Whale Hunting with a Hydrophone

By Charles J. Fish, Professor of Oceanography

Traveling northward in the Sargasso Sea on her first quarterly plankton-bioacoustic cruise on July 17, 1963, RIV Trident came upon a school of sperm whales about 145 miles southeast of Bermuda. Thereafter for several hours the ship moved slowly along with a cetacean escort. The whales in the area at that season were mostly females and young, the adult bulls having already migrated to northern waters to prey on the giant squid , Architeuthis.

Being predatory animals, with little fear of aquatic enemies, these toothed whales exhibited no apparent alarm at our approach. In fact one large specimen rolled lazily almost under the bow and two calves splashed about only a few feet from the hydrophone. After initial curiosity passed, they resumed their unhurried journey. On occasion a large and a medium-sized individual appeared at the surface with a juvenile, about ten to twelve feet in length, between them. Some, when approached, slowly sank beneath the surface and reappeared farther away. However, when sounding, presumably to feed on squid which remain in deeper water by day, they made a more vertical descent and in so doing elevated the flukes above the surface before disappearing.

On three of five Trident biooceanographic cruises to the Sargasso Sea in 1963-64 sperm whales were encountered in numbers ranging from eight to several hundred and tape recordings of their echo-ranging and communication sounds were obtained. All sightings occurred in summer and autumn. None was seen in the western Sargasso Sea during the winter or early spring months (January and March-April).

Whaling log records were collected by the late Dr. Charles H. Townsend on 36,908 sperm wholes caught between 1761 and 1920. He found that small harem-type schools of females and young bulls, led by an old steer, are widespread in tropical and subtropical waters which they rarely leave. Larger aggregations in excess of 100 animals at times comprise both cows and bulls of all ages, but by summer the numbers are reduced when most of the bachelor bulls migrate to the Arctic and Antarctic. With the approach of winter these males return to the tropical mating grounds in the trade wind belt, south of the Sargasso Sea, one of the greatest concentrations according to Townsend occurring on the "Twelve Forty Ground" (Lat. 12° N, Long. 40° W) in March, April and May.

Sperm whaIe echo-location sounds are well known in both the Atlantic and Pacific. In the eastern Pacific other sounds, not positively identified but apparently associated with the presence of sperm whales, have also been reported in several instances. It has been surmised that these sounds occurring during the known breeding season of the species are perhaps comparable to the mating calls of certain fishes. Failure to detect them in the Atlantic may have been attributable to the absence of bioacoustic studies on Atlantic grounds during the sperm whale mating season.

To, further pursue this problem, members of Marie P. Fish's bioacoustic project departed on March 8 to participate in the first sector of the recent Trident cruise, passing through the "Twelve Forty Ground" enroute to Brazil. The ship, in addition to a bow hydrophone and periscope-type directional tracking hydrophone, was equipped with a newIy developed deep water electronic "ear" capable of operating to depths of 3600 or more feet, depths to which the sperm whole is known to descend.

Whale watches were set and periodic sound scanning started when the ship entered the Sargasso Sea and continued throughout daylight hours. During five days of intensive search on the "Twelve Forty Ground,"' but one small group of four sperm whales was observed at the surface and this occurred fifteen minutes after the directional hydrophone had detected and tracked their approach to the ship. The hydrophone further revealed the presence of a large local sperm whale population widely distributed in smaII harem-type schools, usually comprising three or four individuals. They were located and tracked below the surface with sound gear at each of the six test locations in the area; however, during the balance of the journey, which included a traverse of the south Atlantic "coast of Brazil Ground" enroute to Recife, such observations became very rare.

Whale-ship log records, based on visual detection, may therefore be very misleading. The frequent accounts of months of sailing without sighting whales is no assurance that they were not present beneath the keel. Squid, the sperm whales' principal food, descend with plankton by day and the whales, which can remain submerged for more than an hour, spend much time during daylight pursuing them. At the surface, even in relatively calm seas, sperm whales may be rather inconspicuous, swimming with backs awash and frequently exhaling a hardly visible fine spray instead of the characteristic powerful "blow"' after a deep dive.

Townsend's charts, based on whalers' logs, show observations of this species concentrated in the trade wind latitudes and all but absent in the Sargasso Sea region immediately east and south of Bermuda to latitude 30° N where, as already noted, Trident observers found them at times in considerable numbers on all summer and autumn cruises. Here again, old log records may prove misleading unless one realizes that in this portion of the Sargasso Sea the winds are typically variable and rare, and sailing vessels could be expected to avoid waters where there would be danger of being becalmed for extended periods. A bsence of recorded whole sightings in such areas may be attributable to absence of observers.

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