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PACIFIC SCIENCE, Vol XVII, April 1963 
prey. Nevertheless, it seems unlikely that this 
source of food alone is sufficient to support such 
a large shark population. It seems probable, 
then, that they act not only as opportunistic 
scavengers, but also as predators on healthy 
free-moving animals. Eibl-Eibesfeldt and Hass 
(1959) report observing both C. menisorrah 
and C. melanopterus in the Indian Ocean herd- 
ing schools of mullet against the shoreline and 
actively feeding on these fishes. Similarly, Stras- 
burg (1958) observed pelagic whitetips (Pterol- 
amiops longimanus ) herding squid under a 
night light. Strasburg also cites other evidence 
of pelagic sharks apparently capturing what 
would appear to be highly motile elusive prey. 
We observed no such activity among the three 
species in the lagoon at Eniwetok. These sharks 
seemed oblivious to the presence of the numer- 
ous reef fishes which were continually present 
during the shark’s patrolling activities. This be- 
havior might be expected, however, as only un- 
der such conditions would the reef fishes allow 
sharks to move in amongst them without ex- 
hibiting immediate alarm and taking shelter. 
This same apparent oblivion to what appears to 
be potential prey is also standard behavior seen 
in many other reef predators, for example the 
groupers, snappers, and moray eels. It seems 
likely that this behavior on the part of the 
predators is advantageous in allowing them to 
catch their prey unawares with a frequency 
which, while sufficient to maintain life, does 
not destroy the illusion of their non-aggressive- 
ness. It is also probable that the threshold for 
the release of feeding on healthy prey fluctuates 
with the relative availability of more readily 
obtainable food items, such as dead or disabled 
fish. 
The present study is confined to feeding be- 
havior with respect to dead, damaged, and dis- 
tressed prey. An effort is made to determine 
which of several sensory modalities are involved 
and which are dominant in the sequence of 
events between initial stimulation and the act 
of consuming the prey. 
RESPONSE TO OLFACTORY STIMULI 
A number of experiments were conducted 
which elucidated the role of olfaction in detect- 
ing dead and living prey. Only three (II, III, 
and IV) will be reported in detail. 
Experiment II 
This experiment was designed to study the 
response of these sharks to an uninjured fish 
struggling on a line, which might thus produce 
visual, mechanical, auditory, and perhaps olfac- 
tory cues. 
The glass-bottom boat was anchored in 40 ft 
of water on the steep slope of Deep Channel 
where the current ran in one of two directions, 
depending on the tide. Ten trials were con- 
ducted, each at a time of strong flood or ebb 
current when visibility was good. As a precau- 
tion against the sharks becoming conditioned 
to feeding at this location, the trials were spaced 
over a period of several weeks, with only one 
trial on any one day. Each trial involved one 
fish, either a grouper ( Serranidae ) , snapper 
(Lutjanidae) , or mullet (Mugillidae) , 2 to 3 
lb in weight, secured to a line by a piece of 
soft, light cord which passed through the mem- 
brane behind the maxillary. The fish had been 
caught by barbless hook and kept in tanks at 
the laboratory until needed; they appeared to 
be healthy and uninjured. 
Before the fish was lowered to a point 5-10 
ft above the bottom, a 5 -min observation period 
was conducted to insure that no sharks were in 
the area. If sharks were seen the test was de- 
layed until at least 5 min after they had disap- 
peared. 
Observations included species of shark, the 
time each took to locate and take the bait, its 
general behavior, and particularly the nature 
and direction of its approach. It was presumed 
that if the sharks approached directly and con- 
sistently from downstream the attracting stim- 
ulus had been initially olfactory, inasmuch as 
only the olfactory stimulus was affected by the 
current. If the approach was from random di- 
rections, then other sensory cues, such as visual, 
were likely to be involved in the initial attrac- 
tion. 
The results are summarized in Table 1. In 9 
of the 10 trials the sharks appeared from down- 
stream, swimming rapidly and directly toward 
the bait, thus indicating they were responding 
to olfactory cues carried by the current. The 
