260
MAKOTO OMOLLI
Surfwe slioaling is a wrll-known phenomenon in several euphausiid
v p e c h , thr. rwortlq of wliic*h ~ ~ L V P
bcen summarized by Komaki
( I W7e). Acc*ortling to him, t lie mixing of cold water masses with warm
wttstal wthter protlucen contlitionx cotiducive to surface swarming of
Itwphawsiu pacijicu Hailsell, usually tit, the margins of cold masses in
Japanrse c o t ~ s t ~ l
waters. It, is noteworthy that the large shoals of
Sergesks sirnilis in the surface layers usually occur in the transitional
domain just north of the subarctic boundary where the surface isotherms show complicated tneandering and that the species apparently
tends to forin dense swarms at temperatures near the upper limit of
its range (Omori ~t d., 1973). Calunus l o n s u ~ Brady (Kawamura,
unpublished) arid fluphuwiu superba 1)ana ([vanov, 1970) showed in
this connection a similsr beliaviour in the subantarctic region. These
phcnomena are presumably common features of the shoaling of planktoiiic crustaceans. Evidently, the intense mixing of waters having
different environmental temperatures in the surface layers causes
physiological and possibly physical effects stimulating the species to
swarm.
The population density of adult Sergia lucens in the swarms is
tbstimat,ed to be 20-176 inds./m3 at night. That of Sergestes sirnilis,
tlcrivcd from the feeding data of fin whales, is 100-4 500 inds./m3 at
iiight (Oinori P t el., 1972). ('onectitration of Acetes spp. in the neritic
waters or cstuttries iritiy exceed tliose values. but the densitics of the
occ*uiiir shritnp are generdly low compared with those of some of the
c~uphuiisiids. The density of lluphriwia superhn in swarms was spproxiriirttcly 6 1 000 inds./m3 (Marr, lW2) and that of E . krohn,ii (Brandt)
WLS about 31 000 inds./ms (Baker, 1970). Forsyth and Jonex (1966)
recorded a deiisity of about 600 O W indn./rn" in a swarm of Thysnnoe,Ywn
lwigicuudiitu (Kruyer) in the Shetland Islands.
The ecqlogical rolc of the anitnalx in the areas where they form
great shoals niiist be different from that in area8 where they are sparsely
distributed. The shoaling animals may play an important role in
determining the abundance of prey species and competitors, and,
hence, in determining the cornposition of the zooplankton community
within their habitat.
'l'herc. is i~ suggestion that swarming or schooling may reduce
predntioii by decreasing the frequency of contact with predators
t m l by enhimcing evasion aftzer contact (Brock and Riffenburgh,
1W0; Clutter, 1969). This consideration seems to be true, as Brock
imd Kiflinb~rgh (1960) mentioned, when the size of n swarm greatly
tweeds the cluantity th;tt a predator can consume (e.g. sergestids
vcwu8 rnyc~topliittn or wphalopoclx). However, in some areas tho
MAKOTO OMOLLI
Surfwe slioaling is a wrll-known phenomenon in several euphausiid
v p e c h , thr. rwortlq of wliic*h ~ ~ L V P
bcen summarized by Komaki
( I W7e). Acc*ortling to him, t lie mixing of cold water masses with warm
wttstal wthter protlucen contlitionx cotiducive to surface swarming of
Itwphawsiu pacijicu Hailsell, usually tit, the margins of cold masses in
Japanrse c o t ~ s t ~ l
waters. It, is noteworthy that the large shoals of
Sergesks sirnilis in the surface layers usually occur in the transitional
domain just north of the subarctic boundary where the surface isotherms show complicated tneandering and that the species apparently
tends to forin dense swarms at temperatures near the upper limit of
its range (Omori ~t d., 1973). Calunus l o n s u ~ Brady (Kawamura,
unpublished) arid fluphuwiu superba 1)ana ([vanov, 1970) showed in
this connection a similsr beliaviour in the subantarctic region. These
phcnomena are presumably common features of the shoaling of planktoiiic crustaceans. Evidently, the intense mixing of waters having
different environmental temperatures in the surface layers causes
physiological and possibly physical effects stimulating the species to
swarm.
The population density of adult Sergia lucens in the swarms is
tbstimat,ed to be 20-176 inds./m3 at night. That of Sergestes sirnilis,
tlcrivcd from the feeding data of fin whales, is 100-4 500 inds./m3 at
iiight (Oinori P t el., 1972). ('onectitration of Acetes spp. in the neritic
waters or cstuttries iritiy exceed tliose values. but the densitics of the
occ*uiiir shritnp are generdly low compared with those of some of the
c~uphuiisiids. The density of lluphriwia superhn in swarms was spproxiriirttcly 6 1 000 inds./m3 (Marr, lW2) and that of E . krohn,ii (Brandt)
WLS about 31 000 inds./ms (Baker, 1970). Forsyth and Jonex (1966)
recorded a deiisity of about 600 O W indn./rn" in a swarm of Thysnnoe,Ywn
lwigicuudiitu (Kruyer) in the Shetland Islands.
The ecqlogical rolc of the anitnalx in the areas where they form
great shoals niiist be different from that in area8 where they are sparsely
distributed. The shoaling animals may play an important role in
determining the abundance of prey species and competitors, and,
hence, in determining the cornposition of the zooplankton community
within their habitat.
'l'herc. is i~ suggestion that swarming or schooling may reduce
predntioii by decreasing the frequency of contact with predators
t m l by enhimcing evasion aftzer contact (Brock and Riffenburgh,
1W0; Clutter, 1969). This consideration seems to be true, as Brock
imd Kiflinb~rgh (1960) mentioned, when the size of n swarm greatly
tweeds the cluantity th;tt a predator can consume (e.g. sergestids
vcwu8 rnyc~topliittn or wphalopoclx). However, in some areas tho