151st ASA Meeting, Providence, RI
Do Humpback Whales Detect and Classify Fish by Transmitting Sound Through Schools?
Orest Diachok --
Orest.Diachok@jhuapl.edu
Popular version of paper 4aAO5
Humpback whales are the Carusos of the oceans. Their songs have been the subjects of numerous scientific papers, popular books and TV documentaries. It has been established that only males sing the well known "humpback arias." The singers perform in an auditorium that spans an ocean, courteously taking turns "in the spotlight." Their haunting songs last hours and evolve almost imperceptibly over periods of years. Most marine biologists believe that these songs are part of "courtship" rituals. Less well known, but potentially biologically more important, are the tools that humpbacks have developed for catching fish. Marine biologists Cynthia D'Vincent and Fred Sharpe observed that pairs of humpback whales in the Gulf of Alaska capture schools of herring by surrounding them with a "bubble net," a cylindrical wall of bubbles. Such walls consist of large numbers of bubbles of varying sizes, and are typically several meters thick. Fish are reluctant to swim through such bubble screens, and are effectively trapped. According to DVincent, humpbacks form bubble nets by swimming in a spiral (corkscrew) pattern toward the surface making several ever smaller revolutions, as they discharge bursts of air through their blowholes. During these spirals they discharge about one burst of bubbles (a bubble cloud) per second. The diameter of these nets may be as large as 30 meters. This is sufficiently large to capture a large school of herring, which is about the size of a house, and contains close to a million fish. In other regions of the worlds oceans humpbacks are also known to employ bubble nets for catching fish, but have generally been observed to act alone. It is also noteworthy that solitary fin whales have been observed to trap fish with bubble nets, and that highly coordinated groups of dolphins surround large numbers of schools of sardines with a series of bubble screens that form bubble nets. Their nets can exceed 100 meters in diameter. The process employed by humpbacks in the Gulf of Alaska requires cooperation and communication between two or more whales. The two whales position themselves above and below the school of fish, as illustrated in Figure 1. The function of the whale closer to the bottom is to herd the school toward the surface; the function of the whale above the school is to blow a bubble net that encircles and captures the school. As the fish approach the surface several other humpbacks often join the fray; they surround the school and prevent the fish from escaping. The feast begins when the school arrives at the surface. Before and during the process one (or both) of the humpbacks emits sounds, designated "trumpet" sounds, which consist of a series of tones, with a fundamental frequency of about 400 Hz and its harmonics, i.e., 800 Hz, 1200 Hz, 1600 Hz, etc. These tones last about 5 seconds and are repeated several times in quick succession. Controlled measurements in a Norwegian laboratory suggest that the intensity of these signals is insufficient to cause the herring discomfort or precipitate a "panic" response. Some investigators suggest that their purpose is to herd the school. It is possible, however that humpbacks employ these sounds to determine the composition of the school. If acoustic signals emitted by one whale, pass through the school and are detected by the other whale, then the school would create an acoustic "shadow." The method would be analogous to x-ray systems commonly used in hospitals. In principle, the properties of the shadow could be used to infer the biological properties of the fish within the school, analogous to a physician's use of x-rays to infer the presence of water in a patient's lungs. The set of frequencies emitted by humpbacks includes the resonance frequencies of the swim bladders of herring, their favorite prey. Swim bladders are internal air sacs used to regulate buoyancy; they also amplify sound and act as a "hearing aid." The volume of a swim bladder diminishes in response to pressure. The swim bladders of 22 cm (9 in) long herring at a depth of 40 meters (130 ft) resonate at about 1600 Hz, the second highest G on a piano. It has been shown by the author and several other marine biophysicists that sound passing through schools of fish is preferentially absorbed at the resonance frequencies of the fish.
Resonance frequencies increase as the size of the fish decreases. For example, 11 cm (4.5 in) long juvenile herring at a depth of 40 meters resonate at about 3200 Hz, the highest G on a piano. It is well known that schools generally consist of fish belonging to the same species and size. Consequently absorption measurements at multiple frequencies could be used to discriminate between schools composed of adult and juvenile fish, and perhaps between species. Figure 2 illustrates how sound is absorbed by a schools of 22 and 11 cm long herring (passing between the two whales) at the frequencies of the humpbacks' "trumpet sound." In the case of a school of 22 cm long herring (the first school), sounds at 400 Hz pass through the school without any discernable effect (creating no acoustic shadow); whereas the intensity of sounds at 1600 Hz is dramatically diminished (creating a dark acoustic shadow); the intensity at higher frequencies, e.g., 3200 Hz is slightly diminished (creating a slight acoustic shadow). In the case of a school of 11 cm long juvenile herring, the darkest acoustic shadow occurs at a higher frequency, 3200 Hz, due to the juveniles' smaller swim bladder. The duration of these signals, 5 seconds, is too long to be used in an echo sounding system. The typical duration of signals emitted by commercial fisheries echo sounders is much shorter, typically about 0.01 seconds. Humpbacks also emit short duration, high frequency signals, which could be used for detection, but probably not for classification. We know that humpbacks' predominant food in the Gulf of Alaska is herring. We know this from numerous examinations of the stomach contents of slaughtered humpbacks, which were made during the era when whaling of these magnificent creatures was legal. This evidence suggests that humpbacks are capable of discriminating between species. There are many species of fish in the Gulf of Alaska; consequently discrimination prior to "casting" of bubble nets would no doubt expedite the hunt. It is noteworthy that humpbacks in the Gulf of Alaska employ bubble nets under conditions of high, low and no visibility. This observation, first noted by DVincent, is consistent with an acoustic method for discrimination. In summary, the experimental evidence and theoretical calculations to date support the hypothesis that humpback whales exploit fish resonances to detect and classify fish. So, the answer to the question posed in the title is: possibly yes. |
