Ecological Significance of a Drifting Object to Pelagic Fishes 
Reginald M. Gooding and John J. Magnuson 1 
Pelagic fishes frequently gather around drift- 
ing material in the open sea. Commercial and 
sport fishermen regard the immediate vicinity 
of drifting material as a potentially good area 
for trolling. Commercial seine and pole-and- 
line fishermen in Japan, Indonesia, and Malta 
anchor floating material to attract fish. Fish have 
been reported gathered around floating algae, 
coconuts, and pumice (Besednov, I960; Senta, 
1965); floating logs (Inoue, Amano, and Iwa- 
saki, 1963; Kimura, 1954; Yabe and Mori, 
1950) ; coconut fronds and slabs of cork (Har- 
denberg, 1949; Soemarto, I960; Galea, 1961); 
and rafts (Kojima, I960; Heyerdahl, 1950; 
Evans, 1955). In addition to clustering near 
these inanimate objects, the young of many 
pelagic fishes gather beneath jellyfish (Man- 
sueti, 1963); fish- jellyfish associations have 
much in common with the associations studied 
in the present paper. 
Hypotheses suggested to explain the accumu- 
lation of fish around inanimate floating objects 
include: (1) fish seek shelter from predators 
(Soemarto, I960; Suyehiro, 1952); (2) larger 
fish prey on the concentration of smaller fish 
(Kojima, 1956) ; (3) fish feed on algae or 
decaying coconut fronds (Reuter, 1938; Soe- 
marto, I960); (4) fish seek the shade under 
the object (Suyehiro, 1952); (5) fish use float- 
ing objects as a substrate on which to lay their 
eggs (Besednov, I960); (6) the shadow of the 
object makes zooplankton more visible to the 
fish (Damant, 1921). At the beginning of the 
present study we suggested still another hypoth- 
esis: floating objects are cleaning stations, where 
pelagic fishes have their parasites removed by 
other fish. Such symbiotic cleaning associations 
are well documented for fishes in inshore waters 
(Eibl-Eibesfeldt, 1955; Limbaugh, 1955, 1961; 
Randall, 1958). 
To test these hypotheses, studies were made 
1 Bureau of Commercial Fisheries Biological Lab- 
oratory, Honolulu, Hawaii. Manuscript received 
August 19, 1966. 
from a raft with an observation chamber (Fig. 
1) built at the Bureau of Commercial Fisheries 
Biological Laboratory, Honolulu, and set adrift 
in the central Pacific (Gooding, 1965). The 
present paper describes and interprets the ob- 
servations in light of the above hypotheses. 
areas and methods of observation 
Observations were made in two areas, one 
off the leeward coast of the island of Hawaii 
and the other near the Equator in the central 
Pacific (Fig. 2). 
Observations were made in Hawaii between 
September 28 and October 11, 1962, and be- 
tween August 1 and August 26, 1965. This 
area offers two advantages: first, it is sheltered 
from the northeast trade winds and the sea is 
relatively calm; second, essentially pelagic con- 
ditions (water deeper than 800 m) occur within 
1 mile of shore. During 345 hours of drift, 173 
hours of daylight observations and 9 hours of 
night observations were recorded. Eleven drifts 
were made, the longest of which was '52 hours. 
Two drifts were made between February 14 
and March 20, 1964 in the storm-free belt at 
the convergence of the northeast and south- 
east trade winds near the Equator. On the first 
drift the raft was launched 9 nautical miles 
north of the Equator in an area of upwelling. 
During 194 hours of drift, 91 hours of daylight 
observations were made. The second equatorial 
drift began 153 nautical miles south of the 
Equator. During 215 hours of drift, 100 hours 
of daylight observations were made. 
The raft drifted 585 nautical miles west dur- 
ing the first equatorial drift and 395 nautical 
miles west during the second. Most of the drift 
was due to surface currents. To reduce wind- 
induced drift a 28-foot parachute was used as 
a sea anchor during part of the first drift and 
all of the second. (It was also used during 
several of the Hawaiian drifts.) 
While the raft was adrift, wave heights 
ranged from 0 to 1 m at Hawaii and from 1 to 
486 
