white-sided dolphins (Lagenorhynchus obliquidens), gray whales (Eschrichtius robustus), fin and sperm whales (Balaenoptera physalus and Physeter catodon) and with killer whales (Orcinus orca ; Bernard and Reilly, 1999 and refs.
Mazzuca et al. found that in the Hawaiian Archipelago, short-finned pilot whales stranded in the largest groups and experienced the greatest number of stranding events (x = 14 animals, 5 events) of all certaceans recorded from 1957 through 1998. The greatest incidence of odontocete mass strandings occurred on the Island of Maui during the month of June. Mass strandings occurred on all high Hawaiian Islands, except Hawaii; none were reported on the islands, or atolls north of Kauai. Two-thirds of the events occurred on the leeward sides of the islands with similar bottom topography, coastal configuration, and geomagnetic characteristics in all events.
Mignucci et al. report that in waters off Puerto Rico and the US and British Virgin Islands, short-finned pilot whales were one of the most frequently stranded species. A high number of strandings occurring in the winter and spring. The most common natural cause of death category was dependent calf.

 

Food

Although they also take fish, pilot whales are thought to be primarily adapted to feeding on squid. They show the tooth reduction typical of other squid-eating cetaceans. Hernandez-Garcia and Martin found that stomach contents of two short-finned pilot whales found on the Canary Islands were made up entirely of cephalopods:Todarodes sagittatus, Cranchia and juveniles of Megalocranchia.
Baird et al. tested the hypothesis that this species also feeds on vertically migrating prey, with deep dives at dusk and dawn following vertically migrating prey, and near-surface foraging at night, using suction-cup attached time-depth recorders (TDRs) and video camera systems (Crittercam). The deepest dives recorded (typically 600~800m, max. 27 minutes) were during the day. Such deep dives were recorded for all 5 individuals where TDRs remained attached for extended periods. At night, all whales dove regularly to between 300 and 500m, and the rate of deep (>100m) dives at night was almost four times greater than during the day. Long bouts of shallow (<100m) diving occurred only during the day. Video footage from the Crittercams during these shallow dive bouts indicated the whales were engaged in social, rest and travel behaviours, but no feeding was documented. Dive depth differences between day and night presumably reflect vertically migrating prey, though the prey are concentrated at depths of 300~500m during the night.

 

Reproduction

Females become post-reproductive at around 35 years, but may continue to suckle young for up to 15 additional years, suggesting a complex social structure in which older females may give their own or related calves a "reproductive edge" through prolonged suckling. Calving peaks occur in spring and autumn in the Southern Hemisphere, and vary by stock in the Northern Hemisphere.

Migration

The species appears to be generally nomadic, with no fixed migrations, but some north-south movements are related to prey movements or incursions of warm water. Inshore-offshore movements are determined by spawning squid (outside the squid season Globicephala macrorhynchus is usually found offshore). Some populations are present year-round, such as in Hawaii and the Canary Islands.

A marked seasonality in the distribution of pilot whales has been observed in at least three areas : off southern California; in the eastern tropical Pacific ; and off the coast of Japan. In southern California, the seasonal abundance of pilot whales appears to be correlated with the seasonal abundance of spawning squid. During years of low squid abundance, fewer pilot whales were sighted near Catalina Island off the coast of California. In both the coastal and pelagic waters of the eastern tropical Pacific, the density of population centres appears to change seasonally in response to major changes in the current structure of the area. In the southern California Bight, the occurrence of short-finned pilot whales was associated with high relief topography. There seems to also be a seasonal distribution with depth : pilot whales were found in significantly shallower water during winter (depth 375m) than summer (800m).

There have been no systematic studies of home range or migration of individuals of this genus. Opportunistic observations in the southern California Bight have indicated that a pod of 20~30 individuals, identified by scars, unusual marks, etc., lived in the area year-round in the 1970's. Following the strong El Niño event in 1982~83, subsequent surveys throughout the 1980s turned up few sightings, and documented the absence of all but one pod of pilot whales near Catalina Island. Shipboard surveys along the entire California coast using line-transect methodology were conducted in 1991 and 1993 within 550km of shore, documenting an apparent return of this stock. The calculated abundance estimate was 1004 individuals.

Threats

◈ Direct catch

The short-finned pilot whale has been exploited for centuries in the western North Pacific. The largest catches have recently occurred off Japan, where small coastal whaling stations and drive fisheries take a few hundred annually. In recent years, the southern form continues to sustain a higher kill than the northern form. In 1982, the drive fishery at Taiji expanded and harpooning of the northern form was resumed off Sanriku and Hokkaido. Between 1982 and 1985, 1755 whales of the southern form were killed, and 519 of the northern form were taken during this same period. From 1985 to 1989, Japan took a total of 2,326 short-finned pilot whales. The drive fisheries in Japan, as well as the Japanese harpoon fishery continue today. In 1997, Japan recorded a catch of 347 short-finned pilot whales.
Elsewhere, a small, intermittently active fishery takes around 220 pilot whales per year in the Lesser Antilles in the Caribbean at St. Vincent Island, and there are indications of a small fishery at St. Lucia Island.
Dolar et al. report on directed fisheries for marine mammals in central and southern Visayas, northern Mindanao and Palawan, Philippines. Hunters at four of the 7 investigated fishing villages took dolphins for bait or human consumption, including short-finned pilot whales. These are taken by hand harpoons or, increasingly, by togglehead harpoon shafts shot from modified, rubber-powered spear guns. Around 800 cetaceans are taken annually by hunters at the sites investigated, mostly during the inter- monsoon period of February-May. Dolphin meat is consumed or sold in local markets and some dolphin skulls are cleaned and sold as curios. Although the Department of Agriculture issued Fisheries Administrative Order No. 185, in December 1992 'banning the taking or catching, selling, purchasing, possessing, transporting and exporting of dolphins', this did not stop dolphin and whale hunting but seems to have decreased the sale of dolphin meat openly in the market.

◈ Incidental catch

There are probably more pilot whales taken incidentally than is presently documented. In US Atlantic waters, pilot whales have been taken in a variety of fisheries, but not exceeding the allowable annual take under US law. Based on preliminary data, the squid round-haul fishery in southern California waters is estimated to have taken 30 short-finned pilot whales in one year. In the California drift gill net fishery between 1993 and 1995, the mean annual take of short-finned pilot whales was 20. Since the take in US water exceeds the allowable limit, a take reduction plan has been implemented (Olson and Reilly, 2002) . On the other side of the Pacific ocean, an estimated 350~750G. macrorhynchus die annually in passive nets and traps set by the Japanese fishery. The most common human-related cause death categories observed in waters off Puerto Rico and the US and British Virgin Islands were entanglement and accidental captures, followed by animals being shot or speared.