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PACIFIC SCIENCE, Vol XXI, October 1967 
ZONE 
YEAR 
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Fig. 7. Fluctuation in relative abundance of Euphausia pacifica swarming at the surface in the coastal areas 
of the Sea of Japan, (l) a few, (2) less than usual, (3) usual, (4) more abundant than usual, (5) much 
more abundant than usual, (6) extraordinarily abundant. 
sions have a positive slope, which means that 
successive years tended to be warmer during 
this period, and this tendency is greatest in 
March and April (Fig. 9 b and c) when the 
most extensive swarmings take place. 
Predators 
It was learned from fishermen that there is 
a close relationship between the abundance of 
euphausiids at the sea surface and fishes in the 
swarming areas of euphausiids. Various pre- 
vious workers — Smith (1879), Lebour (1924), 
MacDonald (1927), Hjord and Ruud (1929), 
Sheard (1953), Zelickman (1961), and Marr 
( 1962 ) among others — also reported the eu- 
phausiid swarms accompanied by predators 
such as fishes, whales, and birds. In order to 
take this relationship into account, an investi- 
gation was made on representative predators. 
Figure 10 shows the change in annual yield 
of Japanese mackerel, Scomber japonic pis, from 
the Wakasawan area. This was obtained from 
the statistics of the Ministry of Agriculture 
and Forestry. Spring mackerel fishing in the 
area usually is carried out during the period 
approximately corresponding to the swarming 
season of euphausiids, and examination of 
stomach contents of mackerel has shown that 
the mackerel is predatory on euphausiids. A 
conspicuous increase in the yield from 1945 to 
1946 (Fig. 10) probably was caused by resto- 
ration of the local fishing fleet from the war- 
time decline, i.e., the increase of fishing effort. 
In 1949, when an extraordinary abundance of 
