436 
PACIFIC SCIENCE, Vol. XXI, October 1967 
1954-1956. Data on oceanographic observations 
off Iwate Prefecture. 
Japan Sea Regional Fisheries Research Laboratory. 
1953-1959- Monthly report on the out-lined sea 
conditions of the Sea of Japan. 
Ministry of Agriculture and Forestry, Section of 
Statistics. 1946-1956. Fisheries Statistics. 
Miyagi Prefecture, Fisheries Experimental Station. 
1910-1959- Records of hydrometeorological obser- 
vations at Enoshima, Miyagi Prefecture. 
Plankton Samples Collected in the Sea of Japan 
Thousands of plankton samples were col- 
lected during the period 1953-1958 by various 
institutions that participated in the Survey of 
the Warm Tsushima Current and Related Wa- 
ters. Most samples were collected in a strip 
within 100 miles of the coastline. The selected 
samples were examined for the purpose of eco- 
logical studies on euphausiids, and they were 
very useful in the confirmation of euphausiid 
species composing surface swarms. 
RESULTS 
The Species 
Examination of specimens landed at the 
Onagawa fish market revealed that the eu- 
phausiids swarming in the Kinkazan waters 
were large specimens of Euphausia pacifica 
(longer than 20 mm from the tip of the ros- 
trum to the end of the telson). Throughout 
four fishing seasons, only one stray specimen 
of Nematoscelis diffcilis was found among the 
catches of huge quantities of E. pacifica. Com- 
bined swarmings of more than two species, 
such as those reported by H. F. Moore (1898), 
Fish and Johnson (1937), and Zelickman 
(1961), were not encountered in the Kinkazan 
waters. 
The following five species of euphausiids 
inhabit the Sea of Japan (Ponomareva, 1955; 
Komaki and Matsue, 1958) : E. pacifca, 
Thysanoessa raschii, T. inermis, T. longipes, 
and Pseudeuphausia latifrons. 
Plankton samples collected during the sur- 
vey showed that E. pacifca and P. latifrons 
could be obtained from the areas within 100 
miles from the coast. Thysanoessa spp. never 
have been collected from that area, although 
Ponomareva (1959, 1963) has reported that 
the three Thysanoessa species mentioned ap- 
pear at the surface, forming remarkably dense 
swarms in the northernmost part of the Sea 
of Japan. P. latifrons is a small, warm-water 
form, shorter than 10 mm in total length 
(Hansen, 1916; Brinton, 1962^), and appar- 
ently it penetrates into the southernmost part 
of the Sea of Japan from the Tsushima Kaikyo 
only during summer and fall (Komaki and 
Matsue, 1958). Samples from surface swarms 
of a few occasions revealed that the swarms 
were composed of large E. pacifca only. Thus, 
E. pacifca must be the species composing the 
surface swarms in the nearshore waters of the 
Sea of Japan. 
Features of Swarming 
Hardy and Gunther (1935) have described 
beautifully the swarming behavior of E. su- 
perha at the surface, and their descriptions can 
be exactly applied to the surface patches of 
E. pacifca. The animals swim as close as 1-2 
cm from each other, orienting themselves in 
one direction as if commanded by a leader. 
The swarms look like formless clouds, and fre- 
quently change in shape. They are red, brown- 
ish-red, or pale brown in color, depending 
upon their distance from the surface. 
When participating in euphausiid fishing 
operations in the Kinkazan waters, the author 
found that swarming took place intensively at 
intervals of a few days. As shown in Figure 2, 
the daily landings of E. pacifca at the Ona- 
gawa fish market fluctuate with a pulselike 
rhythm. Inasmuch as only surface patches of 
euphausiids are harvested because of the fish- 
ing method, the landing on a given day is 
probably related to the standing crop of 
euphausiids swarming at the surface on that 
day. Daily catch per unit of fishing effort is 
the most suitable term to use in discussing 
such a fluctuation of standing crop, but data 
indicating fishing effort were not available. 
Fishing boats unload their catches at the mar- 
ket two or three times a day while euphausiids 
are abundant. Accordingly, the aggregate num- 
bers of fishing boats in Figure 2 change ap- 
proximately in parallel with the landings of 
euphausiids. 
Efforts were made to relate this pulselike 
occurrence of swarming to the environmental 
factors that vary within short periods, e.g., 
irradiation, wind direction and intensity, and 
