Whales and whaling




старонка1/3
Дата канвертавання24.04.2016
Памер99.5 Kb.
  1   2   3
CHAPTER 10

WHALES AND WHALING
Background
Whales are marine mammals belonging to the order Cetacea.1 Within this order are three suborders, the Odontoceti (toothed whales, including dolphins, porpoises, sperm whales, killer whales, and several other families), the Mysticeti (baleen whales), and the Sirenia (dugongs and manatees). Whales belong to the suborders Odontoceti and Mysticeti, and there is no clear scientific distinction between a “whale” and one of the smaller Cetacea. Dolphins and porpoises, which may attain a length of roughly two meters, are not generally considered to be whales. Virtually all marine mammals generally considered to be whales attain a length of at least four meters, and some grow to be much larger. In this chapter we will be concerned with the larger of the whales, the so-called great whales, which have been the focus of the whaling industry. Table 10.1 summarizes some relevant information.

All whales are descended from land animals. They have vestigial hind limbs buried in their flesh, and they must come to the surface to breathe air. Scientists believe that their terrestrial ancestors looked like small dogs and were related to hippopotamuses. The transition to a strictly marine lifestyle probably took place about 60 million years ago.

Whales are warm blooded, with a body temperature of about 36oC, although they inhabit waters ranging in temperature from 0oC to 20-25oC. Their ability to maintain a 36oC temperature is facilitated by the insulation afforded by a layer of body fat or blubber that may be as much as 30 cm thick.

The suborder Mysticeti are the so-called baleen whales. Baleen whales have no teeth. Instead, a series of hundreds of rows of horny but flexible baleen plates hangs from their upper jaw. The plates consist of a fingernail-like material called keratin and can be anywhere from 0.5 to 3.5 meters long. They are spaced about 1 centimeter apart. Thousands of fine horny tubes emerge from the inner edge of the plates and stick out in all directions, overlapping with the tubes from other plates and effectively forming a continuous mat of bristles that is used to filter food out of the water. When the whales close their mouths, the baleen plates fold into a groove between the tongue and lower jaw. The suborder Odontoceti includes the toothed whales, which have teeth in their lower jaw and no baleen plates. Interestingly, it is unclear that the teeth play any role in catching or masticating prey, and indeed the females of some species of toothed whales have no functional teeth at all.




Table 10.1. Size of the great whales




Scientific name

Length (meters)

Weight (tonnes)

Suborder Mysticeti







Family Balaenopteridae







Blue

Balaenoptera musculus

23-27

100-150

Fin

Balaenoptera physalus

24-26

45-64

Sei

Balaenoptera borealis

14-20

14-36

Bryde

Balaenoptera edeni

12-15

13-26

Minke

Balaenoptera acutorostrata

10

5.4-6.8

Humpback

Megaptera novaeangliae

15-18

30-45

Family Eschrichtiidae







Gray

Eschrichtius robustus

14

35

Family Balaenidae







Bowhead

Balaena mysticetus

15-18.5

70-90

Northern right

Eubalaena glacialis

15

60-65

Southern right

Eubalaena australis

15-16.5

60-65

Suborder Odontoceti







Family Physeteridae







Sperm

Physeter catodon

12-17

55

Sperm whales breathe through a single opening in their blowhole, which is S-shaped and located toward the front of their head on the left side. Baleen whales have two openings in their blowhole, and after breathing will typically stay submerged for 5-20 minutes. Sperm whales however, which are known to dive for squid at depths of 1 km or more, may stay submerged for as much as 90 minutes. All of the great whales are very streamlined for swimming and move by vertical movements of their tail. Typical swimming speeds during migrations are 8-10 knots2, but some whales have been clocked at speeds up to 30 knots when being chased.

Baleen whales are assigned to families based on their feeding strategies. Rorqual whales, family Balaenopteridae, have roughly 25-100 parallel, pleated ventral grooves3 that extend from the throat to the flippers. When they are feeding, the grooves expand, allowing them to take huge gulps of water, which they force through their short baleen plates to filter out food such as small fish and crustaceans.

Right whales, family Balaenidae, have no throat grooves and hence are ill suited for gulping food. Instead, they slowly swim along the surface with their mouths open, continuously filtering the water as it passes into their mouths and out through their long, fine baleen.

Gray whales, family Eschrichtiidae, are benthic feeders that sieve the mud on the ocean floor with their baleen. They apparently tend to feed on their right side, sucking up mouthfuls of mud, filtering out food such as crustaceans, mollusks, and small fish, and then spitting out the mud.

All whales are believed to have limited vision and little or no sense of small. However, they have an acute sense of hearing, and a number of species are known to emit sounds that, depending on the species, have been characterized as whistles, clicks, creaks, and groans. The sounds are produced via air sacs in the blowhole and the passage of air through the larynx. The sounds are believed to have two purposes. One is communication. The other is echo location. In the former case, the nature of the communication may take several forms. The sounds may be used to coordinate group behavior; they may be used for social purposes between pairs of animals; and at least in the case of humpback whales, the sounds or “songs” of males appear to play a role in attracting mates during the breeding season. In the case of echo location, high frequency sounds may be used to locate food. Alternatively, they may be used to avoid obstacles if the whales find themselves in murky water or below the effective depth of light penetration.

In some respects whales socialize in ways that mirror human behavior. Female whales nurture their calves, and small groups of adults may assist the mother in protecting juveniles from potential predators such as sharks. Whales typically travel in small groups of 3-5 animals, but the groups are not permanent associations, and schools may break up and reform in different ways. Whales are believed to be highly intelligent (Herman, et al., 1984). They are able to learn from experience and will modify their behavior accordingly. It seems fair to say that human awareness of the intelligence of whales and their sometimes anthropomorphic behavior has been one of the important factors underlying the sometimes emotional protests to commercial whaling.

Whale species are divided into separate stocks by ocean basins, and in the case of most baleen species, there are separate northern and southern hemisphere populations. The reason for the latter separation arises because most baleen whales migrate to high latitude feeding grounds during the summer months and return to lower latitudes during the breeding season. Because the northern and southern hemisphere seasons are out of phase, there is little commingling between the stocks in the two hemispheres. However, tagging studies4 have shown that not all individual whales migrate according to this pattern. The migrations are far from synchronous. Some individuals are completely out of phase with the rest of the stock, and individuals occasionally cross the equator. Although there is consequently a reasonably homogeneous gene pool, from a fisheries management standpoint the stocks in different ocean basins/hemispheres should be considered separate and distinct.

In the case of sperm whales, there is no segregation into northern and southern hemisphere stocks. Sperm whales are tropical and subtropical in their distribution. So-called bachelor males may migrate to high latitudes during the summer months, but most individuals remain at relatively low latitudes throughout the year. The populations shift toward the northern subtropics during the northern hemisphere summer and toward the southern subtropics during the southern hemisphere summer, but the stock within an ocean basin commingles and is not disjoint.

The breeding grounds for most whale stocks are poorly known. Exceptions are humpback whales, which breed in tropical coastal waters, and gray whales, which breed in coastal lagoons. The availability of food is evidently not a factor in the selection of breeding grounds, since migrating whales are known to pass by areas of known high productivity. The experience of whalers has been that most baleen whales killed in the winter have empty stomachs, and whales returning to the feeding grounds have a low oil content. The time spent on the breeding grounds is typically 1-2 months. The gestation period of baleen whales is typically 10-12 months. Sperm whales have a gestation period of 14-16 months.

Whales are capable of eating remarkable quantities of food during the feeding season. Large blue and fin whales may consume 1-4 tonnes per day, and Clarke (1977) has estimated that the current population of sperm whales eats more than 100 million tonnes of cephalopods per year. Estimates of growth rates and natural mortality rates depend critically on our ability to assign an age to a whale. Baleen plates show periodic irregularities that are believed to occur each year. However, the baleen is ultimately worn away at the ends as fast as it grows from the jaws, so this technique works only up to an age of 4-5 years. In the late 1950s, a method was developed that involves counting the number of growth layers in a plug within the ear canal. Each layer consists of two laminae, one of shed skin cells and the other of ear wax. Initially, it was believed two layers were deposited each year. However, examination of the plugs in whales that had been marked and recaptured after many years indicated that layers were probably deposited annually. Subsequent research has supported this hypothesis. Current understanding is that the light lamina, which contains much fat, forms in the feeding season. The dark lamina, which contains less fat, forms in the breeding season. Counting the growth rings in the ear plug is now the principal method of age determination for baleen whales.

In the case of sperm whales, there is obviously no baleen, and there are no ear plugs. However, ages can be assigned to sperm whales based on periodic (~1 per year) layers on the teeth (Bow and Purdy, 1966).

The reproduction rates of whales depend on the length of the gestation period and the time females spend nursing and nurturing their calves. For baleen whales the gestation period and nursing/nurturing interval last a total of 16-24 months. The interval between births is typically 2-3 years. For sperm whales the corresponding time period is longer. Females nurse their calves for roughly two years and may continue intermittent nursing for up to eight years. The average time between births is 3-5 years. Female whales almost invariably give birth to a single calf. Assuming equal numbers of female and male whales, a time interval of N years between births implies a reproductive rate of per year. Thus a time interval of two years between births would imply a reproductive rate of 25% per year, and a time interval of five years between births would imply a reproductive rate of 10% per year.

With the exception of humans, large adult whales are probably preyed upon only by killer whales. Both sharks and killer whales have been known to prey on whale calves. Other sources of natural mortality include parasites and, in the case of sperm whales, beach strandings. Although numerous parasites are common to whales, they are not believed to be a significant source of natural mortality. Almost all whales that strand are toothed whales (suborder Odontoceti). In some cases whales strand because they are ill, injured, disoriented, or just old, but in many cases the cause of whale strandings, particularly mass strandings, is unclear. In some cases a dying whale strands and is accompanied by other whales that appear to be perfectly healthy. Well intentioned persons have sometimes attempted to return the healthy whales to the sea, only to discover that the whales persist in restranding themselves. It is not uncommon for sperm whales to mass strand, but this behavioral trait is by no means universal to members of the Odontoceti. Killer whales and bottlenose dolphins, for example, virtually never mass strand.


Right Whales. The family Balaenidae includes three species, the bowhead or Greenland right whale, and the northern and southern right whales. In the early days of whaling, these whales were the “right” whales to hunt because they were rich in blubber, were easy to catch (they are unaggressive and slow swimmers), and floated after being killed. Some characteristics of right whales are summarized in Table 10.2.

Bowhead whales have about 350 pairs of baleen plates, and their plates are the longest of any whale, about 4.5 meters in length. Bowheads are confined to the northern hemisphere. They are the only large whales to spend their entire life in polar waters. They do migrate into lower latitudes during the winter, but they remain at all times north of roughly 60oN latitude. The North Pacific stock, for example, spends the summer off the north shore of Alaska, where they feed on plankton in the surface waters. Occasionally they will take benthic prey, but they never dive to depths greater than about 150 meters. In the autumn they migrate through the Bering Straits and spend the winter in the western Bering Sea, which serves as their breeding ground. The gestation period is 12-16 months. Females nurse their calves for about one year, but the mother and calf may remain together for another year or more. Bowheads become sexually mature at about six years of age.





Table 10.2. Some characteristics of Right Whales.

Species

Age of sexual maturity (years)

Lifespan (years)

Northern Right (Black Right)

5-10

70

Southern Right

5-10

70

Bowhead (Greenland Right)

6

100

Bowhead whales have been hunted by Eskimos for several thousand years. However, it is unlikely that Eskimo hunting ever had a serious impact on the adult stock. European whaling did; in fact European whaling almost drove the Bowhead to extinction. The population of Bowheads is currently estimated to be in the range of 8,000 to 12,000 animals, most of which belong to the North Pacific stock. The species has been protected from other than aborigine whaling since 1935.


Northern and Southern Right whales are similar to Bowhead whales but are smaller. They have about 200-270 pairs of long black baleen plates up to 3 meters in length. They become sexually mature at 5-10 years of age. Their geographical distribution overlaps somewhat with Bowhead whales in the northern hemisphere, but they tend to occupy more temperate latitudes. In the North Pacific they are found between 25o and 60oN latitude5 and in the North Atlantic from about 30-75oN. In the southern hemisphere they are found between 20o and 55oS but have occasionally been reported as far as 63oS. Northern Right whales were even more seriously impacted by European whaling than were Bowheads. Hunting of Right whales began as early as the 10th century, and during the 19th century roughly 100,000 Right whales were killed by the whaling industry. Both the Northern and Southern hemisphere stocks were near extinction by the beginning of the 20th century. Along with Bowheads, Northern and Southern Right whales were given international protection in 1935 at the first International Convention for the Regulation of whaling. Their protection was reinforced in 1972 with the passage of the Marine Mammal Protection Act. Despite this protection, Northern Right whales are in serious trouble, with only 500-1,000 animals remaining. The North Atlantic stock, considered by some to be a separate species from the North Pacific stock, numbers no more than about 300 whales and is considered to be headed toward extinction. The southern hemisphere stock numbers about 3,000 animals and is considered to be endangered.



Gray Whales. Gray whales are found only in the northern hemisphere. They are very much a coastal species, and because of this fact have provided many opportunities for human observation. Gray whales are perhaps the best known of the great whales because the annual migrations of the northeastern Pacific stock take them directly past the highly populated west coast of the United States. At one time there were actually three stocks of gray whales, one in the North Atlantic, one in the northeastern Pacific, and one in the northwestern Pacific. The North Atlantic stock was driven to extinction in the 17th century as a result of commercial whaling. The northwestern Pacific stock, which is found during the winter in coastal waters off Korea and Japan, is near extinction, with only 100-250 individuals remaining. The northeastern Pacific stock currently numbers about 26,000 whales and appears to be at the carrying capacity of the ecosystem. However, this stock has twice recovered from the brink of extinction. The Northeastern Pacific stock breeds in coastal lagoons along Baja California. These breeding lagoons were discovered by whalers in the late 19th century, and within a few years the stock had been decimated. Fortunately the stock became economically extinct before it was biologically extinct. The whalers lost interest, and the stock recovered. However, with the transition to pelagic factory ship operations in the early 20th century, there was a new assault on gray whales. The northeastern Pacific population dropped to fewer than 2,000 individuals. Relief finally came in 1946 with the formation of the International Whaling Commission (IWC). The Gray Whale acquired protected status at that time, and since then the northeastern Pacific stock has made a second remarkable recovery. In 1993 it was removed from the National Marine Fisheries Service endangered species list.


Table 10.3. Some characteristics of Gray Whales.

Age of sexual maturity (years)

Lifespan (years)

5-11

50-60
Gray whales feed during the summer months in the shallow coastal waters of the Bering and Chukchi seas. Beginning in October, they begin one of the longest of all mammalian migrations to their breeding grounds in Baja California. The roundtrip is somewhere between 16,000 and 22,000 km.

Whales in general and gray whales in particular are hosts to a sometimes remarkable assemblage of crustacean ectoparasites. It is not uncommon for a gray whale to carry almost 200 kilograms of barnacles and so-called whale lice. The lice in question include three species of amphipods, all belonging to the genus Cyamus. The barnacles that attach to gray whales all belong to the same species, Cryptolepas rhachianecti. The whales apparently become initially infected with lice as a result of direct physical contact with an infected whale, for example during mating or nursing. The barnacles, on the other hand, have a free-living larval stage and hence can be transmitted from one whale to another without direct physical contact. The impact of these ectoparasites on the whale’s well being is unclear, but it can hardly be beneficial.






  1   2   3


База данных защищена авторским правом ©shkola.of.by 2016
звярнуцца да адміністрацыі

    Галоўная старонка