nonharmonic tones which change frequency rapidly. Cries and whistles are pure tones with 

 or without harmonics. Group IV are clicks or pulses which have peak energy at high frequen- 

 cies, often between 20 and 30 kHz. 



Two types of sounds have been recorded from bowhead whales: a short duration and 

 a long duration sound. The sounds' fundamental frequencies are 50-80 Hz and 100-195 Hz, 

 respectively (ref 59). 



The hearing thresholds for large whales have not been measured. If the sounds pro- 

 duced by these whales are indications of sounds they could receive, then the whales' hearing 

 bandwidth extends from 12 Hz to 30 kHz. 



DISCUSSION 



Excess or increased environmental noise could impact animals that rely on acoustic 

 signals to maintain biological functions such as feeding, mating, and protecting and raising 

 young. No standards exist to evaluate the effects of noise on marine mammals and we lack 

 data on the auditory sensitivity for many species of marine mammals, particularly the large 

 whales. Data on the effects of sustained, low levels of noise on biological functions also is 

 sparse. Thus, in this report we cannot quantify the effects of offshore drilling operations on 

 marine mammals. 



The acoustic characteristics of the 20-Hz sound produced by the fin whale, 

 Balaenoptera physalus (ref 60), is described as a signal well suited for long range communi- 

 cations. The authors surmise that a decrease in the signal-to-noise ratio, either at the source 

 or the receiver, could sustantially reduce the detection range. 



Reference 61 showed that as the noise level in the vicinity of an echolocating dolphin 

 increased, the number of clicks increased (echolocation effort). Furthermore, overall detec- 

 tion performance was degraded with increased noise levels. 



Reference 62 suggests that increased shipping activities in Japanese waters have re- 

 sulted in altering the historical migration routes of the Baird's beaked whale, Berardius 

 bairdi. and the minke whale, Balaenoptera acutorostrata. Although additional factors may 

 be affecting these populations, the impact of increased maritime activities in whaling grounds 

 should be considered as a potential disrupting intluence. 



Figure 10 summarizes some possible effects of offshore drilling noise on marme 

 mammal populations. Noise can be classified as either chronic or acute. Chronic noise will 

 either mask signals or induce stress that may become manifest either physiologically or be- 

 haviorally. Acute noise may reduce the animal's ability to perceive a signal. Both acute and 

 chronic noise can cause short-term disruption of critical behaviors or mask intraspecific trans- 

 mission of information. If a population cannot adapt or accommodate to the short-term 



Ljungblad, DK, S Leatherwood and ME Dahleim, Sounds Recorded in the Presence of an Adult and Calf 

 Bowhead Whale, Balaena mysticetus. Naval Ocean Systems Center TR 420, p 1-7, 1979. 



Payne, R and D Webb, Orientation by Means of Long Range Acoustic Signaling in Baleen Whales, New 

 York Acad Sci, 188, p 1 10-141, 1971. 



" Penner, RH and J Kadane, Tursiops Biosonar Detection in Noise, In: Animal Sonar Systems. RF Busnel 

 and JF Fish, eds, p 957-959, Plenum Press, 1980. 



Nishiwake, M and A Sasao, Human Activities Disturbing Natural Migration Routes of Whales, Sci Rep 

 Whales Res Inst, 29, p 1 13-120, 1977. 



H-20 



