Surface Swarming of Euphausiids — Komaki 
E. pacifica is seldom found in the stomachs of 
various fishes caught on the continental shelves 
around Japan. Therefore, the approach of the 
offshore populations of E. pacifca into near- 
shore waters must be the first step of swarm- 
ing in the coastal area around Japan. It may 
be considered as a seasonal expansion of their 
distribution to the coastal areas, inasmuch as 
Beklemishev and Semina (1956) and Semina 
(1958) demonstrated that the seasonal change 
of zoo- and phytogeographical boundaries ac- 
companied the seasonal shift of the conver- 
gence between Kuroshio and Oyashio. 
The size of the approaching offshore popu- 
lations may be strongly influenced by tempera- 
ture conditions from year to year. Uda (1964) 
illustrated and discussed the meanderings of 
the Kuroshio. Masuzawa (I960) discussed the 
annual variation of the Kuroshio axis, and 
showed that the north-south swing of the axis 
around the point of 36°N, 144°E was much 
greater than that of the other portions, and 
also that there is a possibility that the isolation 
of the water bodies may take place at the top 
of the conspicuous current axis curvature. The 
point mentioned above is located in the south- 
east off Kinkazan, and therefore it is quite 
possible that an isolated warm water mass from 
the top of the northward curvature of the 
Kuroshio approaches the coastal region around 
Kinkazan in some cases. The extraordinarily 
high temperature observed in the winter and 
spring months of 1954 possibly was caused by 
such an approach of the isolated water mass, 
although Masuzawa’s data of 1954 through 
1959 showed that the northernmost meander- 
ing of the current axis occurred in 1955 and 
not in 1954. 
Miyata and Shimomura (1959) and Miyata 
(I960) classified several cold water masses in 
the offshore areas of the Sea of Japan and dis- 
cussed their location and transference. Uda 
(1952, 1958) discussed the variation of the 
conditions in this sea, describing the change 
of the position of the polar front from year 
to year. It cannot be doubted that the offshore 
cold water masses come close to the Honshu 
coasts in winter and spring when the monsoon 
blowing from Siberia prevails. 
Actually, the cold water masses defined by 
the above workers are located off the zones 
445 
where the swarming of euphausiids takes place 
most actively, e.g., around Sadogashima (zone 
8), in the Wakasawan area (zones 15 and 16), 
and around Oki (zone 21) (Fig. la). In 
winter and spring these cold water masses may 
protrude against the coastal areas in the man- 
ner of a tongue. The warm water of the 
Tsushima current is very shallow because of 
the shallowness (about 100 m) of the 
Tsushima Kaikyo. Accordingly, it is likely that 
the originally warm Tsushima current water 
will be mixed with cold water masses through 
relatively simple processes. 
The stronger the approach of offshore cold 
water masses to the coast and the stronger the 
mixing in the coastal areas, the more profuse 
will be the s warmings of euphasiids along the 
coasts on the Sea of Japan. As a matter of fact, 
Uda (1958) showed that the polar front came 
very close to the Honshu coast during the 
period from 1946 to 1949, while it was away 
from the coast after 1952. It appears that such 
approach and recession of the polar front corre- 
sponds in time to the change in swarming 
(Fig. 7). 
Thus, low temperature induces the coastward 
approach of the offshore stocks of E. pacifca, 
and it must be one of the indispensable con- 
ditions for surface swarming in Japanese near- 
shore areas. However, it would be premature 
to conclude that it is the only sufficient con- 
dition. One is still unable to explain why 
euphausiids do not swarm in early and middle 
winter months when the temperature is as low 
as it is in spring and vertical mixing is actively 
taking place, and why they come to the surface 
in the daytime when the light intensity may 
be harmful to them. 
There have been several different opinions 
as to the cause of the daytime surface-swarming 
of euphausiids: (a) Predators may drive 
euphausiids to the surface, as previously men- 
tioned. (b) Euphausiids come to the surface 
to search for food in the upper strata where 
phytoplankton is abundant (Paulsen, 1909; 
Manteufel, 1938, 1941). (c) Current condi- 
tions may accumulate euphausiids, or stimulate 
them to swarm at the surface (Fish and John- 
son, 1937; Einarsson, 1945; Peters, 1955). 
(d) Some internal demands related to matu- 
ration or reproduction may drive euphausiids 
