364
J. H. 9. BLAXTER AND F. 0. T. HOLLIDAY
fish reacting at distances usually of about 2 m and swimming on
average a t distances of about 2 m in front of all types of obstacle in
good light conditions. At speeds of 160 cmlsec (2.7 knots) or above,
they were soon overtaken by the gear. At night, shoaling and all
herding ceased, herring not responding even to strings of underwater
lights. The threshold light intensity for loss of the herding response
was between 0.5 and 0.05 photopic lux.
The importance of sound and vibrating and pressure stimuli to
clupeids in avoiding nets is difficult to assess due to the difficulties of
avoiding echoes in tanks and the difficulty of producing suitable
sound sources of low frequency to simulate the sounds made by warps
and boards. However, Schiirfe (1960) suggested that light netting was
desirable for the maiiufacture of the pelagic Engels trawl in order to
prevent too great a pressure wave in front of it. Although the main
object of this would be to reduce drag, it might also reduce the stow in
front of the net to which fish could respond.
3. Reaction to air bubbles
The work on the Maine coast of the U.S.A. on the reaction of young
herrings to “ curtains ” of air bubbles has been summarized by Smith
(1961). The shoals of fish might be led into anchored nets or kept
within the confines of an estuary or bay ; now this seems to be developing
into a commercial technique. According to Smith (personal communication), the curtain was sometimes effective at night. Due to
moonlight or phosphorescence, however, the air bubbles were usually
visible at night, and it was possible that the method was ineffective
on very dark nights. This would agree with the observations of Rlaxter
and Parrish (1959) and Blaxter el al. (i900), that captivo hwring do not
react to air bubble barriers in durkiieHs. Nikonorov ( I 058) found that
Clupeonella in aquaria would pass through air bubble curtains in
daylight.
4. Learning in net avoidance
Our lack of knowledge of this has been discussed by Mohr (1960),
who quoted Aslanova’s observation of sardine avoiding an area where
they had been frightened. Mohr (1961) reported some signs of learning
in captive herring, which were being subjected to the presonco of notn
in their aquarium. Devold (personal communication) pointed to the
gradual increase of the depth of migration of the N;orwr:yian hrmirtg u
it approached the coast in late winter. The main ccmpotionf, of thh
stock is the 1950 year class and various explanations may hc offered to
J. H. 9. BLAXTER AND F. 0. T. HOLLIDAY
fish reacting at distances usually of about 2 m and swimming on
average a t distances of about 2 m in front of all types of obstacle in
good light conditions. At speeds of 160 cmlsec (2.7 knots) or above,
they were soon overtaken by the gear. At night, shoaling and all
herding ceased, herring not responding even to strings of underwater
lights. The threshold light intensity for loss of the herding response
was between 0.5 and 0.05 photopic lux.
The importance of sound and vibrating and pressure stimuli to
clupeids in avoiding nets is difficult to assess due to the difficulties of
avoiding echoes in tanks and the difficulty of producing suitable
sound sources of low frequency to simulate the sounds made by warps
and boards. However, Schiirfe (1960) suggested that light netting was
desirable for the maiiufacture of the pelagic Engels trawl in order to
prevent too great a pressure wave in front of it. Although the main
object of this would be to reduce drag, it might also reduce the stow in
front of the net to which fish could respond.
3. Reaction to air bubbles
The work on the Maine coast of the U.S.A. on the reaction of young
herrings to “ curtains ” of air bubbles has been summarized by Smith
(1961). The shoals of fish might be led into anchored nets or kept
within the confines of an estuary or bay ; now this seems to be developing
into a commercial technique. According to Smith (personal communication), the curtain was sometimes effective at night. Due to
moonlight or phosphorescence, however, the air bubbles were usually
visible at night, and it was possible that the method was ineffective
on very dark nights. This would agree with the observations of Rlaxter
and Parrish (1959) and Blaxter el al. (i900), that captivo hwring do not
react to air bubble barriers in durkiieHs. Nikonorov ( I 058) found that
Clupeonella in aquaria would pass through air bubble curtains in
daylight.
4. Learning in net avoidance
Our lack of knowledge of this has been discussed by Mohr (1960),
who quoted Aslanova’s observation of sardine avoiding an area where
they had been frightened. Mohr (1961) reported some signs of learning
in captive herring, which were being subjected to the presonco of notn
in their aquarium. Devold (personal communication) pointed to the
gradual increase of the depth of migration of the N;orwr:yian hrmirtg u
it approached the coast in late winter. The main ccmpotionf, of thh
stock is the 1950 year class and various explanations may hc offered to