Oceans of noise - Whale and Dolphin Conservation Society

4.3.6. Danger avoidanceMany vertebrate species produce alarm calls, many of which provide information as to the type ofthreat, so that group members can respond appropriately (e.g. Seyfarth et al. 1980; Cheney andSeyfarth 1985). Cetaceans are vulnerable to predation by several marine species such as sharks(Cockcroft 1991) and killer whales (Jefferson et al. 1991). Although there are few data recordingspecific alarms calls produced by cetaceans encountering predators, several studies havedocumented increases in certain odontocete calls believed to be ‘alarm’ calls, in response to boattraffic (Findley et al. 1990; Lesage et al. 1999). The potential biological impacts of a cetaceanunable to hear or distinguish an alarm call are obvious.4.4. Prey stunningAnother use of sound by cetaceans may be to debilitate and stun prey. Several researchers haveremarked upon the fact that production of intense sound sources could be used by cetaceans whenforaging (Berzin 1972; Norris and Møhl 1983; MacKay and Pegg 1988). Certainly, intense soundwaves directed at fish bearing air-filled swim bladders could cause rapid pulsations of the swimbladders and cellular damage. Sound waves could also cause the reverberation of microscopicbubbles contained in seawater, and if this seawater was in a fishes’ lateral line or around the gills,it would cause tissue damage (Miller and Williams 1983). It has, moreover, been demonstratedexperimentally that acoustic pulses can be used to stun fish (Zagaeski 1987). Information oncetaceans using this method to catch prey is limited, but plausible, and would encompass anotherway in which cetaceans use sounds for vital biological processes.4.5. Acoustic sensitivities of cetaceansAt present, the sensitivity of cetacean hearing to various sound frequencies has been conductedby series of behavioural experiments. Simply, these involve playing sounds of variousfrequencies and intensities to captive cetaceans, where the cetaceans indicate whether the soundsare detected. These experiments produce audiograms, u-shaped curves that give the thresholdacoustic intensity (in dB) above which cetaceans can hear sounds of varying frequencies.Toothed cetaceans are capable of hearing sound over a broad frequency range, with aspecialisation for higher frequencies (Nachtigall et al. 1998). Their sensitivity to high frequencysounds is related to their use of relatively high frequency sounds for communication and highfrequency pulses for echolocation.Of the species for which auditory sensitivity has been studied, e.g. beluga whale, false killerwhale, bottlenose dolphin, harbour porpoise and Risso’s dolphin, sensitivity is greatest over10kHz (Au et al. 1997; Kastelein et al. 1997; Ridgeway et al. 1997). The upper limits of auditorysensitivity are believed to range from 31kHz in the killer whale to over 150kHz (Richardson et al.1991).Most man-made noise is below 10kHz in frequency and behavioural experiments have shown thathearing thresholds increase and hence sensitivity to sounds of these frequencies decrease forbeluga whales and bottlenose dolphins. Sensitivity to sounds is poor below 1kHz. Au et al.(1997) and Nachtigall et al. (1998) determined that at 75Hz, false killer whales and Risso’sdolphins have auditory thresholds of 140dB, that is to say sounds with a frequency of 75Hz mustbe over 140dB before they can be heard.48