Drifting Object and Pelagic Fish — Gooding and Magnuson 
495 
Fig. 6. Adult dolphin chafing against a 55-gallon 
drum beneath the raft. 
another juvenile dolphin, Coryphaena sp. Dur- 
ing the display the fish with the ectoparasite 
stopped caudal movements and treaded water 
with its pectorals. It did not lean to one side 
as did the adult dolphin mentioned above. On 
numerous occasions, the juvenile dolphin, 
Coryphaena sp., to which the display was di- 
rected made passes at the caudal fin of the 
parasitized fish. At the end of the day, how- 
ever, the copepod was still attached. 
Several species chafed their sides on the raft, 
skiff, or lines hanging in the water. Adult 
dolphin commonly chafed against the bottom 
of the raft and skiff (Fig. 6). Sanchez Roig 
and Gomez de la Maza (1952) and Heyerdahl 
(1950) have reported similar behavior. Some- 
times dolphin chafe against other fish (Breder, 
1949). In one of our film sequences, a small 
abrasion can be seen on the side that the fish 
was rubbing against the skiff. Other species at 
the raft which were seen chafing were rough 
triggerfish on the bottom of the raft; juvenile 
dolphin on ropes and on the caudal and dorsal 
fins of whitetip shark; whale shark, whitetip 
shark, and scrawled fiiefish on the rope to the 
parachute drogue; and a spiny puffer, on a 
small floating can. This behavior, especially 
common in the coryphaenids, could remove 
parasites or relieve skin irritation. 
Some predation on ectoparasites occurred at 
the raft, but the question remains whether the 
removal of parasites is concentrated near the 
raft and other floating objects. It is obvious 
that removal of parasites by chafing on hard 
objects would be concentrated near floating 
material or larger fishes. In addition, the op- 
portunity to feed on ectoparasites or to solicit 
parasite cleaning would appear to be greater 
near the raft because the fishes usually arrived 
in small groups or alone and formed larger 
aggregations at the raft. 
Other Possible Explanations 
The hypothesis that fishes seek shade under 
floating objects has no substance. Yabe and 
Mori (1950) and Kojima (1956) also reached 
this conclusion. None of the smaller species 
tended to remain in the shade of the raft. 
Larger species such as rough triggerfish, wahoo, 
dolphin, and whitetip shark often ranged far 
from the raft and were seldom in its shadow. 
The hypothesis (Besednov, I960) that fish 
use floating material as a substance on which 
to lay their eggs could not be substantiated. 
Even though fish eggs are frequently found on 
drifting material, no fish deposited eggs on the 
raft nor were any. eggs seen on the undersurface. 
No data were obtained to test the hypothesis 
(Damant, 1921) that the shadow of an object 
makes the zooplankton more visible to fish. 
Four species fed upon zooplankton; the visi- 
bility of these zooplankters may have been 
increased by the raft’s shadow. 
CONCLUSION 
A floating object in the pelagic environment 
provides a relatively rare "superstrate” in an 
environment notable for its horizontal homo- 
geneity. This superstrate has some of the same 
ecological significance to certain pelagic fishes 
that a substrate has to inshore fishes. Obviously, 
no single biological association or adaptive ad- 
vantage can explain the occurrence of fish 
around floating objects at sea. Of the ecological 
hypotheses considered, shelter from predation 
is substantiated best and appears to be the most 
significant factor in the evolution of fish com- 
munities that gather beneath inanimate drifting 
material in the open ocean. 
ACKNOWLEDGMENTS 
We thank Randolph K. C. Chang, Robert 
T. B. Iversen, and Everet C. Jones, Bureau of 
Commercial Fisheries Biological Laboratory, 
Honolulu, who assisted in making the observa- 
tions; Frank J. Mather, Woods Hole Oceano- 
