Marine Mammals and Low-frequency Sound: Progress Since 1994

Abstract

Sound has become a major tool for studying the ocean. Although the ocean relatively opaque to light, it is relatively transparent to sound. Sound having frequencies below 1,000 Hertz (Hz) is often defined as low-frequency sound. The speed of sound is proportional to the temperature of the water through which passes. Therefore, sound speed can be used to infer the average temperature of the water volume through which sound waves have passed. The relationship between water temperature and the speed of sound is the basis for the Acoustic Thermometry of Ocean Climate (ATOC) experiment. The ATOC experiment is designed to monitor the travel time of sound between sources off the coasts of Hawaii and California and several receivers around the Pacific Ocean in order to detect trends in ocean temperature and for other research and monitoring purposes (ATOC Consortium, 1998). The ATOC transmissions are centered at a frequency of 75 Hz, with peak source levels of 180 decibels (dB) re 1 µPa 1m1 this frequency and 195 dB for its broadband source level. Based on well-tested models of signal loss over distance in deep water, the source level should decrease 155 dB within 100 m from the source and to 135 dB at 1 km from the source. Some whales, seals, and fish use low-frequency sound to communicate and sense their environments. For example, baleen whales and some toothed whales are known to use and respond to low-frequency sound emitted by other individuals of their species. Sharks are not known to produce low-frequency sound but are attracted to pulsed low-frequency sounds. Therefore, it is possible that human-generated low-frequency sound could interfere with the natural behavior of whales, sharks, and some other marine animals. In 1994 the Defense Advanced Research Projects Agency (DARPA) requested that the National Research Council (NRC) convene a committee to evaluate the results of ATOCs Marine Mammal Research Program (MMRP) (see Appendix A for Committee biographies). The MMRP was designed to monitor the effects of ATOC transmissions on marine mammals. Although DARPA was the original sponsor, the Office of Naval Research (ONR) is now funding the MMRP and cosponsored this study. The NRC was asked to 1. conduct an updated review of Low-Frequency Sound and Marine Mammals: Current Knowledge and Research Needs (NRC, 1994), based on data obtained from the MMRP and results of any other relevant research, including ONRs research program in low-frequency sound and marine mammals; 2. compare new data with the research needs specified in the 1994 NRC report, focusing on the strengths and weaknesses of the data for answering the important outstanding questions about marine mammal responses to lowfrequency sound; and identify areas where gaps in our knowledge continue to exist. The Committee reviewed numerous written documents and was briefed on the MMRPs progress at the programs midpoint (in 1996) and about 6 months after the completion of its field observations (in 1999). The NRC was asked to prepare an interim report to provide midproject guidance to the MMRP as well as this final report. Some of the recommendations in the interim report (NRC, 1996) were implemented by 1999, but for a variety of reasons others were not. For its update of research priorities related to marine mammals and lowfrequency sound, the Committee augmented the MMRP results with results from the scientific literature, ONRs program on marine mammals, and observations f the reactions of marine mammals to tests of the Navys low-frequency active (LFA) sonar. This report does not examine the effects of all human-generated sound (only low-frequency sound is considered), nor does it include all marine mammals (only whales and seals are included). This report updates all aspects of NRC (1994), including the issue of acoustic harassment and its regulatory definition in light of the 1994 reauthorization of the Marine Mammal Protection Act (MMPA). The publication of the report is particularly timely because the MMPA expired on October 1, 1999 and is in the process of being reauthorized. The Committee focused exclusively on whales and seals, because (1) they are found near the ATOC sources, (2) the effects of low-frequency sound on whales and seals have been studied to a greater extent than effects on other marine mammals (in part, because they live near ATOC sources), and (3) it is thought that low frequency sound is less likely to have a significant impact on other marine mammals, including sea and marine otters, manatees and dugongs, and polar bears. FINDINGS AND RECOMMENDATIONS Some of the MMRP observations, such as movements of humpback whales in near-coastal areas off Kauai and the abundance of some whale species near the Pioneer Seamount source off California, showed no statistically significant effects of ATOC transmissions. For these observations, the Committee could not distinguish among true lack of effect and insufficient observations, small sample sizes, and incorrect statistical treatment of data. A somewhat clearer lack of significant effects of the ATOC transmissions was demonstrated in observations of elephant seals diving behavior near the Pioneer Seamount source. Some statistically significant differences between control and exposure conditions were found for other species, including (1) an increase in average distance of humpback whales from the California source and (2) increased dive duration for humpback whales off Hawaii. The MMRP found no obvious catastrophic short-term effects as a result of transmissions from either source, such as mass strandings or mass desertions of source areas. Statements about whether ATOC should be allowed to continue, based on MMRP and other results, are clearly outside the Committees statement of task. However, the Committee does offer suggestions about how future large-scale acoustic tomography experiments could be designed to accomplish appropriate monitoring for scientific purposes and mitigation measures to decrease the possibility of harm to marine mammals. Progress has been made since 1994 in answering several of the research questions described in the 1994 NRC report. Research funded by ONR and other agencies and the results of the MMRP and LFA tests have contributed new knowledge regarding the effects of low-frequency sound on marine mammals. Research and observations published since 1994 have extended our knowledge of the hearing abilities of marine mammals at lower frequencies, at depth, in the presence of human-generated noise, and among different individuals of the same species. More observations of baleen whale vocalizations and responses to sound have been collected and a greater appreciation has been gained about how the respective locations of a baleen whale and a sound source can affect vocalizations and other behavior. Extensive testing with conventional and new methods, such as computational modeling of ear anatomy, auditory evoked potential techniques, and stimulus-response experiments with trained animals have provided new insights into normal hearing and the levels of sound required to produce shifts in the hearing abilities of individual animals. Most of the research directions recommended by the 1994 report are still relevant. This continued need to answer the questions raised therein is not due to lack of effort but is a result of the complexities of the questions and the difficulties of conducting studies on marine mammals because of the lack of adequate research support, equipment, techniques, and facilities. The 1994 amendments to the MMPA (16 U.S.C. 1361 et seq.) changed the legal definitions of marine mammal harassment as applied to scientific use of sound in the ocean. If the MMPA is to be implemented responsibly, however, additional changes should be made to the act and to the regulations promulgated pursuant to the act by the Office of Protected Resources of the National Marine Fisheries Service (NMFS) in the National Oceanic and Atmospheric Administration (NOAA). There is little disagreement that scientific use of sound in the ocean is a minor component of human-generated sound pollution. Industry (e.g., shipping and hydrocarbon exploration and production) are thought to be the largest sources. Yet, uses of sound by scientists and the Navy are the most stringently regulated. Unfortunately, few data are available to regulators regarding ambient noise levels in the ocean and the relative importance of different sources in contributing to the cumulative human-generated noise. Cooperative funding of research by government and industries responsible for the noise could result in more rapid advance of knowledge about the effects of sound on marine mammals and cooperative solutions to noise problems. This report includes a number of recommendations to Congress, to NOAA in its regulatory role, and to research sponsors, as well as to the scientific community. The recommendations directed to Congress should be implemented in the upcoming reauthorization of the MMPA. The recommendations directed to NOAA in its regulatory role should be implemented as it promulgates new regulations based on the reauthorized MMPA. Finally, agencies that fund marine mammal and acoustic research should begin weighing recommendations about research, monitoring, and facilities against other budget priorities for the fiscal year 2002 budget cycle and beyond. Some of the recommendations to research sponsors should not require reprogramming or new money and could be implemented immediately.