Whales, whose hearing evolved amid natural sounds of ancient oceans, are now regularly bombarded by the noises of shipping, drilling, and sonar. Used by the military, mid-frequency sonar has been implicated in deadly mass strandings of toothed whales. Giant baleen whales such as blue whales, however, have not suffered such stranding. Their deep rumbling communication is at much lower frequencies than sonar, and so they were thought unlikely to be affected by it.
To test that assumption, Jeremy A. Goldbogen of Cascadia Research Collective in Olympia, Washington; Brandon L. Southall at the University of California, Santa Cruz; and a team of eleven other marine researchers equipped a boat to project simulated naval sonar. Their goal was to determine what effect, if any, sonar had on seventeen tagged blue whales off the coast of Southern California. The researchers categorized the whales into three groups according to their behavior: deep feeding, shallow feeding, and non-feeding whales. During periods of thirty minutes of sound projection, the team recorded activities such as feeding, diving, socializing, and traveling, and also measured the depth, speed, and body orientation of each whale, as well as the sound levels received by each.
They found that the experimental sound greatly affected several aspects of the blue whales’ diving behavior, as well as their orientation and horizontal displacement. The response depended on the maximum amount of sound received and especially on what whales were doing at the time. Whales that were feeding near the surface were barely affected by the sound, while deep-feeding and non-feeding whales were significantly affected: some ended their deep foraging dives and others prolonged their midwater dives.
In one case, a whale stopped feeding, swam away, and did not resume eating for more than an hour. Blue whales feed on krill, small crustaceans that occur in huge aggregations. The scientists calculated that at a usual feeding rate, that whale could have missed out on about 2,200 pounds of krill. In an increasingly noisy ocean, disruption of feeding habits could be significantly detrimental to these animals, the largest inhabitants of Earth. (Proceedings of the Royal Society B)