Feeding Behavior of Sharks— -HOBSON 
179 
TABLE 1 
Response of Sharks to Fish Which Are Struggling on a Line (Experiment II) 
TRIAL 
BAIT 
TIDAL CURRENT 
NO. AND LENGTH OF 
SHARKS INVOLVED 
TIME TO 
APPEAR 
DIRECTION 
FROM 
1 
mullet 
flood, moderate 
1 4-ft grey 
17 min 
downstream 
2 
mullet 
flood, strong 
1 4-ft grey 
10 min 
downstream 
3 
grouper 
flood, moderate 
2 6- ft greys 
15 min 
downstream 
4 
snapper 
flood, moderate 
2 4-ft greys 
20 min 
downstream 
5 
grouper 
flood, moderate 
1 6-ft grey 
18 min 
downstream 
6 
mullet 
flood, moderate 
1 4-ft grey 
1 6-ft grey 
5 min 
downstream 
7 
mullet 
ebb, moderate 
2 4-ft greys 
10 min 
downstream 
8 
grouper 
flood, moderate 
1 4-ft grey 
1 3 -ft whitetip 
16 min 
downstream 
9 
grouper 
ebb, moderate 
2 6-ft greys 
1 6-ft whitetip 
14 min 
downstream 
10 
grouper 
ebb, moderate 
2 4-ft greys 
25 min 
upstream 
one test which might at first glance appear to 
deviate from this pattern deserves quotation 
from the field notebook: 
Introduction was made at 1035 . . . bait positioned 
10 ft off the bottom, being carried 20-30 yd astern by 
the current where it struggled vigorously until 1050 
when noticeably tired. By 1055 bait no longer strug- 
gled, but simply maintained position, looking quite 
natural. At 1100, two 4-ft grey sharks appeared from 
upstream, cruising slowly along the bottom toward 
the test area and giving no indication of having sensed 
bait. When still about 20 yd upstream of the boat (and 
40 ft down, on the bottom) they both became notice- 
ably alerted. At this point apparently they had become 
aware of the boat on the surface, as they veered up- 
ward and swam at an accelerated rate directly to within 
5 yd of the boat, turned aside at this distance and 
circled twice. Then, seeming to lose interest, they re- 
turned slowly to the bottom and continued at their 
leisurely pace downstream, passing within 10 yd of 
the bait without apparent notice (as stated, the bait 
was not struggling, but merely maintaining position 
in the current). The sharks continued downstream ap- 
proximately 20 yd below the bait, at which point they 
both obviously and simultaneously became alerted, 
turned around, and with increasing speed raced back 
upstream straight toward the bait, with one of them 
taking it. 
The first response in the above observation 
was apparently one of vision to the boat on the 
surface. The second response, in which the 
sharks returned upstream to the bait, was ob- 
viously one of olfaction. 
In this experiment, the sharks appeared to be 
following an olfactory cue in a direct line to 
an uninjured fish. However, there was no as- 
surance that other stimuli were not also in- 
volved in the detection and approach to the 
bait. The question arises whether these sharks 
can follow an olfactory cue directly to its source 
in the absence of other cues. 
The classical experiments of Parker (1914), 
in which he observed the approach of dogfish 
to bait when both nostrils were free and when 
one was occluded, has been offered as proof that 
directional response to olfactory cues is possible 
by virtue of the ability of each nostril to detect 
minute differences in the concentration of odor- 
ous material. In standing water in the ponds at 
Coconut Island we have observed the hammer- 
head ( Sphyrna lew ini) describing the typical 
figure 8 pattern, described by Parker, in which 
the shark is apparently locating the source of 
stimulation by continually turning toward the 
nostril exposed to the greater concentration of 
the material. Tester ( 1963 ) also describes how 
blinded blacktips spiral down from above in 
converging on bait on the floor of the tanks at 
the Eniwetok Marine Biological Laboratory. It 
seems likely that sharks are capable of following 
an olfactory trail in running water, particularly 
when the current is strong and the trail narrow, 
thus forming what would essentially be an ol- 
factory corridor. Under such conditions they 
could be expected to make a direct-line ap- 
proach by taking advantage of the normal lat- 
eral movements of the head, which are part of 
the swimming motion, in keeping themselves 
oriented in the stimulus trail. 
