300 BULLETIN OF THE BUREAU OF FISHERIES. 
fishes in these lakes? (2) Why are the perch in Lake Mendota larger than those in Lake 
Wingra, and why do the fish of many lakes attain a certain maximum size? (3) What 
effect does the stagnation of a lake like Mendota have on the activities of the fishes in 
it? In seeking answers to these questions we were led to make routine examinations 
of perch of all ages from each lake and for every week in the year; to study the migra- 
tion and distribution of perch in relation to the dissolved gases in the water and to 
determine the gaseous content of the swim bladders; to ascertain the amount of food 
eaten by perch and the rate of digestion; and to investigate the breeding, enemies, and 
other factors which might influence the life cycle of the perch in these lakes. 
During the investigation the authors were under continual obligation to the 
Wisconsin Geological and Natural History Survey and to the University of Wisconsin 
for the loan of apparatus and for assistance in other ways. Chancey Juday, in particular, 
gave many valuable suggestions and read the manuscript for this paper. A. R. 
Cahn and Dr. John Lowe furnished perch from other Wisconsin lakes for comparison 
and otherwise took a helpful interest in the work. Dr. R. A. Muttkowski identified 
the greater part of the insect remains found in the food. Miss Hattie J. Wakeman drew 
all the figures, with the exception of figure 1. 
A few technical matters ought to be mentioned before taking up the habits of the 
perch. All lengths in this paper are given in millimeters and refer to the distance from 
the extreme anterior end to the beginning of the membranous portion of the caudal fin. 
Figures giving estimates of food, except where statement is made to the contrary, mean 
percentages by volume; + indicates an amount too small to be given a percentage value. 
FOOD. 
The yellow perch is more versatile in its feeding habits than its near relatives. The 
pikes are largely piscivorous, and the darters feed for the most part on insect larve, but 
the perch is equipped to secure almost anything edible. Its shape and structures for 
the capture of food are less specialized than those of other members of the family Percide, 
and it is correspondingly more versatile. The stocky body is suited to all sorts of situ- 
ations; the sharp, backwardly directed teeth hold struggling animals; the slender gill 
rakers readily strain microscopic organisms from the water; and the activity of the swim 
bladder renders adjustment to various pressures comparatively easy, so that food may 
be sought at all depths. A perch may grub out insect larve from soft bottom mud, 
snatch crayfishes from their hiding places among the rocks alongshore, strain plankton 
animals from the open waters of lakes, lurk in the shore vegetation in order to capture 
passing fishes, or pull aquatic insects from their retreats among water plants. 
Available food for perch is abundant in the two lakes investigated. Birge (1897) 
has shown that there may be at times more than 3,000,000 microscopic Crustacea per 
square meter of surface in the waters of Lake Mendota. Recent and as yet unpublished 
work by Birge and Juday has demonstrated the presence of more than 30,000 dipterous 
larve per square meter on the bottom of this lake, in addition to many other animals. 
Muttkowski (1918) counted the animals in typical habitats along the shores and com- 
puted the total numbers for the lake. He estimates that there are about 553,220,000 
flatworms, 18,680,000 roundworms, 8,690,530,000 oligochztes, 103,200,000 leeches, 
2,389,740,000 mollusks, 2,221,300,000 macroscopic crustaceans, 815,930,000 water mites, 
and 5,134,190,000 insect larve and adults in the shallow waters of Lake Mendota. 
