270 BULLETIN OF THE BUREAU OP FISHERIES. 



■ It will be noted (Tables 8, 9, and 10) that the foods eaten in excess in Green Lake 

 are largely those associated with the bottom; those most eaten in Lake Mendota are 

 found for the most part in shallow water with plants, or in the open water. These 

 differences are in part accounted for by the stagnation of the deeper water and in part 

 by the greater abundance of food resources in the latter lake. In Lake Mendota there 

 is an abundance of food in the deeper parts, but such supplies are not easily accessible 

 to fishes in summer because there is no oxygen below 8 to 12 m. Birge and Juday 

 have recently made observations with mud dredges which, with the earlier work of 

 Birge (1897) and Marsh (1903), indicate clearly that there is actually less food in Green 

 Lake, as regards both bottom fauna and plankton, than in Lake Mendota. There are 

 some common fishes in Lake Mendota (silversides, crappie, gar, white bass) which are 

 rare or absent in Green Lake. These "extra" fishes feed to a considerable degree on 

 plankton, insects (in or on the surface of the water), and fishes. There is apparently 

 no chance for them to be abundant in Green Lake. 



DISCUSSION AND CONCLUSIONS. 



Green Lake is a fine clear body of water, with sandy and pebbly shores, and great 

 depth. Seventeen species of fishes were caught in it during the summer. The lake 

 stratifies in summer, but the lower water always contains oxygen, and of course remains 

 cool (5° C). 



Lake Mendota has nearly twice the area of Green Lake but is only a third as deep. 

 It stagnates during the summer in its depths and a large part of its water is without 

 oxygen for about three months. Notwithstanding this handicap, Lake Mendota has 

 more than twice as many fishes (as judged by the catch per hour in gill nets) in a unit 

 area. 



During the summer the distribution of the fishes in Green Lake shows definite strat- 

 ification. From the surface down to a depth of 10 m. all species of fishes caught in the 

 lake, except adult ciscoes, were found; from 10 to 20 m. only large pickerel, small- 

 mouth black bass, and suckers occurred; from 20 to 40 m. no fishes were caught; from 

 40 to 70 m. ciscoes were the only fishes caught, and were abundant. Reighard's (1915, 

 p. 246) idea that ciscoes inhabit the intermediate water and are not caught in gill nets 

 set on the bottom is no longer tenable in the light of results presented in this paper. 

 A. R. Cahn has also caught man)' ciscoes in gill nets set on the bottom in Oconomowoc 

 Lake. 



While gill nets were being set in Green Lake, 22 species of fishes were caught in 

 Lake Mendota by the same methods. There are, then, not only more individuals, but 

 a greater number of species in Lake Mendota. There were no fishes caught in gill nets 

 in this lake in the lower, stagnant water, except an occasional perch. Most fishes stay 

 above the thermocline, where oxygen is plentiful but the water warm. The perch 

 apparently congregate just above the thermocline and make short excursions into the 

 stagnated region to take advantage of the food offered by the rich bottom fauna. 



The most abundant species in each lake is one which feeds very largely from the 

 bottom in deep water. In Green Lake this species is the cisco ; in Lake Mendota, it is 

 the perch. Both species are present in both lakes, but a single and different species is 

 dominant in each lake. The cool water in the depths of Green Lake abounds with 

 ciscoes; the perch is not abundant and, in fact, was never caught in deep water. Al- 



