SOME ASPECTS OF PHOTORECEPTION AND VISION IN FISHES
197
pink salmon, Ali and Hoar (1959) have found that crepuscular migration
ie initiated when the retina is only partially dark-adapted (by this they
mean that elongation of cones and withdrawal of retinal pigment are
only partially completed). The light-intensity in the evening falls
rapidly (in 46 min) from 1 to 10-4 ft c while retinomotor changes and
downstream movement are taking place. They have suggested that
the young salmon rise toward the surface at twilight because they are
only partially dark-adapted and can no longer see the bottom, i.e. the
light-intensity is changing faster than the eye can adapt by retinomotor
movements. Once visual contact with the bottom is lost, downstream
transport follows. This interesting theory, baaed on a correlation, does
not take into account the progress of dark-adaptation 8emu stricto, and
there is a notable dearth of information about the movement of the
rods. Indeed, the radial movements of photoreceptors and retinal
pigment obviously are adaptations to high and low levels of illumination,
but their exact significance in terms of visual functioning has still to be
established.
Some fish are affected more by an intermittent light than by a
oontinuous light at night. Paaific sardine in a dark aquarium avoid a
beam of intermittent light (illumination < 1 ft c, flashing rate 60 times/
min), and they display a typical fright reaction when such a light is
used (Loukashkin and Grant, 1959). An intermittent light is med in
California purse-seine fishing in order to keep the fish within the seine
until the net has been closed (California Co-operative Oceanic Fisheries
Investigation, 1968 ; von Brandt, 1969). Juvenile silver salmon
(Oncorhynchw kisutch) give fright reactions when a light is quickly
turned on and off the fish (Dunkan, 1950). And in some experiment8
involving migrating young spring salmon (0. tsuwytscha), a twam of
flashing light was found to be more effective in deflecting the course of
the fish than a continuous light and a curtain of air bubbles (Brett and
MacKinnon, 1953). On the other hand, yellow fish and little tunny, in
aquaria, showed no differences in regponaes to continuous and interrupted lights (Tester, 1959). Special responses to flashing lights may be
significant in connexion with luminous flashes of animals.
Many pelagic fishes diurnally make vertical migrations and these
are probably regulated in large part by daily changes in light-intensity,
as is known to be the caae in planktonic invortobrates. Vertical movoments from deep waters towards the surfnco r b t night, and doscent at
dawn, have beon doteoted in pilohards or surdirios (Sardina pilchardw),
anchovies (Engrauh8 encrmichlw) and herring (Clupea hurengw)
(Dragesund, 1968 ; Cushing, 1959 ; Sara, 1962).
Of quite a different nature is the attractive influence which strong
197
pink salmon, Ali and Hoar (1959) have found that crepuscular migration
ie initiated when the retina is only partially dark-adapted (by this they
mean that elongation of cones and withdrawal of retinal pigment are
only partially completed). The light-intensity in the evening falls
rapidly (in 46 min) from 1 to 10-4 ft c while retinomotor changes and
downstream movement are taking place. They have suggested that
the young salmon rise toward the surface at twilight because they are
only partially dark-adapted and can no longer see the bottom, i.e. the
light-intensity is changing faster than the eye can adapt by retinomotor
movements. Once visual contact with the bottom is lost, downstream
transport follows. This interesting theory, baaed on a correlation, does
not take into account the progress of dark-adaptation 8emu stricto, and
there is a notable dearth of information about the movement of the
rods. Indeed, the radial movements of photoreceptors and retinal
pigment obviously are adaptations to high and low levels of illumination,
but their exact significance in terms of visual functioning has still to be
established.
Some fish are affected more by an intermittent light than by a
oontinuous light at night. Paaific sardine in a dark aquarium avoid a
beam of intermittent light (illumination < 1 ft c, flashing rate 60 times/
min), and they display a typical fright reaction when such a light is
used (Loukashkin and Grant, 1959). An intermittent light is med in
California purse-seine fishing in order to keep the fish within the seine
until the net has been closed (California Co-operative Oceanic Fisheries
Investigation, 1968 ; von Brandt, 1969). Juvenile silver salmon
(Oncorhynchw kisutch) give fright reactions when a light is quickly
turned on and off the fish (Dunkan, 1950). And in some experiment8
involving migrating young spring salmon (0. tsuwytscha), a twam of
flashing light was found to be more effective in deflecting the course of
the fish than a continuous light and a curtain of air bubbles (Brett and
MacKinnon, 1953). On the other hand, yellow fish and little tunny, in
aquaria, showed no differences in regponaes to continuous and interrupted lights (Tester, 1959). Special responses to flashing lights may be
significant in connexion with luminous flashes of animals.
Many pelagic fishes diurnally make vertical migrations and these
are probably regulated in large part by daily changes in light-intensity,
as is known to be the caae in planktonic invortobrates. Vertical movoments from deep waters towards the surfnco r b t night, and doscent at
dawn, have beon doteoted in pilohards or surdirios (Sardina pilchardw),
anchovies (Engrauh8 encrmichlw) and herring (Clupea hurengw)
(Dragesund, 1968 ; Cushing, 1959 ; Sara, 1962).
Of quite a different nature is the attractive influence which strong