348
THE BIOLOGY OF EUPHAUSIIDS
swarming, usually a t the margins of the cold masses. Swarms of
E. pacijica occur most frequently from February to the end of May (the
spring season) off the Pacific coast around Kinhazan, and in the Sea of
Japan around Sadogashima, in Wakasawan and its vicinity, around Oki,
and on the east side of the Tsushima Gunto. Komaki considers that
the state of maturation of the gonads may affect the animals a t this
time but there is little evidence a t present to support this view.
Recently, Forsyth and Jones (1966) have described a swarm of Thysanoessa longicaudata observed in Lerwick Harbour, Shetlands. They could
not suggest an explanation for this phenomenon because Lerwick is
about one hundred miles from the edge of the continental shelf and this
species was not expected to swarm in such a shallow area. Baker
(personal communication) noted surface swarming of Euphausia krohnii
at night off Fuertaventura Island, one of the Canary Islands, in
October 1965; he made some rough calculations and suggests that the
density of individuals in the swarm was about 30 000 per cubic metre.
Most information is available for Meganyctiphanes norvegica and
Euphausia pacijica because they frequently swarm in coastal regions.
Normally, records of surface swarming of a species simply give a time
and geographical position but Aitken (1960) examined a sample of the
swarm of Meganyctiphanes norvegica observed in early January 1960 in
Strangford Lough in Northern Ireland ; he ascertained the distribution
of body length in 300 specimens and from his histogram it is apparent
that the majority of the swarmers, in this case, were animals that had
bred during the previous spring and were now approaching 2 years of
age. A small proportion of the sample was O-group but it is probable
that this swarm consisted of a selected portion of the parent population
because O-group animals normally outnumber I-group animals in a
natural population. The sex ratio within the sample was 169 males t o
131 females, being therefore within the normal range. Baker (personal
communication) found a surface swarm of M . norvegica at 43’38”
15’37’W in August 1967.
These irregular surface swarms are a different phenomenon from
the aggregations formed prior to breeding by North Atlantic species
and possibly also from the swarms formed by Euphausia superba, the
species which is the most notable swarmer of all. Hardy and Gunther
(1935) made a series of consecutive surface hauls with NlOOH nets and
found that E. frigida and especially E. superba have ‘ I patchy distributions ”, one haul having large numbers present while the next haul
might have none a t all. Ozawa and Sat0 (1967) observed surface
patches of E. superba and Marr (1962) has examined the microdistribution of E. superba in some detail and presents descriptions of
THE BIOLOGY OF EUPHAUSIIDS
swarming, usually a t the margins of the cold masses. Swarms of
E. pacijica occur most frequently from February to the end of May (the
spring season) off the Pacific coast around Kinhazan, and in the Sea of
Japan around Sadogashima, in Wakasawan and its vicinity, around Oki,
and on the east side of the Tsushima Gunto. Komaki considers that
the state of maturation of the gonads may affect the animals a t this
time but there is little evidence a t present to support this view.
Recently, Forsyth and Jones (1966) have described a swarm of Thysanoessa longicaudata observed in Lerwick Harbour, Shetlands. They could
not suggest an explanation for this phenomenon because Lerwick is
about one hundred miles from the edge of the continental shelf and this
species was not expected to swarm in such a shallow area. Baker
(personal communication) noted surface swarming of Euphausia krohnii
at night off Fuertaventura Island, one of the Canary Islands, in
October 1965; he made some rough calculations and suggests that the
density of individuals in the swarm was about 30 000 per cubic metre.
Most information is available for Meganyctiphanes norvegica and
Euphausia pacijica because they frequently swarm in coastal regions.
Normally, records of surface swarming of a species simply give a time
and geographical position but Aitken (1960) examined a sample of the
swarm of Meganyctiphanes norvegica observed in early January 1960 in
Strangford Lough in Northern Ireland ; he ascertained the distribution
of body length in 300 specimens and from his histogram it is apparent
that the majority of the swarmers, in this case, were animals that had
bred during the previous spring and were now approaching 2 years of
age. A small proportion of the sample was O-group but it is probable
that this swarm consisted of a selected portion of the parent population
because O-group animals normally outnumber I-group animals in a
natural population. The sex ratio within the sample was 169 males t o
131 females, being therefore within the normal range. Baker (personal
communication) found a surface swarm of M . norvegica at 43’38”
15’37’W in August 1967.
These irregular surface swarms are a different phenomenon from
the aggregations formed prior to breeding by North Atlantic species
and possibly also from the swarms formed by Euphausia superba, the
species which is the most notable swarmer of all. Hardy and Gunther
(1935) made a series of consecutive surface hauls with NlOOH nets and
found that E. frigida and especially E. superba have ‘ I patchy distributions ”, one haul having large numbers present while the next haul
might have none a t all. Ozawa and Sat0 (1967) observed surface
patches of E. superba and Marr (1962) has examined the microdistribution of E. superba in some detail and presents descriptions of