96 ILLINOIS BIOLOGICAL MONOGRAPHS [96 



described by Pearse (1921), and the statement also by Pearse that the 

 cisco is a bottom seeking form (based on the independent work of Pearse 

 and Cahn) nothing has been published regarding the ecology of the fish. 

 Two studies of the Lake Erie cisco (Leucichthys sp., but not artedi) have 

 recently been published by Clemens (1922). This report is, therefore, the 

 first contribution to a life-history study of the cisco, Leucichthys artedi. 



The following ecological study was begun in 1915 at the University 

 of Wisconsin on the species inhabiting Lake Mendota, which is the same 

 as that occuring in the other southern Wisconsin lakes. In 1917 the study 

 was begun in Oconomowoc lake and in other lakes in Waukesha county, 

 and has been carried on each summer since then (except 1919, when the 

 writer was in France), and at intervals during each winter. During the 

 summer the fish have been caught in numbers by means of gill nets; 

 during the winter with hook and line. Live fish were kept in the large 

 concrete tank in the vivarium at the University of Wisconsin during the 

 winter of 1916, and their reactions and habits were under constant observa- 

 tion. Thousands of fish have been examined for age, weight, length, food, 

 parasites, and these examinations and observations are the basis of this 

 report. 



BREEDING HABITS 



The cisco is essentially a deep water fish, spending its life in the deepest 

 part of the lakes during such times of the year as conditions (oxygen content 

 of the water, etc.) permit. This is of interest in view of the statements of 

 Ward, Milner and others to the effect that the fish are surface species in 

 the Great Lakes. This is one of the differences that have been mentioned 

 as separating the small lake forms from the Great Lakes fish. Thus one 

 finds the fish in the deep water of Oconomowoc lake following the fall 

 over turn when the winter conditions of the lake are established. The 

 thermocline has descended and a uniformity of oxygen content exists 

 throughout the water, making the entire lake available to the fish. How- 

 ever, the part selected by the fish is the cold water of the bottom. As the 

 thermocline descends, the temperature of the water also drops slowly. 

 As the water of the lake cools and approaches a mean temperature of 

 5.0°C (41°F) the fish show the first signs of the breeding migration, and 

 approach the shallower waters near the sand bars. With the continued 

 cooling the fish approach the shore still more closely, surmounting the 

 bars and appearing in water from one to three meters deep. If rivers enter 

 the lake, the fish may ascend them. Thus, the ciscos come up the Ocono- 

 mowoc river from Fowler lake, only to be stopped by the locks and dam 

 at the outlet of Oconomowoc lake. Much of this trip is made in water not 

 over 1 m in depth and still shallower water is encountered by the Oconomo- 

 woc lake ciscos in their ascent of the Oconomowoc river toward Okauchee 



