Abstract:

Humpback whales employ bubble nets for catching fish. In the Gulf of Alaska the method involves acoustic communication between whales: one whale, situated below the school, transmits sounds and drives the fish toward the surface; a second whale, situated above the school, deploys a bubble net (as wide as 30 m), which encircles the school as it approaches the surface. Before and during this process one (or more) humpback emits signals, called trumpet sounds, which consists of a series of cw tones, which are harmonics of ~300 Hz. The target species in the Gulf of Alaska is adult, 22-cm-long herring. These signals cover the resonance frequency of the target species swim bladders, which is about 1.5 kHz at 30 m. Resonance frequencies of individual fish are determined by the radius and depth of the swim bladder; significantly lower bubble cloud resonance frequencies of schools are determined by the void ratio and the dimensions of the school. One of the functions of these vocalizations may be detection and classification: the listening whale, near the surface, may be capable of interpreting the high absorption losses at resonance frequencies [Diachok, O. ``Effects of absorptivity due to fish on transmission loss in shallow water,'' J. Acoust. Soc. Am. 105, 2105--2128 (1999)] to infer fish length, species, and size of school.