Vision 217 

 Direct Observation and Sun Orientation 



According to Hasler, et al./'- the eyes of fishes are useful in orientation. 

 These investigators demonstrated that white bass in Lake Mendota 

 (Wisconsin) moved in a definite direction when transported from one 

 part of the lake to another on clear days, but when displaced on cloudy 

 days, or if blinded by eye caps, they moved at random. These investigators 

 demonstrated in laboratorv experiments that the sun was a source for 

 orientation (whether the "sun" was natural or artificial) and that the 

 fish had a biological "clock" that operated in combination with the sun 

 to allow compensation of movement for different times of day. 



In some earlier experiments, Hasler ^^ had discovered that the elritze 

 ( a European minnow ) used minute marks on the wall and floor of its tank 

 for orientation rather than the presence of an artificial "sun." It was only 

 after removing all visible marks in the tank during training over a long 

 period of time that these fish were taught to associate a 90° angle of a 

 "sun" to a feeding position. 



With a combination of sun orientation and/or the demonstrated ability 

 of a fish to orient through observation of inconspicuous "landmarks" under 

 water, it is little wonder that they show considerable ability to return to 

 "home" territory after having been displaced. As will be described later, 

 other sensory organs aid in "homing." 



Light Sensitivity 



The eyes and other light-sensitive organs of fishes control to some 

 extent the relative position of fish in a body of water. Fish placed in a 

 tank containing a light gradient selected the level of illumination most 

 satisfying to them and were restricted in their movements by light above 

 a certain intensity.^^*^ This light intensity selection is upset by hunger, 

 season, and by sex stimulation during the spawning period. Reaction to 

 light also may change as a fish grows from fry to adult size.^^ An important 

 reason why electro-fishing at night is so much more efficient than in day 

 time is because certain kinds and sizes of fishes usually found in deep 

 water move into shallow water with the coming of darkness. 



Many kinds of fishes show diurnal rhythms associated with the chang- 

 ing intensity of light. Elritze kept in a tank were active during the day 

 and quiet at night, but their behavior was reversed if they were given 

 hollow bricks in which to take cover and thus avoid bright light.^^ When 

 cover was available, they were active at sunrise and sunset. Minnows 

 kept in continuous darkness showed no rhythm of locomotor activity. 

 Blinded minnows responded to daily variations in light intensity and were 

 more active at night than during the day. These and many other laboratory 

 experiments demonstrate the high light-sensitivity of most kinds of fishes. 



