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FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013 NOISE-INDUCED LOSS OF HEARING SENSITIVITY. Noise-induced loss of hearing sensitivity 9 or “threshold shift” refers to an ear’s reduced sensitivity to sound following exposure to loud noises: when an ear’s sensitivity to sound has been reduced, sounds must be louder for the individual affected to detect and recognize it. Noise-induced loss of hearing sensitivity is usually represented by the increase in intensity (in decibels) sounds must have to be detected. Although noise-induced losses in hearing sensitivity rarely affect the entire frequency range an ear might be capable of detecting, only a few investigators have reported the frequency range affected by a hearing loss. An animal can experience either temporary threshold shift (TTS) or permanent threshold shift (PTS). TTS can last from minutes or hours to days. When PTS occurs, there is physical damage to the sound receptors in the ear. This can result in total or partial deafness, or an animal’s hearing can be impaired in specific frequency ranges (Box P2 of Figure 3 illustrates the potential consequences of noise-induced loss in hearing sensitivity). Although the published body of science literature contains numerous theoretical studies and discussion papers on hearing impairments that can occur with exposure to a strong sound, only a few studies provide empirical information on noise-induced loss in hearing sensitivity in non-human animals. Richardson et al (1995) concluded that there was no empirical evidence that exposure to active sonar transmissions with the kind of intensity can cause PTS in any marine mammals; instead the probability of PTS has been inferred from studies of TTS. Richardson et al. (1995) hypothesized that marine mammals within less than 100 meters of a sonar dome might be exposed to midfrequency active sonar transmissions at received levels greater than 205 dB re 1 μPa which might cause TTS. Erbe (2002). argued that killer whales would have to stay within 50 meters of a single boat for 8 hours a day, 5 days a week, for up to 50 years to experience a permanent threshold shift of 2 – 5 dB as a result of exposure to engine noise, although exposing killer whales to multiple vessels could cumulatively produce temporary or permanent threshold shifts. Schlundt et al. (2000; see also Finneran et al. 2001, 2003) provided a detailed summary of the behavioral responses of trained marine mammals during TTS tests conducted at the Navy’s SPAWAR Systems Center with 1-second tones. Schlundt et al. (2000) reported on eight individual TTS experiments that were conducted in San Diego Bay. Fatiguing stimuli durations were 1 second. Because of the variable ambient noise in the bay, low-level broadband masking noise was used to keep hearing thresholds consistent despite fluctuations in the ambient noise. Finneran et al. (2001, 2003) conducted TTS experiments using 1-second duration tones at 3 kHz. The test method was similar to that of Schlundt et al. except the tests were conducted in a pool with a very low ambient noise level (below 50 dB re 1 μPa 2 /Hz), and no masking noise was used. The signal was a sinusoidal amplitude modulated tone with a carrier frequency of 12 kHz, modulating frequency of 7 Hz, and SPL of approximately 100 dB re 1 μPa. Two separate experiments were conducted. In the first, fatiguing sound levels were increased from 160 to 201 dB SPL. In the second experiment, fatiguing sound levels between 180 and 200 dB re 1 μPa were randomly presented. 9 Animals can experience losses in hearing sensitivity through other mechanisms. The processes of aging and several diseases cause some humans to experience permanent losses in their hearing sensitivity. Body burdens of toxic chemicals can also cause animals, including humans, to experience permanent and temporary losses in their hearing sensitiviy (for example, see Mills and Going 1982 and Fechter and Pouyanos 2005). 198
FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013 Based on the information available, and given the speeds at which Navy vessels operate during the activities they proposed to conduct in the Hawai'i Range Complex, the protective measures the Navy proposes to employ during an exercise, and the probable avoidance responses of those animals upon exposure, we think it is highly unlikely that large whales would routinely accumulate acoustic energy sufficient to cause noise-induced loss of hearing sensitivity. At the ship speeds involved, collisions would present a greater risk than noise-induced hearing loss; as we have discussed previously, the Navy’s protective measures, which are designed to detect large whales (and other objects) in their path to protect the ships from being damaged during a collision, are also likely to prevent large whales from being exposed to received levels sufficient to cause hearing losses. 5.3.3.2 Acoustic Masking Marine mammals use acoustic signals for a variety of purposes that differ among species, but include communication between individuals, navigation, foraging, reproduction, and learning about their environment (Erbe and Farmer 2000, Tyack 2000). Masking, or auditory interference, generally occurs when sounds in the environment are louder than and of a similar frequency to, auditory signals an animal is trying to receive. Masking, therefore, is a phenomenon that affects animals that are trying to receive acoustic information about their environment, including sounds from other members of their species, predators, prey, and sounds that allow them to orient in their environment (the responses of animals sending acoustic signals are addressed in the next subsection). Masking these acoustic signals can disturb the behavior of individual animals, groups of animals, or entire populations (Box M of Figure 3 illustrates the potential consequences of acoustic masking). Richardson et al. (1995b) argued that the maximum radius of influence of an industrial noise (including broadband low frequency sound transmission) on a marine mammal is the distance from the source to the point at which the noise can barely be heard. This range is determined by either the hearing sensitivity of the animal or the background noise level present. Industrial masking is most likely to affect some species’ ability to detect communication calls and natural sounds (i.e., vocalizations from other members of its species, surf noise, prey noise, etc.; Richardson et al. 1995). Sperm whales have been observed to frequently stop echolocating in the presence of underwater pulses produced by echosounders and submarine sonar (Watkins and Schevill 1975; Watkins et al. 1985). They also stop vocalizing for brief periods when codas are being produced by other individuals, perhaps because they can hear better when not vocalizing themselves (Goold and Jones 1995). Sperm whales have moved out of areas after the start of air gun seismic testing (Davis et al. 1995). Seismic air guns produce loud, broadband, impulsive noise (source levels are son the order of 250 dB) with “shots” every 15 seconds, 240 shots per hour, 24 hours per day during active tests. Because they spend large amounts of time at depth and use low frequency sound sperm whales are likely to be susceptible to low frequency sound in the ocean (Croll et al 1999). Furthermore, because of their apparent role as important predators of mesopelagic squid and fish, changes in their abundance could affect the distribution and abundance of other marine species. The echolocation calls of toothed whales are subject to masking by high frequency sound. Human data indicate low frequency sound can mask high frequency sounds (i.e., upward masking). Studies on captive odontocetes by Au et al. (1974, 1985, 1993) indicate that some species may use various processes to reduce masking effects (e.g., 199
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