352
J. H . S. BLAXTER AND F. 0. T. HOLLIDAY
Mohr (1960, 1961) developed the idea of a relationship between
quantity of fish present (and therefore shoal size) and net avoidance,
small shoals avoiding nets better than large shoals, where the “ shoaling
pressure ” was greater and the fish less able to perceive danger in their
surroundings. The concept of the “ swim ”, a sudden panic reaction of
herring, usually associated with slack tide in shallow depths of water,
was discussed by Graham (1931) and Balls (1951). At the time of the
swim the fish show a panic reaction due, perhaps, to the state of tide,
crowding and sexual or migrating impulses, and they swim into the
nets.
Aquarium observations on captive herring were made by Verheijen
(1953), who mentioned the importance of sight for perceiving obstacles.
Blaxter and Parrish (1059) also found that sight seemed to be the
dominant sense in avoiding nets placed across a tank. The more
contrast the net had (by reason of mesh size, twine thickness, or colour)
in relation to the background, the more it was avoided. The herring
failed to perceive monofilament nylon nets or shects of transparent
plastic, suggesting an inability to react to any form of reflected sound
or pressure waves from obstacles. Fish “ blinded ” by means of opaque
plastic over the eyes, failed to respond to the nets. The reaction
distance of normal fish was of the order of 1 m in a small tank in high
light intensities. Nets which were avoided in daylight were swum
through at night, and it was possible to measure a threshold light
intensity when the herring started to pass through the nets. This was
between 0.001 and 0.0001 photopic lux in small tanks (4 m x 2 m) and
0.01 to 0.001 lux in much larger tanks (16 m x 13 m). Mohr (1961) did
similar experiment8 with nets, but did not meaRim? n i ~ ( : h thrediold
values. He found that thc monofilsrni:rrt riylott n d , ~ wf:r<: r i r A p w w i v d
when the twine was 1e.m than 0.2 m m in diameter. ‘l’h H i m ( : author
(1962) showed that transparent l’crlon netting of 0.3 mm arid mcnh
size 4 cm was avoided by 50% of individual fish, the reaction distance
being 1-10 cm. Small shoals of 40-100 fish almost never swam into the
net while larger shoals frequently did, showing the effect of shoal
pressure ”.
i t
2. Reaction to moving nets
This has been studied at sea by echo-soundcr and tJy indirrxd,
methods such as analysis of catch from nets towed at, tlifferent H J W A ~ .
Scharfe (1955) followed the reaction of herring Hhonln to trcrwln hy
echo-sounder in a small boat and observed no general waring cffwt.
Springer (1957) followed the reactions of Sardinella anchovia to a trawl
by underwater tclevision. The fish showed no strong fright reaction but
J. H . S. BLAXTER AND F. 0. T. HOLLIDAY
Mohr (1960, 1961) developed the idea of a relationship between
quantity of fish present (and therefore shoal size) and net avoidance,
small shoals avoiding nets better than large shoals, where the “ shoaling
pressure ” was greater and the fish less able to perceive danger in their
surroundings. The concept of the “ swim ”, a sudden panic reaction of
herring, usually associated with slack tide in shallow depths of water,
was discussed by Graham (1931) and Balls (1951). At the time of the
swim the fish show a panic reaction due, perhaps, to the state of tide,
crowding and sexual or migrating impulses, and they swim into the
nets.
Aquarium observations on captive herring were made by Verheijen
(1953), who mentioned the importance of sight for perceiving obstacles.
Blaxter and Parrish (1059) also found that sight seemed to be the
dominant sense in avoiding nets placed across a tank. The more
contrast the net had (by reason of mesh size, twine thickness, or colour)
in relation to the background, the more it was avoided. The herring
failed to perceive monofilament nylon nets or shects of transparent
plastic, suggesting an inability to react to any form of reflected sound
or pressure waves from obstacles. Fish “ blinded ” by means of opaque
plastic over the eyes, failed to respond to the nets. The reaction
distance of normal fish was of the order of 1 m in a small tank in high
light intensities. Nets which were avoided in daylight were swum
through at night, and it was possible to measure a threshold light
intensity when the herring started to pass through the nets. This was
between 0.001 and 0.0001 photopic lux in small tanks (4 m x 2 m) and
0.01 to 0.001 lux in much larger tanks (16 m x 13 m). Mohr (1961) did
similar experiment8 with nets, but did not meaRim? n i ~ ( : h thrediold
values. He found that thc monofilsrni:rrt riylott n d , ~ wf:r<: r i r A p w w i v d
when the twine was 1e.m than 0.2 m m in diameter. ‘l’h H i m ( : author
(1962) showed that transparent l’crlon netting of 0.3 mm arid mcnh
size 4 cm was avoided by 50% of individual fish, the reaction distance
being 1-10 cm. Small shoals of 40-100 fish almost never swam into the
net while larger shoals frequently did, showing the effect of shoal
pressure ”.
i t
2. Reaction to moving nets
This has been studied at sea by echo-soundcr and tJy indirrxd,
methods such as analysis of catch from nets towed at, tlifferent H J W A ~ .
Scharfe (1955) followed the reaction of herring Hhonln to trcrwln hy
echo-sounder in a small boat and observed no general waring cffwt.
Springer (1957) followed the reactions of Sardinella anchovia to a trawl
by underwater tclevision. The fish showed no strong fright reaction but