Drifting Object and Pelagic Fish — Gooding and Magnuson 
493 
and had not previously been recorded (for other 
colorations of this species, see Murchison and 
Magnuson, 1966). The dorsal half of the body 
turned a dark brownish-black. A sharp separa- 
tion extended longitudinally along the side 
between the dark dorsal area and the silvery 
ventral half of the body. The above behavior 
and coloration, observed only when 1 or 2 
dolphins were at the raft, were different from 
those seen on similar occasions when 13 or 
more dolphins were present. Then the group 
of dolphins swam immediately behind a bill- 
fish, a whitetip shark, bottlenose dolphin, and 
a swimmer near the raft. A position im- 
mediately behind a potential predator may be 
of advantage to the prey provided the animal 
has the speed and maneuverability to maintain 
such a position. 
Large schools of goatfish attempted to avoid 
dolphins and amber jacks by swimming to the 
other side of the raft, but only rarely did in- 
dividuals use the maximum shelter of the raft 
by swimming under it. As a consequence both 
predators were able to prey upon them success- 
fully. 
One of the most clearcut examples of preda- 
tor avoidance occurred when a golden jack was 
chased to the raft by the feeding attacks of five 
dolphins. The dolphins stopped their feeding 
passes after the jack swam under the raft. For 
several hours the jack swam within inches of 
the chamber. The observer on deck could reach 
into the water and touch the fish without driving 
it away. After several hours it began to swim 
under the flotation drums, but not away from 
the raft. About 8 hours after it arrived the jack 
joined a whitetip shark and six pilotfish which 
swam close by, and left the raft in their com- 
pany. The dolphins took on their feeding colora- 
tion, but did not attack the jack as it swam off 
with the shark. This incident provided evidence 
for the protective role that both floating objects 
and large animals such as sharks play for the 
fish that accompany them. 
Concentration of Food Supply 
It has often been said that floating material 
concentrates the food supply — smaller fish, zoo- 
plankton, or sessile biota. Most piscivores did 
net successfully prey on fish that sought shelter 
beneath the raft, but they did prey extensively 
on those that gathered at the raft but did not 
take shelter beneath it. Zooplankton was not 
concentrated at the raft, nor did large numbers 
of sessile organisms attach themselves to it. 
Kojima (1956) suggested that dolphins were 
found near floating objects because more food 
was available there, but was unable to demon- 
strate that they fed substantially on other fishes 
gathered at anchored bamboo rafts (Kojima, 
I960, 1961). Yabe and Mori (1950) argued 
that abundance of food was an inadequate ex- 
planation for the presence of yellowfin and 
skipjack tuna near floating logs because the 
fish took bait readily and did not have much 
food in their stomachs. The simultaneous pres- 
ence of piscivores and potential prey near the 
raft was well documented, yet, as mentioned 
above, only amber jack successfully preyed on 
the small fish that took shelter there. We saw 
them chase and eat freckled driftfish. The 
stomach of the only amber jack taken at the raft 
contained three driftfish. The only other species 
we saw catch smaller fish was the adult dolphin. 
Both it and the amber jack, as has been men- 
tioned, preyed on schools of goatfish that were 
near the raft, but not under it. The stomachs 
of 53 dolphins caught near the raft contained 
only 5 scrawled filefish; 1 sargassum trigger- 
fish, Xanthichthys r in gens (Linnaeus); and 1 
puffer, Diodon holocanthus Linnaeus. All were 
juveniles. Once we saw an adult dolphin seize 
and eat a freckled driftfish which was attempt- 
ing to reach the raft. This incident suggests 
that dolphins sometimes intercepted driftfish 
seeking shelter. Possible supporting evidence 
for this supposition came from observations off 
Hawaii. While the raft was anchored for several 
days, numerous freckled driftfish, 19 dolphins, 
and 1 amber jack accumulated. The raft was 
then towed by the ship 30 miles down the coast 
and set adrift. During the tow the driftfish 
were outdistanced and all were lost; only the 
dolphins and amber jack remained. Thus, un- 
like other drifts, this drift began with a number 
of fish — 19 dolphins and 1 amberjack — at the 
raft. During 52 hours of drifting no freckled 
driftfish appeared. Yet in the same area, two 
weeks earlier, approximately 500 and 200 drift- 
fish gathered at the raft on two drifts of 50 
and 32 hours, during which only 2 and 7 
dolphins had accumulated. 
