28 
PATTERNS OF OCCURRENCE 
The results shown in figures 10 and 11 indicate in 
how many days of the 8-day total the individual sounds 
were present during a particular hour of the day, but do 
not indicate the frequency of occurrence within the sample, 
the number of sources involved, or any measure of signal 
level. Because of the great fluctuation in the means from 
one hour to the next the curves were smoothed, using the 
method of running averages. ‘+ Three of the sounds tabu- 
lated were heard exclusively from the shallow hydrophone: 
the Click Chorus, Motorboating, and Growl. As suggested 
by figure 10, the cyclic Click Chorus was always present 
during night hours, while in only about half of the days was 
it present at all around midday. This type of graph does 
not do justice to the diurnal nature of this sound, since the 
number of sources and density of the sound are so much 
greater during the night hours. As can be seen in figure 10, 
the Rhythmic Grunt diurnal characteristic was different at 
the two hydrophones. The sound's frequency of occurrence 
increased gradually from early afternoon until early eve- 
ning at the shallow hydrophone and until late evening at the 
deep hydrophone. It seldom was noted after 0100 at the 
shallow hydrophone and after 0400 at the deep hydrophone. 
A sound that was always present was Barking. 
Barking was heard in all samples from both hydrophones. 
On the other hand, the least common of the sounds tabulated 
was the Growl sound. It was present in only 23 of the 
samples and all of these were from the shallow hydrophone. 
These data are shown in figure 11. The Growl sound 
seemed to be one of a large variety of pinniped-type sounds 
and was of interest because of its possible functional im- 
portance, for instance, to a pinniped pulse train Wsorcncem 
The fact that this sound was almost never heard during the 
night hours may be due to the higher ambient background 
levels at night, together with the relatively low signal level 
of the Growl. Weak pulse trains of similar pulse repetition 
rate and frequency range, but without the characteristic 
