TJIE BEHAVIOUR AND PHYSIOLOGY OF HERRING AND OTHER CLUPEIDS 353
they increased their swimming speed slightly to avoid the different
parts of the gear. Schiirfe (1960), in a review paper, reported the
tendency of herring to swim downwards when frightened by a pelagic
trawl, reacting 2-4 m from the headline or ground rope. At 3 knots
(170 cmlsec) the herring appeared to swim ahead of the net, but at
3.5+ knots (200+ cmlsec) they were overtaken. Okonski and Konkol
(1957), however, reported an upward movement by herring in the
Skagerak when scared by a pelagic trawl. Clearly the direction of
movement depends on the position of the shoal in relation to the net as
well as on other possible factors. Galkov (1958) stressed that the
capture of Atlanto-Scandian herring by pelagic trawl depended very
much on their mobility, catches being best in November and December
when mobility was low. Mohr (1962) compared the echo traces obtained
from a transmitter on board ship with those from a transmitter on the
headline of a herring trawl. The reaction of the herring differed,
depending on their concentration. Small groups of fish kept about 5 m
away from the headline and ground rope ; large groups appeared not
to react at all, due to greater shoaling " pressure ". Mohr pointed out
the variability of behaviour of herring to nets with such factors aa
maturity stage, shoal sizes and time of day. Manteufel and Radakov
(1960) reported that tightly packed shoals of herring were more difficult to catch by trawl in the Baltic, suggesting that shoaling assists
the fish in reacting to the net and avoiding it. Manteufel et al. (1958)
showed that herring shoals in the Baltic wcro frightened by the boards,
warps and net of a trawl, and also by the fishing vessel itself 30 m above.
The shoals soon returned to their previous position after the net had
passed. In contrast, Itadakov ( 1 962) obnervcd tihoal~ of ClupeoneZh
duZicatu2a ;it i t 1 Bn,!yravli.Y encm.uicho1u.u H wi it1 rrr i trg, clpparci t t I y w i tt io tit
agitation, within a trawl towed at 1.5 knots.
Lucas (1936), Krefft and Schuler (1951) and Richardson (1960)
showed that trawl catches of North Sea herring were better by day,
there being size and maturity differences in the catches. While vertical
migration may play the principal part here, it is also posdlh: that
herring catches are poorer at night becaufic: th: finh arc more qmw1 orif,
(if they are not shoaling), or because they arc not hc;rdcd by t h
extremities of the net, the net having, therefore, a loww cffi:ctive
width.
In relation to this, the aquarium observations of Blaxter at ul. (1960)
showed the importance of sight to hcrring in avoiding moving obstacles.
'L'hcy t m t d thr hrrding ability of ground ropes, floats, panels of
Ilctting, underwater lights and air bubbles. The more conspicuous the
ob8tacle and the greater its effective height, the better tho herding, the
they increased their swimming speed slightly to avoid the different
parts of the gear. Schiirfe (1960), in a review paper, reported the
tendency of herring to swim downwards when frightened by a pelagic
trawl, reacting 2-4 m from the headline or ground rope. At 3 knots
(170 cmlsec) the herring appeared to swim ahead of the net, but at
3.5+ knots (200+ cmlsec) they were overtaken. Okonski and Konkol
(1957), however, reported an upward movement by herring in the
Skagerak when scared by a pelagic trawl. Clearly the direction of
movement depends on the position of the shoal in relation to the net as
well as on other possible factors. Galkov (1958) stressed that the
capture of Atlanto-Scandian herring by pelagic trawl depended very
much on their mobility, catches being best in November and December
when mobility was low. Mohr (1962) compared the echo traces obtained
from a transmitter on board ship with those from a transmitter on the
headline of a herring trawl. The reaction of the herring differed,
depending on their concentration. Small groups of fish kept about 5 m
away from the headline and ground rope ; large groups appeared not
to react at all, due to greater shoaling " pressure ". Mohr pointed out
the variability of behaviour of herring to nets with such factors aa
maturity stage, shoal sizes and time of day. Manteufel and Radakov
(1960) reported that tightly packed shoals of herring were more difficult to catch by trawl in the Baltic, suggesting that shoaling assists
the fish in reacting to the net and avoiding it. Manteufel et al. (1958)
showed that herring shoals in the Baltic wcro frightened by the boards,
warps and net of a trawl, and also by the fishing vessel itself 30 m above.
The shoals soon returned to their previous position after the net had
passed. In contrast, Itadakov ( 1 962) obnervcd tihoal~ of ClupeoneZh
duZicatu2a ;it i t 1 Bn,!yravli.Y encm.uicho1u.u H wi it1 rrr i trg, clpparci t t I y w i tt io tit
agitation, within a trawl towed at 1.5 knots.
Lucas (1936), Krefft and Schuler (1951) and Richardson (1960)
showed that trawl catches of North Sea herring were better by day,
there being size and maturity differences in the catches. While vertical
migration may play the principal part here, it is also posdlh: that
herring catches are poorer at night becaufic: th: finh arc more qmw1 orif,
(if they are not shoaling), or because they arc not hc;rdcd by t h
extremities of the net, the net having, therefore, a loww cffi:ctive
width.
In relation to this, the aquarium observations of Blaxter at ul. (1960)
showed the importance of sight to hcrring in avoiding moving obstacles.
'L'hcy t m t d thr hrrding ability of ground ropes, floats, panels of
Ilctting, underwater lights and air bubbles. The more conspicuous the
ob8tacle and the greater its effective height, the better tho herding, the