Seiji Osumi

The Mysterious Ecology of Whales

Whales living in the vast extent of the sea have an extremely wide biosphere, and hence it is very difficult to keep track of them over the long term. Moreover, a peculiar environment, namely ¡Èsea water,¡É poses various problems for our survey (please see Table 1). This article addresses the history of studies on whale migration and the challenges presented to them.

Whale migrations can be categorized into periodical migrations, horizontal migrations, and vertical (depth-wise) migrations. And periodical migrations can be further broken down into daily, monthly, and annual migrations, all of which vary with the species of whales. Take baleen whales as an example, they migrate (recurrent migration) on a global scale annually. These migrations are comprised of trips between breeding areas and feeding areas, and those between coastal areas and offshore areas; with daily migrations generally falling into the latter.

Whale migrations, however, still remain in the realm of mystery. In fact, the situation is such that even breeding areas of blue whales have yet to be tracked down.

Tracing Whales Through the Use of Satellite Beacons

Table 2 shows measures currently available for monitoring whale migrations.

During the period between the 1920s and the 1980s, a ¡Èmarking survey¡É had been standard practice for tracing whale migrations: distance information was obtained by catching whales previously marked with marking harpoons. Track A in Fig. 1 is an example of a mink whale that had been marked in the Antarctic Ocean in summer, and was caught later in a whaling ground off Brazil in winter. This method, however, was abolished in 1986 along with a moratorium being imposed on whaling.

Nowadays, surveys based on ¡Ènatural markings¡É are widely conducted for various species of whales across the globe. The shape and color pattern on the humpback whale¡Çs flukes vary among individuals ? a feature that can be used to identify each individual by checking it against pictures previously taken. Track B in Fig. 1 shows an example of a humpback whale that had been recognized in the Antarctic Ocean based on its unique markings, and later witnessed again in the coastal area of N02* latitude.

In addition to this method, meanwhile, the use of satellite tags is receiving attention these days. This particular method employs transmitters attached to whales, the signals of which are received and relayed via satellites and analyzed. Table 3 shows issues inherent in this method. Measures for attaching transmitters to whales, one of those issues, can be broadly categorized into two; one that requires the capture of whales, and the other that does not. In the case of the former, transmitters are attached to subjects either by a sash (around a dorsal fin), bolts, suction cups, or adhesives. These measures, however, are applied only to small whales such as dolphins since large whales are difficult to capture.

The latter, on the other hand, is being practiced by a group led by Bruce Mate of Oregon State University; they use a bow gun to stick a pin equipped with a transmitter into a subject. For whales that are relatively slow in movement, these pins can be directly stuck into the subjects, approaching them on a small boat.

The method of using transmitters for marking has long been experimented with in Japan. Track C and D in Fig. 1 are the results of surveys carried out by Tokai University in 1985 and 1986; they attached transmitters to several bottlenose dolphins at Taiji on the Kii Peninsula, and traced them for seven days through the use of a satellite. It turned out that one of them crossed the Japan Current to migrate southward.

Bruce Mate is actively conducting satellite tracking. Their transmitters, powered by lithium batteries, send signals indicating both locations and depths of whales to NOAA TIROS-N, a weather satellite; the ARGOS system aboard NOAA receives those signals. Locations of a whale can be tracked down (the range of error: less than 150 m) if the periodical signals sent by the transmitter are received three times during each surfacing of the whale.

Track E and F in Fig. 1 show the migration routes of two northern right whales marked by Bruce Mate in the Bay of Fundy located off the East Coast. While Subject F remained in the Bay, subject E migrated southward, keeping away from ??warm core ring??; it also avoided the warm current of the Gulf Stream and then headed northward.

Track G represents the migration route of a Greenland right whale marked in the Arctic Ocean in fall. Track H is a valuable record of a humpback whale marked off the coast of Maui; the subject headed northward along the Aleutian Islands and ended off the coast of the southern tip of Kamchatka, taking two months for the entire journey.

As mentioned above, surveys based on satellite tracking are widely practiced in various sea areas across the globe. But judging from data currently available, the longest survey has been that of an Atlantic white-sided dolphin, which continued for 105 days; there have been no surveys conducted for over three months (there is an example reported in some literature that allegedly continued for 200 days, but its original data have yet to be confirmed). The bottleneck in long-term surveys is the absence of a proper method of attaching transmitters to subjects. Surveys that extend over a period of at least six months, preferably one to two years, are indispensable for unlocking the secrets of whale migrations. Further developments in studies and surveys are thus needed in the future.

Applied research on satellite tracking of whales is just beginning to take shape. We therefore expect many researchers, especially young researchers, to take part in this particular field to shed light on the mystery of whales.