340
J . H. S. BLAXTER AND F. Q. T. HOLLIDAY
for spawning and then drift with the current back to the Skagerak to
overwinter. But this approach may be too simple, particularly in
relation to the further southward movement of the southern North Sea
spawners, some of which pass through Scottish waters, and in relation
to the migration of Norwegian herring from Icelandic waters.
Another factor in migration is the sudden movement of fish from
productive fishing grounds often without any apparent cause. Sometimes these movements may be correlated with unfavourable weather
and they may also be due to lack of food or the arrival of predators.
There is a wide field open for the experimental study of phenomena
relating to kineses in fish and how their speed of movement may be
controlled by the environment. For instance, Loukashkin and Grant
(1959) found that Sardinops caerulea swam more rapidly in unfavourable
red light.
3. Directed orientation
Much more difficult is the explanation of directed orientation which
may well be of great importance. Can clupeids detect, and react to,
gradients of hydrographical conditions or of food or of some unspecified
chemical and thus actively seek an optimal environment? Glover (1957)
suggested that herring might be able to detect concentration gradients
of food organisms, though it is not clear what the mechanism would be.
Presumably the fish .would move slower and slower as food became more
abundant, which would be more of a kinetic response. It seems unlikely
that gradients of temperature or salinity would ever be abrupt enough,
except perhaps vertically, for single fish to detect them along the
length of their bodies. Shelford and Powers (1915) reported that
Clupea pallasii in tanks could detect a temperature change of 0*6"C,
but their temperature preference experiments were inconclusive.
Anon. (1956) reported that Sardinops caerulea acclimated t o 17-19"C
usually occupied a region of temperature 18-22°C in a tall tank with a
gradient from 4°C at the bottom to 34°C at the top. Engraulie w d u x
acclimated to 12°C remained in 12-14"C, acclimated to 17°C in 15-18°C
and acclimated to 20°C in 18-20°C. It is conceivable that a very large
shoal could occupy an area where a gradient existed such that one
side of the shoal experienced more favourable conditions than the
other. In this event, the fish in the favourable conditions might start
to move more slowly, introducing a " wheeling " effect and a turn of
the shoal towards the more favourable area (Steele, 1961).
Contranatant migration seems unlikely in a pelagic fish out of
contact with stationary reference points (see Section VI, 0) but a
denatant drift, which would essentially be passive, seems quite probable.
J . H. S. BLAXTER AND F. Q. T. HOLLIDAY
for spawning and then drift with the current back to the Skagerak to
overwinter. But this approach may be too simple, particularly in
relation to the further southward movement of the southern North Sea
spawners, some of which pass through Scottish waters, and in relation
to the migration of Norwegian herring from Icelandic waters.
Another factor in migration is the sudden movement of fish from
productive fishing grounds often without any apparent cause. Sometimes these movements may be correlated with unfavourable weather
and they may also be due to lack of food or the arrival of predators.
There is a wide field open for the experimental study of phenomena
relating to kineses in fish and how their speed of movement may be
controlled by the environment. For instance, Loukashkin and Grant
(1959) found that Sardinops caerulea swam more rapidly in unfavourable
red light.
3. Directed orientation
Much more difficult is the explanation of directed orientation which
may well be of great importance. Can clupeids detect, and react to,
gradients of hydrographical conditions or of food or of some unspecified
chemical and thus actively seek an optimal environment? Glover (1957)
suggested that herring might be able to detect concentration gradients
of food organisms, though it is not clear what the mechanism would be.
Presumably the fish .would move slower and slower as food became more
abundant, which would be more of a kinetic response. It seems unlikely
that gradients of temperature or salinity would ever be abrupt enough,
except perhaps vertically, for single fish to detect them along the
length of their bodies. Shelford and Powers (1915) reported that
Clupea pallasii in tanks could detect a temperature change of 0*6"C,
but their temperature preference experiments were inconclusive.
Anon. (1956) reported that Sardinops caerulea acclimated t o 17-19"C
usually occupied a region of temperature 18-22°C in a tall tank with a
gradient from 4°C at the bottom to 34°C at the top. Engraulie w d u x
acclimated to 12°C remained in 12-14"C, acclimated to 17°C in 15-18°C
and acclimated to 20°C in 18-20°C. It is conceivable that a very large
shoal could occupy an area where a gradient existed such that one
side of the shoal experienced more favourable conditions than the
other. In this event, the fish in the favourable conditions might start
to move more slowly, introducing a " wheeling " effect and a turn of
the shoal towards the more favourable area (Steele, 1961).
Contranatant migration seems unlikely in a pelagic fish out of
contact with stationary reference points (see Section VI, 0) but a
denatant drift, which would essentially be passive, seems quite probable.