388
THE BIOLOGY OF EUPHAUSIIDS
however, swarms regularly a t the surface and exploitation of these
swarms by novel fishing methods might yield an economic return
(Hardy, 1966) ; this, however, probably depends on the ships prosecuting
some other fishery in that area at the same time so that when swarms
are scarce the ships can change to the alternative fishery until further
swarms are located.
Many species of euphausiids form pre-breeding, subsurface aggregations that might be exploited. This is especially true of the large northern
species, Meganyctiphanes norvegica, and these aggregations have been
fished in the Clyde Sea area. Using a midwater 6-ft Isaacs-Kidd trawl,
this species has been caught a t a rate of 60-100 kg wet wt per 24 hr
working, but this rate of catching is only possible during the period
December to March or April. This rate of catching is too low to be of
commercial interest because the cost of operation of the ship plus a
margin of profit would require the market price of these euphausiids
to be about ten shillings per pound at the quayside. What methods of
catching are likely to produce the largest return? Towed nets constructed of stramin, a strong openly woven fabric, tend to have their
filtering efficiency seriously impaired because the meshes become
" clogged ) ) by part of the catch so that the hauls must be timed to
prevent this and allow cleaning of the nets when on board the vessel.
This, however, decreases the amount of time the nets are in the sea and
fishing and so increases costs. Towed nets with larger meshes, about
1 em2, are eminently suitable for catching M . norvegica when a good
proportion of the population are I and 11 group animals, that is of
total length greater than 25 mm. Further, this size of mesh allows
copepods and many other planktonic organisms to escape so that the
catch consists predominantly of euphausiids. The largest catches are
obtained during the day in continental shelf areas by towing the net
close to the bottom because a t night the animals, during their vertical
migration, become dispersed between the bottom layers and the surface
and it is more difficult to find the layer or layers of greatest concentration. A fishery for Meganyctiphanes norvegica, used by fishermen as
bait, is prosecuted in the post of Monaco in the Mediterranean (Fisher
et al., 1953) mainly during the period January to February when
breeding aggregations are present; the animals swarm in the harbour
and are attracted to light lures. Wiborg (1966) is at present investigating the possibility of fishing M . norvegica and Thysanoessa inerrnis
commercially in the fjords of Norway. He is experimenting with light
luresand finds that the euphausiids appear close to the light shortly after
it has been switched on. Using a dip net, one man has been able to
collect as much as 500 kg (half a ton) in one night-an economic
THE BIOLOGY OF EUPHAUSIIDS
however, swarms regularly a t the surface and exploitation of these
swarms by novel fishing methods might yield an economic return
(Hardy, 1966) ; this, however, probably depends on the ships prosecuting
some other fishery in that area at the same time so that when swarms
are scarce the ships can change to the alternative fishery until further
swarms are located.
Many species of euphausiids form pre-breeding, subsurface aggregations that might be exploited. This is especially true of the large northern
species, Meganyctiphanes norvegica, and these aggregations have been
fished in the Clyde Sea area. Using a midwater 6-ft Isaacs-Kidd trawl,
this species has been caught a t a rate of 60-100 kg wet wt per 24 hr
working, but this rate of catching is only possible during the period
December to March or April. This rate of catching is too low to be of
commercial interest because the cost of operation of the ship plus a
margin of profit would require the market price of these euphausiids
to be about ten shillings per pound at the quayside. What methods of
catching are likely to produce the largest return? Towed nets constructed of stramin, a strong openly woven fabric, tend to have their
filtering efficiency seriously impaired because the meshes become
" clogged ) ) by part of the catch so that the hauls must be timed to
prevent this and allow cleaning of the nets when on board the vessel.
This, however, decreases the amount of time the nets are in the sea and
fishing and so increases costs. Towed nets with larger meshes, about
1 em2, are eminently suitable for catching M . norvegica when a good
proportion of the population are I and 11 group animals, that is of
total length greater than 25 mm. Further, this size of mesh allows
copepods and many other planktonic organisms to escape so that the
catch consists predominantly of euphausiids. The largest catches are
obtained during the day in continental shelf areas by towing the net
close to the bottom because a t night the animals, during their vertical
migration, become dispersed between the bottom layers and the surface
and it is more difficult to find the layer or layers of greatest concentration. A fishery for Meganyctiphanes norvegica, used by fishermen as
bait, is prosecuted in the post of Monaco in the Mediterranean (Fisher
et al., 1953) mainly during the period January to February when
breeding aggregations are present; the animals swarm in the harbour
and are attracted to light lures. Wiborg (1966) is at present investigating the possibility of fishing M . norvegica and Thysanoessa inerrnis
commercially in the fjords of Norway. He is experimenting with light
luresand finds that the euphausiids appear close to the light shortly after
it has been switched on. Using a dip net, one man has been able to
collect as much as 500 kg (half a ton) in one night-an economic