SOME ASPECTS OF PHOTORECEPTION AND VISION I" FISHES
199
lengths, mentioned in Section V, may have their counterparts in overt
behavioural responses.
The sensory cues and factors which influence migrating fish when
moving to and from their breeding and feeding a r e a have fascinated
biologists, and evidence is now accumulating that fieh can orientate on
the sun and thereby follow a fixed compass direction. In one investigation some white bass (Roccw chysops), a freshwater fish, were captured
on their spawning grounds near shore, transported to the middle of the
lake (a distance of 2.4 km) and there released. When this was done on
clear days they subsequently made their way back to their particular
spawning areas, but they failed to orientate and return on days that
were overcast (Hasler et al., 1958). The same investigators found that
sunfish (Lepomis) could be trained to enter a box in a certain compass
directioii at any time of day. Both these fishca can certainly orient by
the sun ; in fact, they cannot oriont themselves when it is not visible,
and they must be able to compensate for movement of the sun during
the course of the day. Moreover, it has been discovered by training
experiments with Lepomis that they also adapt their diurnal sunorientation rhythm to conform to seasonal changes in connexion with
alterations in day length. This latter compensation, of course, is
necessary to enable a fish to make use of sun-compass reactions throughout the year in the same latitude. Sun-compass reactions have becn
demonstrated in young silver salmon and other freshwater fishes
(Hasler, 19608 ; Schwassmann, 1960 ; Schwassmann and Braemer,
1961).
The experiments just mentioned deal with homing in local areas.
Braemer (1 960) simulated longitudinal diqhcement by conditioning
freshwater fish to light-cycles that were delayed or advanced from
those of the normal day. When they were tested subsequently out of
doors, their sun-compass reaction now deviated by a certain angle from
the previous direction, and this doviation was roughly cquivnlcnt to
the amount of Hhift in time or longitudo. If II f i ~ h i H dinplaced in latitude,
it encounters n chrrngo iii lcrigth of tho (lay and IL ohango in tlm inulinntion of tlio ~ 1 1 1 i ' t i ILW. SiiiilidI traincd to orient, to tho Hun in WiaconHin
(43"N) were tlowii to Iirazil ( I "S). A t tho oqiiiitor tho eunfish oriontctl
during the fortwoon hut becamo disoriented in the aftornoon, apparently
because of the high position of the sun. A sunfish flown to Montevideo
(30'5) continued to compensate for the azimuth curve of the sun in the
way that would have been correct in the northern hemisphere. Fishes
living in the tropics must reverse the direction of the sun compenaating
mechanism during the course of the year, owing to the annual reversal
of the position of the sun's azimuth. CichlidA (tropical and subtropical
199
lengths, mentioned in Section V, may have their counterparts in overt
behavioural responses.
The sensory cues and factors which influence migrating fish when
moving to and from their breeding and feeding a r e a have fascinated
biologists, and evidence is now accumulating that fieh can orientate on
the sun and thereby follow a fixed compass direction. In one investigation some white bass (Roccw chysops), a freshwater fish, were captured
on their spawning grounds near shore, transported to the middle of the
lake (a distance of 2.4 km) and there released. When this was done on
clear days they subsequently made their way back to their particular
spawning areas, but they failed to orientate and return on days that
were overcast (Hasler et al., 1958). The same investigators found that
sunfish (Lepomis) could be trained to enter a box in a certain compass
directioii at any time of day. Both these fishca can certainly orient by
the sun ; in fact, they cannot oriont themselves when it is not visible,
and they must be able to compensate for movement of the sun during
the course of the day. Moreover, it has been discovered by training
experiments with Lepomis that they also adapt their diurnal sunorientation rhythm to conform to seasonal changes in connexion with
alterations in day length. This latter compensation, of course, is
necessary to enable a fish to make use of sun-compass reactions throughout the year in the same latitude. Sun-compass reactions have becn
demonstrated in young silver salmon and other freshwater fishes
(Hasler, 19608 ; Schwassmann, 1960 ; Schwassmann and Braemer,
1961).
The experiments just mentioned deal with homing in local areas.
Braemer (1 960) simulated longitudinal diqhcement by conditioning
freshwater fish to light-cycles that were delayed or advanced from
those of the normal day. When they were tested subsequently out of
doors, their sun-compass reaction now deviated by a certain angle from
the previous direction, and this doviation was roughly cquivnlcnt to
the amount of Hhift in time or longitudo. If II f i ~ h i H dinplaced in latitude,
it encounters n chrrngo iii lcrigth of tho (lay and IL ohango in tlm inulinntion of tlio ~ 1 1 1 i ' t i ILW. SiiiilidI traincd to orient, to tho Hun in WiaconHin
(43"N) were tlowii to Iirazil ( I "S). A t tho oqiiiitor tho eunfish oriontctl
during the fortwoon hut becamo disoriented in the aftornoon, apparently
because of the high position of the sun. A sunfish flown to Montevideo
(30'5) continued to compensate for the azimuth curve of the sun in the
way that would have been correct in the northern hemisphere. Fishes
living in the tropics must reverse the direction of the sun compenaating
mechanism during the course of the year, owing to the annual reversal
of the position of the sun's azimuth. CichlidA (tropical and subtropical