Occurrence And Status Of Marine Mammals Whales

The Southern Hemisphere fossil record shows that whales have inhabited the region for over 40 million years from the Late Eocene to the present (Fordyce, 1985). The earliest cetaceans (archeocetes) were toothed whales, but opinions differ as to whether they gave rise to the modern toothed whales (odontocetes) and baleen whales (mysticetes) which appeared in the Oligocene (Barnes, 1984). Baleen whales are those that feed by straining krill or small fishes from the sea water through a filtering system of baleen plates — an energy-efficient method that has proved very successful, as demonstrated by the baleen whales' large size. Baleen whales probably evolved from the odontocete whales, which are active predators possessing teeth used for grasping larger prey such as fishes and squids.

Fordyce (1977) has postulated that the mid-Oligocene seas over eastern New Zealand and the Campbell Plateau region could have provided a focal point for the evolution of the filter-feeding mysticete whales. Productivity changes in the Southern Ocean associated with the initiation of the Circum-Antarctic Current in the mid-Oligocene possibly triggered the evolution of the new feeding adaptation. As Australia separated from Antarctica after the late Eocene, the Circum-Antarctic Current brought cool, nutrient-rich waters into the shallow seas east of New Zealand. The large supply of nutrients in this temperate region would have resulted in a huge increase in plankton biomass, able to support rapidly evolving and radiating organisms.

Cetaceans now living in the Southern Ocean can be classified into 6 families : right whales (2 species), other baleen whales (5 species), sperm whales (1 species), beaked whales (9 species), dolphins (8 species) and porpoises (2 species).

Whales that have been exploited in the Antarctic Sector of the Pacific include the right whale, humpback, the rorquals (blue, fin, sei and minke), which are all baleen whales, and the sperm whale, which is the largest tooth whale. The term "rorqual" has been traditionally used as a common family name for those whales belonging to the genus Balaenoptera. The general natural history of the great whales is well covered by Ellis (1981) and Evans (1987), and the distributional ecology by Gulland (1974) and Gaskin (1976).

Existing in the Southern Ocean are a further 20 species of smaller cetaceans that have never been seriously considered as commercial propositions by whalers. They include the pygmy right whale, the beaked whales and the dolphins and porpoises. Descriptions and illustrations of the whales, dolphins and porpoises of the Antarctic Sector of the Pacific can be found in Leatherwood and Reeves (1983), all regions; Sielfeld (1980), South American region; and Daniel and Baker (1986), New Zealand region.

Baleen Whales and the Sperm Whale

Southern right whales occur throughout the Antarctic Sector of the Pacific, usually north of 55°S. Although now one of the world's rarest whales, a few can still be seen regularly south of New Zealand at Campbell Island and the Auckland Islands, and on the southern and eastern Australian, New Zealand, and Chilean coasts. They are slow swimmers, and often frequent shallow coastal waters. Right whales have been recorded in schools of 11-12, but mostly occur singly or with a calf.

The prime target for early shore-based whaling in New Zealand, Australia and Chile, the right whale was easy prey because of its coastal habit and slow swimming speed. Some 193,000 southern right whales are thought to have been caught in the southwestern Pacific between 1804 and 1817 (Harmer, 1931). Records show that, between 1827 and 1848, 23,060 right whales were taken from the waters of eastern and southern Australia and New Zealand by both "bay whalers" and the pelagic fleets (Dawbin, 1987). American whalers took 6,262 right whales from Chilean waters between 30°S and 50°S, during the years 1785-1913 (Aguayo, 1974). The whale's value in those early days lay in high-quality oil and baleen (used then for corset stays, umbrella ribs, etc.) and the natural flotation of the carcasses, which made retrieval by small boats relatively easy.

By 1848, right whale stocks in the southwestern Pacific had been heavily depleted, and the species accounted for only a small proportion of the whale catch thereafter, until it was fully protected in 1931. In Chile, right whales continued to be caught up until 1966, but the total number caught between 1920 and 1966 was only 119 whales. The size of the current southern right whale population is thought to be small, and estimates range from 1,030 to 4,300 individuals (Cum-mings, 1985).

Humpback whales were prominent in the early South Pacific whale catches despite their widespread general distribution because they tend to concentrate on winter and summer grounds, making them easy targets. In the South Pacific, there are several largely isolated stocks of humpbacks, which spend the summer in Antarctic waters and migrate north past the coasts of Australia, New Zealand and Chile to winter in subtropical waters (see summary in Gaskin, 1968, 1972). Like the right whale, humpbacks are not fast swimmers, and could be taken on their migratory routes by shore-based whalers. In the 1930s, humpbacks were heavily depleted in Antarctic waters by the pelagic whaling fleets. Although shore stations in Australia and New Zealand continued to catch them through the

1950s (Fig. 9.7), the stocks had declined to a few hundred individuals by the early 1960s making whaling uneconomic (Mackintosh, 1965). In 1939, the humpback was protected in Antarctic waters, but this was only temporary, as catching resumed in 1949. Final protection in southern waters came in 1963 through the International Whaling Commission.

Around 9,300 humpbacks are estimated to remain worldwide, including about 1,000 in the South Pacific (Winn and Reichley, 1985). The latter number may be a minimum, however, for a recent photo-identification survey of humpbacks in the Antarctic Peninsula area (Stone and Hamner, 1988) showed that these whales were more numerous there than have been previously estimated. There is a need to extend such surveys to other parts of the Antarctic to clarify the distribution and abundance of humpback whales.

Humpbacks travel alone or in small groups and may congregate in groups of up to 15 when feeding on krill.

Rorquals — blue whales, fin whales, sei whales and minkes — were the mainstay of pelagic whaling in the Antarctic Sector of the Pacific (Mackintosh, 1965). Blue whales are found in all oceans and, in the South Pacific, usually spend the summer south of 40°S. It is believed that the population of blue whales in Antarctic seas was reduced from some 150,000 whales in the 1930s, to between 5,000

Fig. 9.7. A humpback whale being flensed at the former shore whaling station at Whangamumu, New Zealand (Photo : National Museum of New Zealand).

and 10,000 by the late 1950s (Chapman, 1974). They are still extremely rare in the Southern Hemisphere despite complete protection since 1966. Gulland (1981) estimated the total blue whale population in the Southern Ocean to be about 10,000, and increasing at the rate of 4-5% each year.

The largest of living animals, blue whales of the Southern Ocean, were recorded at 30 m length in the early days of whaling but, as catching increased, their maximum lengths decreased. Blue whales do not form schools; they are usually found alone or in pairs. A detailed summary of the blue whale's natural history is given by Yochem and Leatherwood (1985).

Fin whales also occur world-wide, and were probably the most abundant whales in Antarctic waters when whaling began there. Like the blue and sei whales, the fin feeds mostly on krill in southern waters. It is more gregarious than the blue whale, and is sometimes seen in groups of about 100, but usually travels in pods of less than 10. In the South Pacific, fin whales migrate annually from the Antarctic feeding grounds north to temperate and subtropical seas for breeding in the winter. It is thought that separated breeding stocks of fin whales exist broadly on each side of the South Pacific and that they overlap to some degree in the Antarctic feeding grounds (Gambell, 1985a).

Although the fin whale was hunted only occasionally in Antarctic seas from the early 1900s, it became the most sought-after commercial species once the blue whale had been virtually eliminated from Antarctic waters, and was heavily depleted during the years 1946-65. Off the coast of Chile, the fin whale was caught consistently between 1920 and 1968, accounting for 4,503 whales (Aguayo, 1974). There is still argument over assessments of the numbers of fin whales presently living, and their population trends, but it has been estimated that 125,000 may still exist in the Southern Hemisphere (Gambell, 1987).

Sei whales are found in all major oceans and, in the Southern Hemisphere, they occur mostly south of 40°S in the summer, but do not live as close to polar waters as the other rorquals. They possibly migrate north to about 30°S in winter (Aguayo, 1974, gives some evidence of a northward movement to at least 39°S in Chilean waters), but their true wintering grounds are at present unknown (Gambell, 1985b). Sei whales usually travel alone or in pairs but sometimes form larger feeding aggregations. These whales were also hunted in the Antarctic from the earliest days of mechanized whaling, but became the most important target for whalers much later in the 1960s as fin whale numbers were declining rapidly. They were fully protected in 1977. The Southern Hemisphere stocks of sei whales have been calculated at 191,000 before whaling began, and about 37,000 at present (Gambell, 1985b).

Minke whales are the smallest baleen whales to have been commercially exploited, reaching only 10 m in length and 0.98 tonnes weight. Although a few minkes were caught in the Antarctic earlier this century, the main exploitation began in the 1960s — 22,427 were taken between 1963 and 1978 (Stewart and Leatherwood, 1985). Since then, catches have fluctuated between about 800 and 2,000 whales each season. Following the International Whaling Commission's 1986 moratorium, several hundred have been caught for research purposes. The present Antarctic population is estimated by the IWC to be between 440,000 and

690,000 whales. It has been suggested that more should be caught to reduce competition with other whales and so allow recovery of the blue, fin and humpbacks (Oshumi, 1979).

Minkes are gregarious (Fig. 9.8) and may form groups of several hundred. There are two distinct forms of minke whale in the Southern Hemisphere, one being a dwarf, reaching only about 7 m in length, and possibly having a more northerly distribution than its larger relative (Arnold et al., 1987).

Sperm whales are found in all oceans and have a somewhat clumped distribution. Best (1974) used catch statistics for sperm whales to demonstrate that certain regions had the high densities of whales. South of 40°S in the Pacific, there are probably several segregated stocks, which he termed the "New Zealand and East Australian Stock" (140°-160°W), the "Central South Pacific Stock" (160°-100°W), and the "Western South American Stock (100°-60°W). In the South Pacific, female sperm whales are not found south of 45°-50°S, whereas the males occur south to between 60°S and 70°S. They do not approach the polar pack ice, and are generally regarded as an oceanic species. They were hunted in the Southern Ocean in the last century (Harmer, 1931), but the main exploitation by modern whalers began after World War II, peaked in 1973-74 (3,765 whales), and ceased in 1979. Sperm whales may be found alone or in large groups of 50 or more, sometimes sexually segregated. Gambell (1987) estimated the total Southern Hemisphere population to be 750,000.

Fig. 9.8. Minke whales in Antarctic waters: this species is the subject of "scientific whaling" (Photo: P. Ensor).

Small cetaceans

The remaining species of cetaceans found in the Antarctic Sector of the Pacific are regarded as the "small cetaceans" and are not commerical species, although a few have been taken by modern whalers from time to time for research purposes. With the exception of the pygmy right whale, which is a krill filter-feeder, the small cetaceans are primarily squid and fish eaters. They are mostly known from occasional sea sightings and strandings and some, for example the beaked whales, are extremely rare. Brownell (1974) gave a good summary of Antarctic small cetaceans. Goodall and Galeazzi (1985) have reviewed the distribution and food habits of small cetaceans from both Antarctic and Subantarctic waters, and Good-all and Cameron (1980) the exploitation of small cetaceans off southern South America.

The only baleen whale not to have received the attention of whalers is the pygmy right whale, a relative of the large right whale. The pygmy right whale lives only in the Southern Hemisphere between about 31 °S and 52°S, and reaches about 6.5 m in length. It is rare in the South Pacific, where it has only been recorded from the New Zealand region and Tasmania (Baker, 1985).

Arnoux's beaked whale, the largest of the southern beaked whales, has a circumpolar distribution from about 34°S to the Antarctic ice edge. In the South Pacific, it is known from New Zealand and Antarctic waters, where it has been observed well south of the Antarctic Convergence (Miyazaki and Kato, 1988), but is not common. This whale can be gregarious, occurring in groups of 6-10, and occasionally up to about 80 (K. Balcomb, pers. comm.), but is also solitary.

The southern bottlenose whale has a similar distribution to that of Arnoux's whale, is similar in general appearance, and is probably more common in Antarctic waters (Miyazaki and Kato, 1988). Some specimens have been caught by whalers in the Antarctic Sector of the Pacific (Goodall and Galeazzi, 1985).

Shepherd's beaked whale is known from only 18 specimens south of 35°S (Ross et al., in press), mostly from New Zealand. The southernmost record is from Tierra del Fuego at 55°S (Goodall and Galeazzi, 1985), and it has not been reported from Antarctic waters.

Cuvier's beaked whale is widely distributed in most oceans, and has been recorded many times from the South Pacific through sea sightings and strandings. It has been reported from Subantarctic South American waters at 60°S by R.N.P. Goodall (pers. comm.), and south of New Zealand at 52°S by Baker (1977).

Five species of beaked whales of the genus Mesoplodon have been recorded from south of 45°S in the Pacific, but none close to the Antarctic continent. They range in length from 4 to 6 m, are rarely sighted, and are notoriously difficult to identify at sea.

The biggest Mesoplodon species is the strap-toothed whale, which seems to be a truly Southern Hemisphere whale, being found off all the southern temperate continents, and in the Subantarctic it has been recorded from Tierra del Fuego (Sielfeld, 1980) and Heard Island (Guiler et al., 1987). The male of this species, which reaches just over 6 m in length, is the most easily recognized beaked whale because of its pair of long, strap-like teeth.

The dense-beaked whale, normally an inhabitant of tropical oceans, is included in this summary of southern Pacific cetaceans because of records of specimens from Tasmania and Macquarie Island (Guiler, 1966; Ross et al., in press). Notable for its raised lower jaws and, in the male, a pair of massive teeth, the dense-beaked whale must be regarded as an occasional straggler into southern waters.

Gray's beaked whale is the most commonly reported species of the remaining three small species of Mesoplodon beaked whale known from the Southern Ocean (Gray's, Hector's and Andrew's). About 60 specimens have been recorded, all north of 55°S, mostly from strandings, although there are some sea sightings east of New Zealand (Ross et al., in press). Gray's beaked whale is unusual amongst the species of Mesoplodon for its sometime habit of travelling in small (< 25) schools. Hector's beaked whale is known from less than 20 specimens from mainly temperate waters of both Pacific hemispheres, but a stranded specimen has been recovered from southern Chile at about 55°S (Sielfeld, 1979; Mead and Baker, 1987). Andrew's whale, known mainly from the Australasian region, is even rarer but skeletal remains have been found on the Subantarctic islands to the south of New Zealand (Baker, 1983; Goodall and Galeazzi, 1985).

Eight species of dolphins and two true porpoises complete the list of Southern Ocean cetaceans. Only the killer whale, hourglass dolphin, and the southern right whale dolphin are known to travel south of the Antarctic Convergence. The killer whale is commonly seen amongst the pack ice in summer, but the dolphins are oceanic, the hourglass dolphin (Fig. 9.9) approaching the pack ice south of 60°S, but not actually entering the ice, and the right whale dolphin being found further north, beyond 60°S (Miyazaki and Kato, 1988). The remaining dolphins (pilot, dusky, Peale's, Hector's and Chilean) are cool temperature species and are mostly associated with coastal waters of the South Pacific. In the Subantarctic, the pilot whale and dusky dolphin are known from the Auckland Islands and Campbell Islands respectively (Baker, 1977), and pilot whales have been recorded close to Antarctica at 64°24'S (Miyazaki and Kato, 1988). The Chilean and Peale's dolphins are known from the southern coast of Chile, and are common around Chiloe Island. Peale's dolphin is frequently seen along the Pacific coast south to Cape Horn (J. Oporto, pers. comm.). Hector's dolphin, a close relative of the Chilean dolphin, is known only from the inshore waters of southern New Zealand, where it is relatively common to 48°S in certain localities, but the total population may be less than 4,000 (Dawson and Slooten, 1988).

Of the two porpoises known from the Southern Ocean, the spectacled porpoise is truly Subantarctic, having been recorded from the Auckland Islands (Baker, 1977). Macquarie Island (Fordyce et al., 1984), Heard Island (Guiler et al., 1987), Tierra del Fuego, and the southwestern Atlantic islands (Goodall, 1978). Burmeister's porpoise is included here because of its rare occurrence in the Cape Horn area of South America; otherwise its Pacific distribution is northward along the coast of Chile, particuarly north of Chiloe Island (43°S). A report of this porpoise at Heard Island in the Antarctic (Guiler et al., 1987) is, however, a mis-identification of the spectacled porpoise.

Fig. 9.9. An hourglass dolphin south of the Antarctic Convergence (Photo : P. Ensor).

0 0

Post a comment