91] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 91 



shows that the pH of the blood of the young fish is lower than that of the 

 adults, and it will be remembered that the young fish on coming into the 

 shallow water, are entering a region of higher pH values: the older they 

 are the more permanently do they select the shallow water with its higher 

 pH values, and the older they are the higher the pH of the blood becomes. 

 Thus the change in habitat selection appears as a reaction to a physio- 

 logical change in the body which results in a change in the H-ion concentra- 

 tion of the body fluid. 



THE ADULT FISH 



Once the fish establish themselves inshore toward the end of the sum- 

 mer, their lives assume a much more even and less exciting level. With 

 the coming of the cold weather and the coincident cooling of the water, 

 the activity of the species becomes conspicuously reduced. The species 

 remain inshore all winter, with the exception of a slight tendency to wan- 

 der about, due probably to the uniform conditions of the water. Occasion- 

 ally they drift slowly through the openings in the ice where the cisco 

 fishermen are at work over the deep water. Much more frequently, 

 however, they are seen along the shore over water up to two meters deep 

 where the ice cutters have cleared their fields. Movements are slow and 

 sluggish and in marked contrast to the summer activity as already noted. 

 The food habits revert back to those of the immature, principally because 

 of the almost total absence of other food in the upper stratum of water. In 

 fact, it is quite a difficult task at times to obtain sufficient fish during the 

 winter to get a fair idea of what they are feeding upon, so scantily do they 

 appear to feed. This is borne out by the fact that growth ceases entirely 

 and the fish are living on a maintenance ration only. With the coming of 

 spring and the warming of the water, the normal activity slowly returns 

 and the species goes back to its insectivorous diet. With this increase in 

 activity and the excess of rations above the point of mere maintenance, a 

 new growth period is initiated and the fish attain their full size during the 

 early summer (Figs. 11, 12). In June comes again the spawning season, 

 the young of the previous summer go through their breeding performances, 

 and the life cycle begins over again in the new generation. 



As to the old generation, their fate is apparently sealed, for they die 

 before the second winter is upon them. The problem of their death and 

 its causes has not been attempted by the writer. It is evident, however, 

 that the adults die sometime during the last half of their second summer, 

 for neither Hubbs nor the writer has ever seen a silversides showing two 

 annuli on the scales. In an examination of 478 adults, no second winter 

 ring has ever been found. Then too, as the summer advances, the adults 

 grow notably less common. On the night of July 18, 1923, I was able to 

 pick up 26 adults along a few hundred feet of shore line in a few minutes; 



