103] STUDY OF SOUTHERN WISCONSIN FISHES-CAEN 103 



hole in the ice, and have been utterly astonished at their numbers: a thick 

 mass of fish from just below the ice as far down as the eye could see. An 

 examination of the water of Pine lake shows that the plankton organisms 

 are relatively less abundant per liter of water than in Oconomowoc lake. 

 Thus one can explain the weight discrepancy on the basis of inadequate 

 food supply: there are too many fish for the amount of food available. 

 A similar condition is to be found in Otis lake in the case of the large 

 mouthed black bass (Micropterus salmoides) this is a small lake to which 

 the public does not have access, and it is full of bass, many times more 

 than the food will suffice for anything over a strict maintenance ration. 

 The result is a large number of bass of light weight and scanty measure- 

 ments. In a similar way one occasionally catches a cisco in Pine lake 

 that weighs over a pound and still is but five or six years old; but such 

 a one must be looked upon as a particularly fortunate individual in so far 

 as food supply is concerned. 



FOOD HABITS 



The cisco is one of the most exclusive plankton eaters among our fishes. 

 The food of the immature fish — up to about 16 cm. in length — is 100% 

 plankton organisms, being composed of Cyclops, Daphnia, Diaptomus, 

 Bosmina, Chydorus, rotifers and other animals of a similar nature. Bottom 

 material is freely taken into the alimentary canal and as this contains a 

 considerable amount of vegetable matter, the smaller algae, diatoms etc. 

 form a part of the diet. As the fish reach a larger size the food assumes a 

 wider range, and various mollusks, insect larvae, Crustacea and small 

 fish appear in the food list. There is a slight tendency on the part of the 

 fish to feed more upon the micro-crustacea of the Daphnia-Cyclops type 

 during the winter than during the summer, but in Oconomowoc lake, as 

 in all of the lakes in Waukesha county which the writer has investigated, 

 these organisms form normally a large part of the diet the year around. 

 This is at variance with findings of Pearse (1921a) in the case of Leucichthys 

 birgei in Green lake in summer, where he found 61.2% of the food to be 

 amphipods. However, his numbers are hardly large enough to base con- 

 clusions upon, nor is the seasonal range of his investigations of sufficient 

 duration: he had but 30 fish caught between August 13 and 19. In table 

 26 the writer presents a summary of 941 stomach examinations grouped 

 according to months in sets of two months to a group. In this table the 

 Cladocera are represented by species of Bosmina, Daphnia and Chydorus; 

 the Copepoda by Dioptomus and Cyclops; the Ostracoda by Cypris. The 

 mollusks include various species of Amnicola, Valvata, Sphaerium, 

 Pisidium and Planorbis. Sayomia (formerly Corethra) has been separated 

 from the other insect food because of its abundance and frequency. The 

 heading "other insects" includes various larvae and pupae picked up from 



