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statement. Results recently published by Popper et al. (2007) indicate that freshwater rainbow trout did not have any audi- tory or non-auditory tissue damage even though they experi- enced a significant amount of TTS after continuous expo- sures to LFA transmissions for 216 seconds. Additional data on other species are still being analyzed. The Chief of Naval Operations Environmental Readiness Division (CNO N45) sponsored a Workshop on Mid-Frequency Sonar and Marine Fishes in April 2007 to reach consensus among the scientific community and other stakeholders on research recommen- dations to address this issue (Read et al., 2007). Presently a CEE with mid-frequency sonar, similar to that conducted for SURTASS LFA, is in progress at Seneca Lake.
Fish have suffered hearing loss and damage to auditory sensory cells when exposed to seismic air gun emissions. Previous studies indicated that lengthy exposure to low fre- quency continuous tones (Hastings et al., 1996) with an SPL of 180 dB re 1 μPa or multiple emissions from a seismic air gun at close range (McCauley et al., 2003) could destroy sen- sory hair cells in the inner ears of fish. But in a recent study by Popper et al. (2005), fish that received a cumulative sound exposure similar to that reported in the McCauley et al. (2003) study when exposed to multiple emissions from a small air gun array in a river delta, experienced only TTS that recovered within 18-24 hours without hair cell damage. Understanding the differences among results of these studies is a topic of current research.
Studying the behavior of marine animals in the wild is very difficult, but much progress has been achieved in understand- ing both natural behaviors and the effects of sound on short- term behaviors of many marine animals. Because of potential effects on commercial fisheries, the behavior of fish in response to exposure to seismic air gun emissions has been studied for many years primarily via visual observation or underwater video (Falk and Lawrence, 1973; Pearson et al., 1987 and 1992; Løkkeborg and Soldal, 1993; Wardle et al., 2001, Thomsen 2002; Gausland, 2003; Hassel et al., 2004). Although catch rates are reported to decrease after air gun shooting and some fish have shown aversive reactions to the sound, overall the data are not easily extrapolated to other field operations. Research in this area as well as in behavioral responses of fish to other types of underwater sound is ongoing.
Richardson et al. (1995) provide the most comprehensive summary of short-term behavioral responses to sound by marine mammals for a number of different offshore industri- al activities. Changes in behavior attributed to underwater sound vary with age, sex, activity engaged in at the time of exposure (e.g., resting, foraging, socializing), perceived motion of the sound, and the nature of the sound source. Two CEE studies with SURTASS LFA signals indicated tem- porary alterations in behavior of marine mammals. Migrating gray whales avoided a stationary underwater sound projector playing back SURTASS LFA sonar signals when the source was located in their migratory path off the California coast (Tyack and Clark, 1998; NRC, 2003). But the whales seemed to ignore the sound source when it was locat- ed seaward of their migratory path, even when received lev- els were higher, indicating that the location of the sound source, not just its level, was critical to their behavioral
Fig. 2. Rainbow trout in test tank being removed from Seneca Lake after exposure to the U.S. Navy’s Surveillance Towed Array Sensor System, Low Frequency Active (SURTASS LFA) transmissions. Photo from Popper et al. (2007).
response. In the second study, Miller et al. (2000) found that some, but not all, humpback whales exposed to SURTASS LFA signals made louder and longer songs during exposure. But the song duration and loudness returned to normal lev- els immediately afterwards. The long term significance of these changes in behavior is unknown.
In the mid-1990’s advances in satellite tag technology enabled several large scale natural behavioral studies of marine mammals (Mate et al., 1998 and 1999; Lagerquist et al., 2000). Satellite-monitored tags transmit a radio signal that allows tracking day-to-day movements of animals that migrate over tens of thousands of miles in the ocean each year. The tags are safely implanted in the skin-blubber layer of marine mammals (Fig. 3), and similar externally mounted tags have been used on turtles, fish, and even seabirds. This information has established previously unknown migratory corridors, feeding grounds, and movement patterns of sever- al populations of whales. Perhaps the most ambitious and successful project is Tagging of Pacific Predators (TOPP), which began in 2000 and is part of the Census of Marine Life (www.coml.org), a 10-year worldwide effort to assess diversi- ty and abundance of life in the oceans. TOPP is managed by a team of scientists from Stanford University’s Hopkins Marine Lab, NOAA Southwest Fisheries Science Center, and UCSC (see www.topp.org for more information). The use of these tags has dramatically improved understanding of the range and habitat use of large whales, sharks, tuna, turtles, and many other marine species. These data are critical for planning and mitigating potential adverse impacts of sound- producing activities in the ocean.
During the last decade many passive acoustic monitor- ing (PAM) tools have been developed that are useful for large scale natural behavioral studies. These include low cost, hand-deployable listening arrays and ‘leave-behind’ retriev- able devices that are now widely used by scientists to assess the underwater acoustic environment and study animal
26 Acoustics Today, April 2008