97J STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 97 



lake. True, the ascent of the rivers is a rheotropic reaction, but too much 

 weight must not be placed on this because of the fact that in reality only 

 a small part of the fish of the lake actually travel up the streams, the 

 majority of the individuals, all of whom had the same chance to go up- 

 stream, content themselves with approaching the shore and lay their eggs 

 there. This is perhaps well for the species, for it is very doubtful, indeed, 

 if any of the eggs laid in the shallow water of the rivers ever hatch and 

 return young fish to the lake: there are too many voracious small fish 

 seeking to devour the eggs and fry. 



The first fish to reach the shallows are the males, and they are function- 

 ally ripe when they arrive. These early arrivals strip easily and microscopic 

 examination shows abundant normally active spermatozoa. The first 

 arrivals usually come in at night, but this is not always the case. They 

 swim leisurely about, occasionally wandering toward the deep water, but 

 show a distinct tendency to follow the shoreline. This movement exhibits 

 no co-ordination as to direction, about equal numbers traveling in each 

 direction, which is in marked contrast to the behavior of the fish when they 

 are "schooled up" in the deep water. The arrival of the males precedes 

 that of the females by from two to four or five days. As the controlling 

 factor for the arrival of each sex is the water temperature, no definite 

 statement can be made as to the intervening time; it depends on the 

 weather and the resulting influence on the water temperature. With the 

 arrival of the females the breeding begins. Several males follow behind a 

 single female at a distance of about one foot, the number of males to a 

 female varying considerably. The first females to arrive may have over a 

 dozen males in attendance, while later on two males is more usual. The 

 actions of the fish are at all times slow and deliberate; there is no chasing or 

 darting about, no evidence at all of excitement or pugnacity. The deposi- 

 tion of eggs takes place in water varying from one to two meters in depth, 

 and the region is free from heavy aquatic vegetation. The female descends 

 to within six or eight inches of the bottom as the eggs are deposited, the 

 male following close behind and along side, his head about even with the 

 anal opening of the female. It is not at all infrequent to find two males 

 present during the fertilization process. As the eggs appear, coming from 

 the female in a thin cylindrical band, the males discharge the spermatozoa 

 freely in the water, forming a conspicuous milky cloud as they swim slowly 

 along. The spawning fish does not cease swimming, with the result that 

 the total egg complement is well scattered over a considerable area. The 

 eggs are slightly viscous, the result being that they become fixed to the 

 rocks, vegetation or debris on the bottom, where they remain during their 

 developmental period. 



As to the conditions surrounding the nuptial migration, they have 

 been carefully worked out. It was early discovered that the arrival of the 



