Remarks on the Distribution and Migration of Whales, based on Observations from Netherlands Ships

A. GENERAL REMARKS During three years 4500 reports of whales sighted from Netherlands ships were collected, bearing on approximately 11.000 individual animals. Most of the observations were made in the Atlantic and Indian Ocean. It was supposed that the species could be determined with a fairly high degree of reliability in the case of Humpback Whales, Sperm Whales and Right Whales. No distinction could be made between Blue, Fin, Sei and Bryde Whales. They were collected under the heading Rorquals. Catches of land stations and strandings of whales, however, indicate that in all areas, at least a part of these Rorquals must have been Blue or Fin Whales. Probably the majority of this part were Fin Whales. Nevertheless it must he emphasized that the observations give no exact figures but only indications. It would be highly desirable if the results could be controlled by observations made by experienced whale biologists or gunners, especially in tropical and subtropical waters. We have the impression that for the time being no better results can be obtained with the present type of research. On the other hand, the fact that the generally known facts about the annual migration of the big whales were also clearly shown by this research, may be an indication for a certain degree of reliability of the research. The monthly number of animals of each species observed per 1000 hours steamed in daylight was plotted on charts in ten degrees squares. The reliability of the converted data is highest in the black dots. B. RORQUALS AND HUMPBACKS 1. Distribution The animals involved are not evenly distributed over the Oceans. There are big concentrations in certain areas, whereas in other areas practically no whales occur. Broadly outlined the highly populated areas coincide with the areas of greatest biological productivity of the sea, as shown by WALFORD (1958). In the tropics and subtropics important areas with a great number of sighted whales are: the Caribbean, the North African west coast, the Atlantic coast of South Africa, the Arabian Sea, the Gulf of Aden, the Bay of Bengal, the Indonesian Archipelago and the African east coast between 30° S and 40° S. It could be demonstrated that in the Indian Ocean southern Rorquals migrate over the entire breadth of the Ocean south of 30° S. North of 30° they migrate only at the eastern and the western side, apparently in order to avoid the waters with low biological productivity in the central part of the Ocean. No special relationship was found between the distribution or the migratory routes of the whales and the course of the big Ocean currents with regard to the locomotory aspect. There was a special relationship only in those cases where the big currents show a great biological productivity, as for example the Gulf Stream and the currents in the northern part of the Indian Ocean. 2. Migration, general remarks With regard to Rorquals in the North and South Atlantic, the Indian Ocean and the Pacific Ocean, as well as with regard to Humpbacks in the Atlantic and in the Indian Ocean it could be demonstrated that during the summer a part of the population does not migrate into Arctic or Antarctic waters, but that it stays in tropical, subtropical or temperate waters. In Humpbacks the phenomenon is less pronounced than in Rorquals. In Rorquals the phenomenon is not caused by observations of Sei or Bryde Whales only, because catches of land stations and strandings show that Blue and Fin Whales are present during the summer in the waters involved as well. The percentage of the stock of Blue and Fin Whales staying behind in warm and temperate waters is not known, but the authors have the impression that it is not unsignificant. The number of Rorquals staying behind during the summer appears to be larger in the North than in the South Atlantic, probably because Fin Whales in the North Atlantic feed on fish. The phenomenon of staying behind of a part of the population confirms the assumption that estimations of the Antarctic population of Blue, Fin or Humpback Whales never bear on the total stock of the species involved. The phenomenon may also cause that the number of periods or laminations in baleen plates or ear plugs, used in determining the age of Whalebone Whales, is not a reliable indication for the actual age of the animals. The actual age may be higher than the number of periods, because the staying behind in warm waters causes irregularities in their formation. It could, however, be demonstrated, that in most areas the majority of the populations showed the generally accepted type of annual migration. 3. Migration, Rorquals In the North Atlantic the principal northward migration of Rorquals takes place in April-July, the southward migration in September-November. In the South Atlantic the period of migration southward is September-December, that of the northward migration March-June. The majority of the Rorqual population (which may be principally the Fin Whale population) lives in the North Atlantic during the northern winter between 0° and 40° N and during the northern summer between 30° N and the border of the pack ice. With regard to the South Atlantic these areas are: in the southern winter between 20° N and 50° S, in the southern summer between the equator and the pack ice, but mainly in Antarctic waters. The northern and southern population apparently meet in the Caribbean, in waters off the North African west coast and probably also in the central part of the Ocean between 0° and 20° N. In the Indian Ocean large concentrations of Rorquals have been encountered in the northern part of the Ocean during the southern summer, whereas the number of sightings during the southern winter is surprisingly small. During this season the majority of the Rorquals is concentrated in waters of Madagascar and off the Australian west coast. This suggests, that during the southern summer (northern winter) the northern part of the Ocean is populated by Rorquals coming from the northern part of the Pacific Ocean. Probably these whales enter the Indian Ocean by passing the waters of the Indonesian Archipelago and the straits between these waters and the Indian Ocean. This supposition is supported by the fact that in the northern part of the Indian Ocean calves have been sighted in almost equal monthly numbers during the whole year, whereas in the Atlantic Ocean seasonal peaks in the number of sightings have been demonstrated. On the other hand, the possibility of a local stock in the northern part of the Indian Ocean may not be excluded. Although a number of southern Rorquals certainly migrate into the northern part of the Ocean during the southern winter, the majority of the population probably live in this season between the equator (or 10° S) and 30° S. In the southern summer the majority of the population is found in Antarctic. In the North Pacific Ocean the majority of the population is found during the northern summer between 20° N and the pack ice and in the northern winter between 10° N and 30° N. The South Pacific population apparently migrates northward during the southern winter up to about 10° N. 4. Migration, Humpbacks Humpbacks appear to migrate principally in coastal waters with the exception of the crossing part of the Gulf Stream in the North Atlantic (30° N to 50° N) where they are found during the northern winter over the entire breadth of the Ocean. In the northern part of the Indian Ocean they are spread over a large part of the Ocean as well. In the North Atlantic the majority of the population is found during the northern summer between 30° N and 50° N, and during the northern winter between 40° N and 10° S (especially in the Caribbean and off the North African west coast). Probably all Humpbacks in the Caribbean belong to the northern stock, because the southern population appears to live during the southern winter between 30° S and 20° N at the African side of the Ocean, but between 30° S and the equator at the American side. During the southern summer they are found between 30° S and the pack ice (mostly in Antarctic waters). In former days the North Atlantic Humpback population probably lived further northward (in summer as well as in winter) than nowadays. This may be connected with changes in feeding conditions or with the general decrease of the stock. Just as has been shown with regard to Rorquals, a part of the North Pacific Humpback population seems to migrate into the northern part of the Indian Ocean during the northern winter. The southern population of the Indian Ocean lives during the southern winter between the continent and 30° S. During the southern summer the animals are found between 45° S and the border of the pack ice. The northern and southern stocks of the Pacific Ocean meet in waters of the Indonesian Archipelago. At the eastern (American) side of the Ocean the northern population lives during the summer between 30° N and 50° N (or farther northward). During the winter they live between 10° N and 30° N. The southern stock appears to migrate as far to the North as 10° N. 5. Calves Sightings of calves of Rorquals (probably the majority of them being Fin Whales) in the Atlantic Ocean point to a peak in the number of births in December-January for the northern population and in May-June for the southern stock. North Atlantic Humpbacks appear to be born principally in the southern part of the North Atlantic in April, whereas births of the southern stocks apparently occur in tropical waters with a peak in September. C. SPERM WHALES 1. North Atlantic Sperm Whales are always present in the North Atlantic between 10° S and 30° N, but on the African side the population appears to be much larger than on the American side. A great number of animals are sighted in the Gulf Stream during the summer. The northward migration starts in April, the animals return to the South in autumn. The majority of the females do not go farther to the North than 40° N (a minority probably up to 50° N). The males migrate into Arctic waters. During the northern winter the majority of the males and females apparently live between 10° S and 30° N (the American stock mostly in the Caribbean), but some males may stay behind in colder waters as far as 60° N. 2. South Atlantic Practically no sightings of Sperm Whales have been reported from the South American east coast, although these waters show a reasonable biological productivity and although a great number of Rorquals have been sighted there. In former days great numbers of Sperm Whales have been caught in these waters. During the summer the males migrate into Antarctic waters, the females migrate up to about 40° S. During the winter most of the animals live in tropical waters but some males and females are present up to 40° S. 3. Indian Ocean With regard to the Indian Ocean there is a very significant correlation between the distribution of Sperm Whales and the biological productivity of the sea. In the northern part of the Ocean there are many more Sperm Whales sighted during the northern winter than during the northern summer. The general seasonal movements described with regard to the Atlantic and Pacific Oceans could not be demonstrated in the Indian Ocean. Apparently the Sperm Whales in this region show very special migratory movements which may be correlated with special conditions, caused by the fact that the Monsoon-stream in the northern part flows in an opposite direction in the two halves of the year. 4. Pacific Ocean Sperm Whales are encountered in the Indonesian Archipelago the whole year round. In the South Pacific they are not evenly distributed but apparently they are restricted to certain areas. The normal seasonal migratory movements could be demonstrated with regard to the South Pacific. D. OTHER SPECIES 1. Little Piked Whales Fairly large numbers of this species were sighted throughout the whole year in tropical waters of all oceans. Large herds were also seen in the northern hemisphere. They show concentrations in areas with a high biological productivity of the sea. During the winter the majority of the animals apparently live in tropical and subtropical waters. During the spring and the autumn they show the usual migratory movements, just as Rorquals and Humpbacks. During the winter, however, some animals stay behind in northern waters, whereas during the summer there are some stragglers in warm waters. The species has been observed in the northern part of the Indian Ocean during the northern winter. In the North Atlantic births take place in warm or temperate waters, probably from November to March. 2. Californian Grey Whales Sightings in the North Pacific were quite in accordance with the generally accepted opinion about the migration of this species. 3. Right Whales North Atlantic as well as Southern Right Whales have been reported. The majority of the animals do not migrate into waters between 20° N and 20° S, but there are indications that a few animals may also visit these tropical waters. With regard to the North Atlantic no sightings have been reported from regions north of 50° N, whereas there was a large number of sightings between 20° N and 50° N during the northern summer. In the Indian Ocean and in the Indian Archipelago two sightings were reported from waters between 10° N and 10° S. These observations, however, need further confirmation.