346
J. E. 8. BLAXTER AND F. Q. T. HOLLlDAY
through a sharp salinity barrier, except temporarily, to get food. It
is also clear that vertical migration might also be modified by such
factors as tide ; Valdez and Cushing (1959) showcd that tho hroakirig
and re-forming of what was thought to be herring shoals depended on
the state of tide and the amount of light. Catches of herring vary with
moon phase and the depth of shoals may be lower where there is a
moon (see Section VI, T).
It seems likely that the control of vertical migration is extremely
complex, depending on a number of external factors, the effect of which
will be modified by internal factors such aa the age and physiological
state of the fish and the size of shoals. Any prediction of the depth to
which shoals will rise at night or sink by day needs to bc based on data
from a given area at a given time.
6. Abptive value of vertical migration
Zusser (1968a) described the advantage to the fish of changing their
environment twice per day, giving them a greater choice of food and the
possibility of better distribution by currents. Wynne-Edwards (1962)
developed the idea that the numerical density of animals was regulated
at certain specified times, in the case of plankton, when they migrated
to the surface at night. This hypothesis could also be extended in a
modified way to vertical migration of clupeids.
7 . #peed of vertical migration and pressure changes (see also Section VI, N)
Runnstrcam (1941b) reported an upward movement of 100 m in
1 hr in Atlanto-Scandian herring and Dragesund (1958) an upward
movement of sprats (2) of 16 m in 46 min. Krr:fff; and 3 c h ~ i h r (1051 j
gave speeds of 75 m/hr up and 200 m/hr down in North Sea herring.
On a more experimental basis, Blaxter and Parrish (1958) memured tha
speed of clupeids following the vertical movements of artificial lightrc.
Herring 15-25 cm long moved up at speeds up to 13.5 m/min and down
at speeds up to 36 m/min ; sprats, 5-15 cm long, swam up at npeed8 up
to 30 m/min. The pressure changes involved ranged from increarccn of'
16-180% and decrease of pressure from 18 to 73% for horring, and
increases of 22 to 225% and decreaseR of 11 to 07%, for RpratR. Drawn
(1962) put immature herring from Passamaquoddy Bay into c a p s
which she raised and lowered. They withstood being lowercd from the
surface to about 50 m (change of' prossure 0 to 4.7 kg/cm2) at a rate of
0.06 kg/cm2/sec, and boing rnisod to the surface again at a spoed of
0.07-0.08 kglcm21sec. The herring wcre also subjected to greatcr
stressw i i i a Rtorilizer. Aftor a prosRuro incronm of 1.4 kg/cma they
J. E. 8. BLAXTER AND F. Q. T. HOLLlDAY
through a sharp salinity barrier, except temporarily, to get food. It
is also clear that vertical migration might also be modified by such
factors as tide ; Valdez and Cushing (1959) showcd that tho hroakirig
and re-forming of what was thought to be herring shoals depended on
the state of tide and the amount of light. Catches of herring vary with
moon phase and the depth of shoals may be lower where there is a
moon (see Section VI, T).
It seems likely that the control of vertical migration is extremely
complex, depending on a number of external factors, the effect of which
will be modified by internal factors such aa the age and physiological
state of the fish and the size of shoals. Any prediction of the depth to
which shoals will rise at night or sink by day needs to bc based on data
from a given area at a given time.
6. Abptive value of vertical migration
Zusser (1968a) described the advantage to the fish of changing their
environment twice per day, giving them a greater choice of food and the
possibility of better distribution by currents. Wynne-Edwards (1962)
developed the idea that the numerical density of animals was regulated
at certain specified times, in the case of plankton, when they migrated
to the surface at night. This hypothesis could also be extended in a
modified way to vertical migration of clupeids.
7 . #peed of vertical migration and pressure changes (see also Section VI, N)
Runnstrcam (1941b) reported an upward movement of 100 m in
1 hr in Atlanto-Scandian herring and Dragesund (1958) an upward
movement of sprats (2) of 16 m in 46 min. Krr:fff; and 3 c h ~ i h r (1051 j
gave speeds of 75 m/hr up and 200 m/hr down in North Sea herring.
On a more experimental basis, Blaxter and Parrish (1958) memured tha
speed of clupeids following the vertical movements of artificial lightrc.
Herring 15-25 cm long moved up at speeds up to 13.5 m/min and down
at speeds up to 36 m/min ; sprats, 5-15 cm long, swam up at npeed8 up
to 30 m/min. The pressure changes involved ranged from increarccn of'
16-180% and decrease of pressure from 18 to 73% for horring, and
increases of 22 to 225% and decreaseR of 11 to 07%, for RpratR. Drawn
(1962) put immature herring from Passamaquoddy Bay into c a p s
which she raised and lowered. They withstood being lowercd from the
surface to about 50 m (change of' prossure 0 to 4.7 kg/cm2) at a rate of
0.06 kg/cm2/sec, and boing rnisod to the surface again at a spoed of
0.07-0.08 kglcm21sec. The herring wcre also subjected to greatcr
stressw i i i a Rtorilizer. Aftor a prosRuro incronm of 1.4 kg/cma they