318 BULLETIN OF THE BUREAU OF FISHERIES. 
VARIATION IN FOOD AT DIFFERENT AGES. 
In order to determine what foods were eaten by young perch during the first summer 
after they hatched, collections were made with a minnow seine at intervals during 1916 
in shallow water east of the base of Picnic Point in Lake Mendota. The results of the 
food examinations are summarized in Table 18. The perch were very uniform as to size, 
and it will be noted that the average length showed a regular increase as the season 
advanced. The table shows that Cyclops, other small crustaceans, and minute insect 
larve are replaced to a large extent by Hyalella and good-sized insect larve as the perch 
increase in size. 
To compare the food of the perch summarized in Table 18 with the food of those 
from another place in Lake Mendota on a date close to one of those utilized in the usual 
locality, 10 small perch were collected from the mouth of Six Mile Creek on August 8, 
1916. They had eaten of Tanypus monilis larve, 2.5 per cent; Chironomous lobiferus 
larve, 7.8 per cent; mayfly nymphs, 18.5 per cent; Betis nymphs, 8 per cent; Cenis 
diminuta nymphs, 23.4 per cent; Corixa nymphs, 16.3 per cent; Chironomus lobiferus 
pup, 13.1 per cent; Hyalella azteca, 10.3 per cent; ostracods, +. If these results be 
compared with those for August 7, in Table 18, it is apparent that only three of the same 
items have been eaten in the two localities, yet there is general similarity. About the 
same types of foods are eaten in about the same proportions. 
Through the kindness of A. R. Cahn we were able to examine small perch from 
Oconomowoc Lake. Though the individuals were more variable in size than those 
examined in Lake Mendota, the same food changes are evident (Table 19). Small 
insect larvee and entomostracans are succeeded by larger larvee and Hyalella. 
If these two tables showing the food of small perch are compared with Table 9, 
which gives the results for adults, it is evident that at the close of the first summer the 
food of the young has become like that of adults. 
RATE OF GROWTH ON DIFFERENT FOODS. 
To be of most significance, the determinations of the rate of growth should be 
made on perch of various ages at different temperatures. Kmnauthe (1898) performed 
experiments which indicated that metabolism is more rapid in young fish than in old 
and that more protein food is necessary during youth. Older fish need apparently 
more mineral than young. Piitter (1909) says that the smelt and herring require 
nearly twice as much food after growing for a month. He found that a carp after 
two summers weighed about 500 g. and that it would increase to 1,250 g. by the middle 
of the next August. In this paper it has already been shown that digestion is more 
rapid in perch at higher temperatures (p. 313). 
We have been able to test the rate of growth in perch of one size only and at one 
temperature. From August 19 to September 18, 1916, when the temperature of the 
water varied from 20 to 16.8° C., 26 small perch were placed in separate glass jars having 
a capacity of 4 liters each, and in lots of 3 were fed, as follows: : 
1. Fish liver and flour mashed and mixed together. 
2. Hyalella azteca alive. F 
3- Plankton fresh from Lake Mendota. It consisted of Daphnia, 95 per cent; Leptodora, 4.5 per 
cent; alge, chiefly Lyngbya, 0.5 per cent. 
4. Earthworms alive. 
