On the Surface Swarming of Euphausiid Crustaceans 1 
Yuzo Komaki 2 
ABSTRACT: A general aspect of the daytime surface swarming of Euphausia 
pact pea in Japanese nearshore waters is described in connection with the water 
temperature. Swarming usually starts with a local minimum temperature around 
7°C and terminates with a temperature just below 16°C. The swarming season is 
essentially in spring, from February through May, with little difference among 
regions. The main swarming areas are on the Pacific coast around Kinkazan, and 
on the coast of the Sea of Japan around Sadogashima, in Wakasawan and its 
vicinity, around Oki, and on the east side of the Tsushima Gunto. 
It is shown that the swarming is closely related to cold water masses, and that 
the approach of offshore cold water masses to the nearshore areas and the mixing 
process in the coastal areas may provide favorable conditions for swarming. 
Swarming of E. pacipca is a phenomenon that occurs at the margins of the cold 
water bodies, and is related to the seasonal change in the geographical distribution 
of those euphausiids. 
A uniformly low water temperature profile must be the necessary condition for 
swarming, but other possible factors stimulating euphausiids to swarm are 
enumerated. 
It has been reported frequently from vari- 
ous parts of the world that conspicuous day- 
time aggregation of euphausiids takes place at 
the surface in rather nearshore waters (Table 
1). The animals swarm in such large numbers 
that the sea surface turns red or brownish-red 
from their red and/or orange pigments. This 
phenomenon, as Bigelow (1926) pointed out, 
differs from their usual vertical migratory be- 
havior because it occurs in the daytime, inde- 
pendent of the light intensity. 
Most previous reports on this peculiar be- 
havior of euphausiids have been descriptive, 
and very few observations have been made on 
its relationship to environmental factors. This 
may be attributed not only to the complexity 
of animal behavior in general, but also to the 
capriciousness of such phenomena. As is the 
1 Contribution No. 400 from the Department of 
Oceanography, University of Washington, Seattle. 
The manuscript was prepared with support from 
National Science Foundation Grant GB-3360. Manu- 
script received July 23, 1966. 
2 Present Address: Department of Fisheries, Fac- 
ulty of Agriculture, University of Tokyo, Tokyo, 
Japan. 
case with red tides, the surface swarming of 
euphausiids is not predictable from physical 
measurements. 
Such a swarming phenomenon with Euphausia 
pacipca occurs in the nearshore waters sur- 
rounding Japan. A fishery based on E. pacipca 
is maintained in certain areas — around Kin- 
kazan (Komaki, 1957), in Wakasawan Bay 
and vicinity, and along the coast of northwest- 
ern Kyushu. The local fishermen scoop E. 
pacipca swarming at the surface with a pyra- 
mid-shaped net. The surface swarming of 
euphausiids is regarded as sporadic, but the 
existence of the euphausiid fishery means that 
the surface swarming of E. pacipca must be at 
least an ordinary annual phenomenon. 
While participating in the Survey of the 
Warm Tsushima Current and Related Waters 
during the period from 1953 to 1958, the 
author carried out ecological studies on eu- 
phausiids and reported briefly on the relation- 
ship between water temperature and the swarm- 
ing of E. pacipca (Komaki and Matsue, 1958). 
The present paper emphasizes this relationship 
more extensively, employing additional data, 
information, and references. 
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