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an area for extended periods of time. For example, Engås et al. (1996) showed a significant catch decrease in a fishing area after use of air-guns, suggesting that fish moved from the ensonified area and only returned days later. There is also some evidence low-frequency noise produced by fishing vessels and their associated gear may cause fish to avoid the vessels (Maniwa, 1971; Konagaya et al., 1980). While all of these data need replication, they do suggest that sounds may change the behavior of fish. Movement of fish from a feeding area of marine mammals (or fishing areas for humans) could have an adverse impact on the higher members of a food chain and therefore have long-term implications despite the fish themselves not being killed or maimed.
Another concern is the impact of high-level anthropogenic sounds on overall behavior. Since many species of fish use sound for attracting mates and for other behaviors, any masking of these sounds could alter behavior. Increased environmental sounds in the vicinity of coral reefs may have a substantial impact on settling of larval fish on the reefs. Larval reef fish of many species spend part of their lives offshore and away from reefs, and then need to find a reef where they will live for the remainders of their lives (Leis et al., 1996). Recent evidence suggests that at least some larval fish are likely to use the reef sounds to find the reefs and that the fish will go to regions of higher-level sounds (Tolimieri et al., 2003). Thus, if there are intense offshore sounds, larval fish may be confused and not be able to find the reef. Alternatively, such sound may mask reef sounds, again preventing larval fish from finding the reef.
Potentially, anthropogenic sounds can have effects on marine life at a number of different levels, from short-term effects on individuals to long-term effects on populations and even species. Effects that can be dramatic, even lethal, at the level of the individual may have negligible consequences at the population level if, for example, small numbers of a large healthy population are affected. Conversely, effects that may seem insignificant for the well-being of individuals could have important conservation consequences for populations that are depleted and under stress. For example, a decrease in feeding rate that might equate to a year’s delay in attaining sexual maturity, a small increase in infant mortality, or a slightly shorter life span may not be overly significant to an individual animal but could mark the difference between extinction and recovery for a critically endangered species. It is important to emphasize that whether or not a particular impact could be of conservation significance will depend on the status of the population; thus, the conservation significance of particular impacts must be assessed on a case-by-case basis. While much legislation and scientific work focuses on conservation goals, it is important to recognize that the well-being and welfare of individual wild animals is also a concern for many members of the public and harassment of any individual marine mammal is prohibited by the Marine Mammal Protection Act.