A Sperm Whale blowhole.
Photo: Chris Johnson
July 12, 2002
Sperm Whales & Orcas
This is Genevieve Johnson speaking to you from the western side of the Chagos Archipelago in the central Indian Ocean.
Six days ago, members of the Odyssey crew heard a sperm whale on the acoustic array. The clicks trains ( visual representations of sound displayed on a computer), have increased each day. Currently, we have tracked these animals over more than 45 miles a day for 230 miles. These whales are travelling at a speed of 4 knots and are heading on a general northerly course.
The whales first appeared to be moving above, or just west of, a 4,000-meter contour line that shows on our bathymetric chart. It is the edge of a trench, which could indicate that the animals are foraging since these features usually account for increased primary productivity caused by up welling, making for a productive hunting ground. The staple diet of the sperm whale throughout the world is thought to consist primarily of mesopelagic squid weighing 1kg on average.
After the fourth day, rather than follow the canyon around to the northeast, the whales continued going over what seemed to be comparatively flat, featureless terrain. It also appeared as though their pace was quickening. Bob noticed while tracking the animals at over 5 knots on our Rainbow Click program, that the animals off the bow were moving farther ahead of Odyssey, while those off our stern were actually catching up to us. It appears as though these animals may be intentionally heading north for some reason, and at a faster pace than we normally find them going.
We are always left guessing the sex of a whale unless it is a mature bull - an immense animal that is easy to identify by its great size. Genetic tests must first be carried out on the small tissue samples we take from the animal before we can know for sure. From past experience, it is likely the animals we have been following may be young males. They are widely scattered, all are fairly similar in size, though none are close to the size of a mature bull, and all seem relatively unscathed, with few scars, and tail flukes margins in tact. If they were females, we would expect to see them in closer group associations during at least some of the time, and probably with some juveniles present.
After dispersal from their natal family units at about 7 - 9 years of age, young male sperm whales generally move to higher latitudes as they age, migrating seasonally toward the equator in winter (since we are south of the equator it is winter here). Perhaps this was a group of younger males. However the movement of sperm whales also depends very much on the abundance of food, which in any area can show great variation from year to year. When feeding success, as indicated by defecation rates is high, sperm whales tend to stay within geographic areas a few tens of kilometres wide, zig-zagging back and forth across them, whereas when there is little food available, they move in fairly straight lines for longer distances, covering over 40 miles a day. (Whithead, 1996.) Perhaps this group was simply moving to a better feeding ground!
Generally, a large home range will contain areas where food is reasonably abundant. The sperm whales' ability to move long distances in short periods of time - which may be what we have just seen-allows these animals to survive. Conditions affecting food supply in specific areas could prove to be disastrous for a less mobile animal.
Eventually their new course took the whales to the limits of the Exclusive Economic zone (EEZ) of the Chagos Archipelago, and into the territorial waters of the Maldives. As we have not applied for a permit to conduct research in the Maldives, we were forced to bid the group farewell.
Only a few hours later we headed south and began to hear an unfamiliar whistle on the array.
- Listen to some of the orca sounds recorded on the Odyssey underwater acoustic array -
These whistles did not belong to the dolphins and pilot whales we are so used to hearing on a daily basis, no, these sounds belonged to something altogether different and we all knew it. 'I think that's an Orca,' Chris said, and we all raced up onto the observation platform. The whistles coming through the speakers in the pilothouse increased steadily in volume. Chris saw it first, striking in coloration and immediately recognisable, a huge black and white body, robust and barrel-shaped, lying over on its side looking up at the Odyssey before drifting below our keel-an Orca, at least 16 feet long. We never saw the animal emerge on the other side, but two tall, unmistakable dorsals at 300 meters and moving away, let us know that the orcas' apparent but fleeting curiosity had been satisfied.
These were probably 'transient' animals, Orcas that roam the oceans in small cooperative groups, in search of their preferred prey - other marine mammals.
Our thoughts drifted back to the sperm whales we had left only hours ago. Sperm whales are sometimes attacked and killed by Orcas, although the size of mature sperm whale bulls, and the communal defence by healthy groups of females or young males usually keep them safe from predation. However our whales where not mature bulls, nor were they travelling in a group. We hope they are still moving quickly and that they are now too far away to be detected by the orcas.