FISHERY BULLETIN: VOL. 84, NO. 2 



categories, nor did we record his nonvocal, sharp im- 

 pulsive category. 



Our experience with finback and Bryde whales in 

 the Gulf of California showed that underwater- 

 generated sounds were not produced when visible 

 animals were at or very close to the surface. Excep- 

 tions were those sounds which, although principally 

 airborne (eg., blow and snort sounds), established 

 a physical coupling with the water medium allow- 

 ing detection by hydrophone The typical short 

 "20-Hz signals" noted from finback whales in other 

 locations (Northrop et al. 1968) appear in trains that 

 are interrupted after 3 to 22 min of pulsing (equi- 

 valent to expected dive times, Fig. 8). We believe that 

 these interruptions that last from 1 to 6 min 

 represented surface time. Blue whale sounds in 

 southeast Pacific waters had silent interruptions that 

 were associated with surfacing and ventilation (Cum- 



mings and Thompson 1971). Winn et al. (1970) cor- 

 related certain "cries" and "ratchet" sounds with sur- 

 facing behavior of humpback whales. Data from the 

 present cruise, our recordings of typical short "20-Hz 

 signals", our recordings from blue whales, and from 

 work on humpback whales, apparently reveal sur- 

 face and dive times as learned through monitoring 

 underwater whale sounds. 



Possible explanations for our lack of 20-Hz short 

 pulses in the presently described recordings and for 

 the absence of other classes of sounds that Watkins 

 (1981) has commonly recorded from finbacks are 

 seasonality and insufficient sampling. We now know 

 that seasonality is involved. 



Watkins (1981) recorded the pulses in the North 

 Atlantic only from late October to early May. Cum- 

 mings and Thompson (fn. 4) recorded them in the 

 North Pacific from September to April, and Thomp- 



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Figure 8— (a) Spectrogram of short "20-Hz signals" from finback whales; the effective analyzing filter bandwidth was 0.4 Hz. (b) Strip 

 chart showing 11 trains of short "20-Hz signals" with interruptions between; the filter passband was 12.5-25 Hz. 



368 



