65] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 65 



be associated with a single female during the height of the spawning 

 season, but this poly-association usually terminates by one of the males 

 driving away the others. During the early part of the breeding season the 

 progress of the fish is leisurely, a foot being traversed in from three to four 

 seconds. As the breeding season advances and the water warms up still 

 more, the speed of progression increases until the fish travel in spurts, 

 covering a foot in a fraction of a second and often breaking water. At the 

 same time the interval of alignment has been decreased to about two 

 inches, and finally, as the breeding season reached its peak, the position is 

 shifted to a horizontal one, the male following the female which travels at 

 a furious pace. A school of silversides reveals a wild sight when the spawn- 

 ing activities are in full sway. In and out dart the females, pursued by one 

 or more males, darting this way and that, shooting an inch or more out 

 of the water and landing again three or four inches from the spot of their 

 emergence amid a spatter of spray, followed immediately by the attending 

 male retinue. Suddenly the female slows down her pace and comes to 

 what amounts to comparative rest. The first male to reach her approaches 

 from the rear and draws up along side. This apparently is the signal for 

 the departure of any other males that may be pursuing that particular 

 female, for never have I seen any disturbance once a male is associated 

 along side of a female. Other males simply disperse and join in the chase 

 of other females. The paired fish now begin a downward glide, approach- 

 ing the bottom at an angle of approximately 30°. During the descent, the 

 fish bring the edges of their abdomens into repeated momentary contacts — 

 from eight to twenty-one times being the extreme numbers observed, with 

 fourteen as an average of forty-six observations. During the descent 

 the eggs are extruded from the body of the female and may be seen slowly 

 settling toward the bottom in the wake of the descending pair. Fertilization 

 takes place in the water immediately after the eggs leave the female, the 

 spermatozoa being extruded by the male coincident with the momentary 

 contact with the abdomen of the female. By the time the pair reach the 

 bottom the egg complement has been deposited. Observations on twenty- 

 six females captured immediately after the completion of the descent, 

 show the ovaries empty, with only occasionally (three cases) some half 

 dozen eggs still retained. Normally the female is entirely spent on the 

 completion of a single breeding performance, and hereafter she is com- 

 pletely ignored by the still active males. The males, on the other hand, 

 hesitate not at all on the completion of the breeding descent, but ascend 

 at once to the surface and are off in pursuit of other females. 



A series of observations over four breeding seasons in which a series of 

 temperature readings of the water was taken during the period of breeding 

 activity, shows that the vertical pairing of the fish begins when the water 

 has reached a temperature of 18°C (64.4°F), that the spawning begins at a 



