Feeding Behavior of Sharks — Hobson 
183 
The presence of the disabled fish is mentioned 
almost incidentally, although in such a situation 
it is impossible to dismiss the olfactory and 
visual cues emitted by these fish. Similarly, 
the presence of olfactory and visual cues also 
complicate reports of sharks being attracted to 
a struggling fish in response to compression 
waves. In regard to the repelling effect of the 
shouts of divers, we did not see any such re- 
sponse in the species studied in Eniwetok. How- 
ever, sudden movements and/or a sudden burst 
of bubbles from the aqua-lung, such as might 
readily accompany a shout, often startled the 
blacktip and put it to flight. 
It is important to consider the ability of 
sharks not only to sense these stimuli but also 
to locate their sources. Parker (1912), investi- 
gating sound as a directing influence on the 
movements of some teleosts, noted that there 
was a directing effect only during the duration 
of the sound. These results cast some doubt on 
the ability of such noncontinuous stimuli as a 
single underwater explosion to attract sharks. 
Two experiments on the perception of com- 
pression waves by sharks were conducted in the 
field at Eniwetok. 
Experiment V 
The aim of this experiment was to investi- 
gate the effects of compression waves of a variety 
of sonic frequencies on these sharks. The fol- 
lowing were presented: 
A. Continuous pure tones, covering a fre- 
quency range in steps from 100-1,000 cycles 
per sec. 
B. Sounds of mixed frequencies recorded on 
a circular tape, including: 
1. wood struck against wood, 1/ sec and 6/ sec; 
2. rock struck against rock, 2/sec and 6/sec; 
3. rasping effect, continuous; 
4. rattling of nuts and bolts in a bottle, 
continuous. 
C. Shouts by the observer, directly projected 
through the apparatus. 
The experiment was conducted at the Engebi 
location, using a tape recorder equipped with an 
underwater speaker lowered 6-8 ft below the 
surface. Each sound was presented during 20- 
min trials, with each trial immediately follow- 
ing a 20 -min control period and conducted 
under two sets of conditions: (1) following a 
different experiment in which sharks had been 
drawn into the experimental area and were still 
present, and (2) upon arrival at the site, with 
no sharks present. All sounds were audible to a 
submerged human at distances greater than 
150 ft. 
Although we watched closely for signs of any 
sort of response, for example curiosity, we 
saw nothing in the behavior of the sharks which 
suggested that they were able to perceive the 
sounds. 
Experiment VI 
This experiment was designed to investigate 
the effects of various subsonic compression 
waves on these sharks. 
A metal hoop, having a diameter of 36 inches, 
was fitted with a rubber diaphragm held in posi- 
tion by a series of surgical tubing lacings. This 
piece of apparatus was installed midway through 
a natural tunnel, approximately 20 ft long, in 
the coral ridge bordering Deep Channel beneath 
the raft. A line was secured to the center of the 
diaphragm on that side facing away from the 
channel, and this line was run out of the tunnel 
and up to the surface where a small skiff rode 
at anchor. The diaphragm could thus be vibrated 
by jerking on the line. 
Two trials were conducted in which the dia- 
phragm was vibrated irregularly for a period of 
20 min immediately following another experi- 
ment in which a number of grey sharks had 
been drawn into the area and were still present. 
Throughout both trials there were grey sharks 
swimming about calmly in the area. At times 
they passed within 5 yd of the tunnel’s entrance. 
In observations from the raft we looked for 
any type of response in the sharks which might 
indicate that they were aware of the vibrating 
diaphragm. We saw no such response. 
The negative results in these two experiments 
do not mean that the sharks could not perceive 
these stimuli. If releasers of feeding activity are 
emitted by struggling fish in the form of com- 
pression waves, they probably are of a specific 
nature or involve a characteristic pattern. It 
would have been a rare stroke of luck if we had 
duplicated a specific releaser with our crude 
experiments. The problem of delimiting the 
