320 BULLETIN OF THE BUREAU OF FISHERIES. 
of the water. In late summer the water in Lake Wingra is murky with myriads of 
alge. ‘The perch are pale in color and apparently in poor condition. 
The chief respect in which the perch of Wingra differ from those in Mendota, in 
regard to food, is (1) that they eat more insect larve and less of entomostracans, 
(2) that through most of the year they apparently feed more among water plants, 
and (3) that they have longer periods when little or no food is eaten. 
COMPARISON OF FOOD OF PERCH AND CRAPPIE IN LAKE WINGRA. 
From February to November, 1916, the food of crappies (Pomoxis sparoides) from 
Lake Wingra was studied, and the results for the nine months may be compared with 
those for perch. The total percentages of foods eaten by both was as follows, the 
perch being placed first in each case: Fish, 12.7 to 8.8; insect larve, 52.8 to 25.5; 
insect pupz, 11.4 to 7.9; adult insects, 1.3 to 4.8; mites, 2 to +; amphipods, 0.3 to +; 
clams, 0.05 to 0; leeches, 0.2 to +; oligochates, 0.1 to 0; plants, 3 to 0.4; débris, 
1.2 tox. In other words, the perch eats more of fish, insect larve, insect pupz, mites, 
amphipods, snails, clams, leeches, oligochztes, plants and débris; the crappie more of 
adult insects, ostracods, copepods, cladocerans. These proportions clearly indicate 
that perch feed largely on or near the bottom, while crappies hunt more toward the 
surface and among water plants. 
The structure of the crappie is more specialized than that of the perch and would 
indicate greater adaptation to particular conditions. Its mouth is more upturned, 
suggesting feeding toward the surface of the water, and the body is more compressed, 
indicating a habitat among aquatic vegetation. A full account of the observations on 
the crappies of Lake Wingra has already been published (Pearse, 1919). 
RESPIRATION. 
A perch must obtain the oxygen necessary for its metabolic activities from the 
water in which it lives. Water free to absorb gases from the atmosphere will contain 
about 35 per cent oxygen, 65 per cent nitrogen, and a trace of carbon dioxide. The 
total amount of gas which may be absorbed varies with the temperature of the water. 
At 0° C. aliter of water, when the pressure is 760 mm., can absorb 41.14 c. c. of oxygen, 
1,796.7 ¢. c. of carbon dioxide, and 20.35 c. c. of nitrogen. At 20° C. the amounts will 
be: Oxygen, 28.38 c. ¢.; carbon dioxide, 901.4 c. c.; and nitrogen, 14.03 c. c.; at the 
boiling point of water none of the gases will be absorbed. When there are many living 
plants present, the amount of oxygen may rise above the saturation point; when 
oxygen is used up (decomposition, respiration, etc.), it may fall much below saturation 
or even be absent altogether. A perch, then, normally lives in water which may vary 
greatly in its gaseous content at different seasons. 
Supersaturation with oxygen appears to offer no particular difficulties for fish, 
but when this gas is scanty there may be trouble in obtaining a sufficient supply for 
respiratory needs; yet some species are able to live in water containing a very small 
amount. Winterstein (1908) states that 0.7 c. c. of oxygen per liter was enough to 
sustain life in Leuciscus erythropthalmus, but when the amount was decreased as low 
as 0.4 to 0.5 c. c., death ensued. Most fishes show signs of distress when the oxygen 
is 1 to 4c. c. per liter. In natural bodies of water the carbon dioxide usually increases 
as the oxygen decreases, but in amounts such as occur in nature its presence does not 
