5 . VERTICAL DISTRIBUTION AND MIGRATION
155
these animals, and of their vertical movements, active avoidance of tho siirface
net by the oldor individuals in daylight is a factor that must always be reckoned
with. From these strikingly contrasting rcsults, obtained in almost, biit not
quite, the samo water, and almost, but not quite, simultancously, we can fairly
in fact conclude that throughout daylight in the surface zone such older and more
active animals as may be there, though not always able to avoid the stramin net
with its relatively large mouth-opening, with their rapid dodging movemcnt can
readily, it seems, spring clear of a smaller net with approximately half the catching area, avoiding it evidently outright.
It is obvious from these conclusions that there is very considerable
variation in the behaviour of this species. Marr considers surface
swarming during the day as relatively common although he states that
Sir Alister Hardy believes it to be less common.
The problem of euphausiids escaping capture by nets has received
little positive attention. Mauchline (unpublished) towed 1 m stramin
nets a t different speeds and found that Meganyctiphanes norvegica of
body length more than 30 mm tended to escape capture by nets moving
through the water a t speeds less than about 1 m per sec ; there was no
significant difference between numbers of smaller animals caught in
slow and fast hauls. Jerde (1967) compared the fishing efficiency of the
micronekton net with that of the 1 m net. The micronekton net, of
mouth opening 2.3 m2, is made of nylon netting with a uniform mesh
aperture of 5.5 by 2.5 mm fitted with a cod end having a mesh aperture
of 0.31 mm. The 1 m net, of mouth opening 0.785 m2, is made of grit
gauze with a mesh aperture of 0.65mm in the forward section and
0.31 mm in the rear section and cod end. He obtained evidence of
avoidance of the 1 m net by euphausiids in the size range 22-28 mm ;
animals smaller than 13 mm tended to escape through the mesh of the
larger net, as might be expected. Brinton (1967) found that Nyctiphanes
simplex, Thysanoessa gregaria, Xtylocheiron suhmii, and S. afine effectively avoided nets in the surface layers while Euphausia recurva, E .
mutica, Nematoscelis dificilis, Nematobrachion jlexipes, Stylocheiron
elongatum, S. abbreviatum, and X . maximum effectively avoided nets in
the deeper layers. He obtained no evidence of avoidance of nets by
Thysanopoda aequalis, Euphausia brevis, E . hemigibba, Nematoscelis
tenelb, N . atlantica, and Xtylocheiron carinatum. No other data on
avoidance of towed nets by euphausiids are available.
Surface swarming of species of euphausiids has been recorded but
they are relatively rare occurrences except in one or two geographical areas. Meganyctiphanes norvegica forms surface patches but they
are irregular in occurrence and may be present either in the open sea or
in coastal waters. Komaki (1967a) and Mauchline and Fisher (1967)
have summarized the known instances of surface swarming in this
155
these animals, and of their vertical movements, active avoidance of tho siirface
net by the oldor individuals in daylight is a factor that must always be reckoned
with. From these strikingly contrasting rcsults, obtained in almost, biit not
quite, the samo water, and almost, but not quite, simultancously, we can fairly
in fact conclude that throughout daylight in the surface zone such older and more
active animals as may be there, though not always able to avoid the stramin net
with its relatively large mouth-opening, with their rapid dodging movemcnt can
readily, it seems, spring clear of a smaller net with approximately half the catching area, avoiding it evidently outright.
It is obvious from these conclusions that there is very considerable
variation in the behaviour of this species. Marr considers surface
swarming during the day as relatively common although he states that
Sir Alister Hardy believes it to be less common.
The problem of euphausiids escaping capture by nets has received
little positive attention. Mauchline (unpublished) towed 1 m stramin
nets a t different speeds and found that Meganyctiphanes norvegica of
body length more than 30 mm tended to escape capture by nets moving
through the water a t speeds less than about 1 m per sec ; there was no
significant difference between numbers of smaller animals caught in
slow and fast hauls. Jerde (1967) compared the fishing efficiency of the
micronekton net with that of the 1 m net. The micronekton net, of
mouth opening 2.3 m2, is made of nylon netting with a uniform mesh
aperture of 5.5 by 2.5 mm fitted with a cod end having a mesh aperture
of 0.31 mm. The 1 m net, of mouth opening 0.785 m2, is made of grit
gauze with a mesh aperture of 0.65mm in the forward section and
0.31 mm in the rear section and cod end. He obtained evidence of
avoidance of the 1 m net by euphausiids in the size range 22-28 mm ;
animals smaller than 13 mm tended to escape through the mesh of the
larger net, as might be expected. Brinton (1967) found that Nyctiphanes
simplex, Thysanoessa gregaria, Xtylocheiron suhmii, and S. afine effectively avoided nets in the surface layers while Euphausia recurva, E .
mutica, Nematoscelis dificilis, Nematobrachion jlexipes, Stylocheiron
elongatum, S. abbreviatum, and X . maximum effectively avoided nets in
the deeper layers. He obtained no evidence of avoidance of nets by
Thysanopoda aequalis, Euphausia brevis, E . hemigibba, Nematoscelis
tenelb, N . atlantica, and Xtylocheiron carinatum. No other data on
avoidance of towed nets by euphausiids are available.
Surface swarming of species of euphausiids has been recorded but
they are relatively rare occurrences except in one or two geographical areas. Meganyctiphanes norvegica forms surface patches but they
are irregular in occurrence and may be present either in the open sea or
in coastal waters. Komaki (1967a) and Mauchline and Fisher (1967)
have summarized the known instances of surface swarming in this