THE BEHAWOUR AND ~HYSIOLOQY OF HERRINO AND OTHER CLUPEIDS 331
small groups disappeaqd at night, the larger ones were kept together
by a chemical sense reacting to a substance produoed by the herring
themselves, only large groups producing enough to keep the individuals
together. This might explain why clupeids at sea could keep together
in the dark, whereas they could not in aquarium tanks, where the
smell would be pervasive. It w m s , however, basically unlikely that the
fish could polarize by smell, and unorientated aggregations at night
would be perhaps more likely. The importance of the lateral line in
shoaling is completely unknown, but it might play some part in keeping
fish together in darkness.
Galkov (1958) reported that small dense schools of herring, sometimes aa deep as 600 m, have been seen to migrate actively across the
Norwegian current by day and night. Craig and Priestley (1961) also
photographed herring by flash on a spawning ground off the west coast
of Scotland and found that the fish were orientated sometimes by day
and night, the light on the bottom being below threshold. These fish
were near the bottom and could all have been reacting to an external
stimulus, such as tide, in the same way and not to each other. Indeed
Valdez and Cushing (1969) reported that layer t r w s in the southern
North Sea, which were probably herring, formed up in general at the
first slaok water after dusk and remained aa a layer until the first slack
water after dawn. It seemed aa if the herring shoals could not break up
until slack water and once dispersed in the tidal stream, could not reform into shoals until the next slack water with good light conditions.
No evidence is available that sound might be important for clupeids
keeping together, but recently Freytag (1961) has reported that herring
may make sounds, and Moulton (1 960) hm suggested that Amhaviella
shoals may keep together by mund.
At the present time it may be said that viRion appoam tAJ h: of grt:at
importance to the maintenance of the shoal but the exhnt to which
clupeid shoals may keep together, or remain polarized, as a result of
intervtion between the individuals due to other stimuli must remain
an open question.
The more important question for the fisheries biologist is the control
of shoal size, for while herring may be present in fairly high density, it
is the aim of shoal which may decide whether certain fisheries are to be
successful or not. Some factors such as spawning, predation and
feeding on slow-moving organisms, as well as gradients in hydrographical conditions or certain bottom configurations may keep H ~ O R I R together, or even catalym the formation (Jf I q p r whoaln by thr: fnniotl of
smaller ones. Other factors such &H fcedirig on ftuit-movirjg orgariirrrrrrr,
darknew and uniform hydrographic conditiona may oithor tc!trd hsrtrly
small groups disappeaqd at night, the larger ones were kept together
by a chemical sense reacting to a substance produoed by the herring
themselves, only large groups producing enough to keep the individuals
together. This might explain why clupeids at sea could keep together
in the dark, whereas they could not in aquarium tanks, where the
smell would be pervasive. It w m s , however, basically unlikely that the
fish could polarize by smell, and unorientated aggregations at night
would be perhaps more likely. The importance of the lateral line in
shoaling is completely unknown, but it might play some part in keeping
fish together in darkness.
Galkov (1958) reported that small dense schools of herring, sometimes aa deep as 600 m, have been seen to migrate actively across the
Norwegian current by day and night. Craig and Priestley (1961) also
photographed herring by flash on a spawning ground off the west coast
of Scotland and found that the fish were orientated sometimes by day
and night, the light on the bottom being below threshold. These fish
were near the bottom and could all have been reacting to an external
stimulus, such as tide, in the same way and not to each other. Indeed
Valdez and Cushing (1969) reported that layer t r w s in the southern
North Sea, which were probably herring, formed up in general at the
first slaok water after dusk and remained aa a layer until the first slack
water after dawn. It seemed aa if the herring shoals could not break up
until slack water and once dispersed in the tidal stream, could not reform into shoals until the next slack water with good light conditions.
No evidence is available that sound might be important for clupeids
keeping together, but recently Freytag (1961) has reported that herring
may make sounds, and Moulton (1 960) hm suggested that Amhaviella
shoals may keep together by mund.
At the present time it may be said that viRion appoam tAJ h: of grt:at
importance to the maintenance of the shoal but the exhnt to which
clupeid shoals may keep together, or remain polarized, as a result of
intervtion between the individuals due to other stimuli must remain
an open question.
The more important question for the fisheries biologist is the control
of shoal size, for while herring may be present in fairly high density, it
is the aim of shoal which may decide whether certain fisheries are to be
successful or not. Some factors such as spawning, predation and
feeding on slow-moving organisms, as well as gradients in hydrographical conditions or certain bottom configurations may keep H ~ O R I R together, or even catalym the formation (Jf I q p r whoaln by thr: fnniotl of
smaller ones. Other factors such &H fcedirig on ftuit-movirjg orgariirrrrrrr,
darknew and uniform hydrographic conditiona may oithor tc!trd hsrtrly