Chapter VI — Effects of Pollution 



result in crowding, overexploited food resources, 

 increased mortality, and decreased productivity. 

 Certain sounds also may affect the distribution, 

 density, movements, or productivity of important prey 

 species, making it more difficult for marine mammals 

 to find suitable prey. Certain sounds also may cause 

 physiological or psychological stress and make ani- 

 mals more vulnerable to parasites, diseases, and 

 predation. Also, some sounds may attract marine 

 mammals and make them more vulnerable to hunting, 

 harassment, entanglement in fishing gear, and colli- 

 sion with boats. Further, high-intensity sounds and 

 pressure waves, such as those produced by underwater 

 explosions or acoustic harassment devices used to 

 keep marine mammals away from fishing pens, can 

 cause temporary or permanent hearing loss and, in 

 some cases, seriously injure or kill marine mammals. 



How and to what extent sounds affect marine 

 mammals depends on a number of variables. The 

 variables include the nature and intensity (loudness) of 

 the sound, whether the source is stationary or moving, 

 and the species, age, sex, reproductive status, activity, 

 and previous experience of the animals exposed to the 

 sound. Blue whales, for example, produce and 

 apparently use low-frequency sounds for long-distance 

 communications and therefore are more likely to be 

 affected by low-frequency sounds from anthropogenic 

 sources than species that produce and use higher- 

 frequency sounds. 



The distribution, diet, and behavior of some 

 marine mammal species differ between sexes and age 

 groups, and responses to sound likewise may differ. 

 For example, temale northern fur seals that pup and 

 breed on the Pribilof Islands migrate to waters off 

 central California during the winter while most adult 

 males migrate only as far south as the Gulf of Alaska. 

 Further, newly weaned pups and possibly yearlings 

 are not able to dive as deep or as long as adults and 

 therefore may have a more restricted diet and be 

 affected more by sound-caused changes in prey 

 availability. Also, pregnant females and females with 

 dependent young may have habitat -use patterns, food 

 preferences, feeding behaviors, and response thresh- 

 olds that make them more or less sensitive to anthro- 

 pogenic sounds than are juveniles, males, or females 

 that are not pregnant or nursing. 



In some cases, responses to anthropogenic sounds 

 may be accentuated or dampened by prior exposure. 

 If a sound is associated with a painful experience 

 {e.g., hearing and then being hit by a boat), exposure 

 to that sound may evoke a more rapid or greater 

 response in "experienced" animals than in "naive" 

 animals. Conversely, if a sound evokes a startle 

 response because it is unusual, repeated exposure may 

 evoke less and less response {i.e., animals may 

 become so used to the sound that they no longer 

 respond to it). 



Response may also vary depending on the environ- 

 ment. For example, animals may respond differently 

 in deep water than in shallow water, in murky water 

 than in clear water, and in enbayments than in the 

 open ocean. In some cases, differences in response 

 may be due to differences in ambient noise levels, 

 which in turn are affected by vessel traffic, wind, 

 weather, the presence of ice, and other variables. In 

 other cases, the differences may be due to the animal 

 itself {e.g., an animal in an unfamiliar environment 

 may respond to a sound differently than it would in a 

 familiar environment). Similarly, response to a 

 particular sound may depend on the activity in which 

 the animal is involved at the time. For example, 

 some species and individuals may be nearly oblivious 

 to external stimuli when engaged in activities such as 

 feeding and courtship, but other species and individ- 

 uals may be particularly sensitive to disturbance when 

 engaged in such activities. 



There is growing awareness that sounds from 

 various anthropogenic sources could be having ad- 

 verse effects on marine mammals and other marine 

 species. There also is growing awareness that certain 

 types of sound can be used to help elucidate geologic 

 resources beneath the ocean, the structure and dynam- 

 ics of ocean water masses, and how ocean processes 

 affect and reflect weather and climatic conditions. 

 There also is increasing interest in using sound to 

 influence marine mammal behavior {e.g., keep them 

 away from fishing gear) or to detect vessels and 

 marine life, particularly fish. As this interest has 

 increased, so too have the sources and pervasiveness 

 of anthropogenic sound throughout the marine envi- 

 ronment. 



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