FISHERY BULLETIN: VOL. 69, NO. 3 



ed off Dabob Bay, Wash.; a matching sequence 

 of two simultaneous pure tones of 500 and 2000 

 Hz that resembled the major frequency com- 

 ponents in most of the recorded killer whale 

 "screams" and with the same on-off times as 

 the "screams"; and a similarly timed sequence 

 of random noise in the band fiom 500 to 2000 

 Hz. Random noise and pure tone were used 

 as control stimuli in addition to a third control 

 wherein there was no playback or any intended 

 stimulus, and the whales were allowed to pass 

 relatively undisturbed. The ship's equipment 

 was silenced throughout the experiments and 

 we made no unnecessary noises. 



The sound projector (designed and built at 

 our laboratory) was lowered to 12 m below the 

 surface of the water. It was powered with a 

 250-w amplifier (Optimation)' connected to the 

 tape recorder (Uher 4200) used for playback. 

 We monitored the output from a receiving hydro- 



phone ( Wilcoxon, Type M-H90-A) located at the 

 same depth as the projector, 12 m away. Cal- 

 deron and Wenz (1967) have described this cal- 

 ibrated monitoring system. Underwater signals 

 picked up by the receiving hydrophone led to 

 one track of another Uher stereo tape recorder. 

 The other track carried our running commentary 

 of the whales' behavior. Peak source level of 

 the killer whale signals (Figure 1) and the play- 

 back control stimuli was nearly constant- — 151 db 

 re 1 /xNewton m' ( = 51 db i-e 1 dyne'cm') at 1 m 

 in the 1969 experiments, and 176 db in 1970. 

 Because of the natural propagation losses mea- 

 sured in the area, we expected sound pressure 

 levels of the projected sounds to reach the pre- 

 vailing ambient sea noise level in the third-oc- 

 tave band at 500 Hz, at about 1100 to 1400 m. 



' The use of trade names is merely to facilitate de- 

 scription; no endorsement is implied. 



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T — I — I — I — i — I — I — I — I — i — I — I — I — I — i — I — I — r— 1 — r 



1 2 



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Time [seconds ] 



Time [seconds] 



FiGl'RE 1. — Spectrograms of killer whale "screams" that were played back to migrating gray whales. Of 141 

 signals on the playback, 83 were very similar to the one at top, left. The othcr.s shown are in clockwise order 

 according to their frequency of occurrence on the tape (30, 19, 9). The analyzing filter bandwidth was 20 Hz. 



526 



