Most species of marine mammals seem highly reliant on and sensitive to underwater sounds. Sounds important to marine mammals may include calls from conspecifics, odontocete echolocation sounds, predator and prey sounds, and environmental sounds (e.g. surf or ice noise). Some man-made noises are known or suspected to have negative effects on marine mammals, including noise-induced masking, disturbance, hearing impairment, and possibly stress. However, marine mammals are adapted to a variable and often naturally noisy environment. Also, even when levels of man-made noise are well above natural ambient levels, negative effects on marine mammals are not always obvious. Data available up to early 1995 were summarised in the book "Marine Mammals and Noise'' (Richardson et al. 1995, Academic Press). Since then, advances have occurred in some but not all areas of particular concern:
(1) When can marine mammals hear man-made noise? Additional data are becoming available for some small- and moderate-sized odontocetes, pinnipeds, and manatees. There is still an urgent need for direct audiometric data from baleen and sperm whales.
(2) Does man-made noise mask important natural sounds? Data are available on masking in a few species of captive odontocetes and pinnipeds. However, we need data on masking processes and significance when free-ranging marine mammals are exposed to typical man-made sounds, including variable, non-tonal, and directional sounds.
(3) When does man-made noise disturb mammals, and when is disturbance strong enough to constitute harassment? Disturbance effects are graduated, not ''all or none''. Sometimes no disturbance is apparent even at short ranges with high received levels (RQ. At other times there is strong disturbance even at long ranges with low RLs. Strong and/or prolonged disturbance may have negative biological elects even if there is no physical damage. However, infrequent brief disturbances may have no biological significance, and if so should not be considered "harassment''. Additional controlled studies, both field and captive, are needed.
(4) What are the thresholds for noise-induced auditory impairment and non- auditory effects, and what types of man-made sounds could elicit them under field conditions? The first data on Temporary Threshold Shift (TTS) in marine mammals have been released. recently. TTS work with additional species and exposure conditions is needed. However, TTS results have limitations in establishing damage risk criteria (DRC), and relationships between TTS and harassment are uncertain.
(5) Noise-induced stress in marine mammals is almost entirely unstudied.
Mitigation measures sometimes used to reduce noise effects include seasonal and geographic restrictions, ramping up, and real-time monitoring plus localised mitigation. We need more data on the effectiveness of ramping up, visual and/or acoustic monitoring, and localised measures such as minimum approach distances, minimum altitudes, and shutdown radii. Progress is being made toward understanding noise effects on marine mammals, in focusing on the most serious issues, and in devising mitigation approaches. However, the issues are complex and the needed studies are often difficult. Some major emitters of underwater sound remain reluctant . to become involved in the process. It will take time, money and cooperation to conduct the needed studies, to determine which situations need mitigation, and to devise, test and implement effective yet practical mitigation measure.