HABITS OF YELLOW PERCH. 315 
February, March, and April, 1916, the perch in Lake Mendota and the crappies in Lake 
Wingra had considerable amounts of beautifully regular calcium carbonate crystals 
(which were ‘usually embedded in a brownish, amorphous matrix) in their intestines. 
The food of 10 perch examined on each of the following dates contained the amount of 
crystals indicated: February 1, 0.5 per cent; March 1, 8.1 per cent; March 29, 12.3 per 
cent; April 14, 1.2 per cent; April 28, 2 per cent. At the same time the crappies 
(Pomoxis sparoides) in Lake Wingra showed the following amounts: March 11, 4 per 
cent; March 18, 0.2 per cent; April 22, 0.2 per cent; but the intestines of the perch in 
Lake Wingra contained none. ‘Two of the perch from Lake Mendota examined on 
March 29 contained 30 per cent of the calcium crystals. The remainder of the.food in 
one of these consisted of 45 per cent silt and bottom débris, 5 per cent chironomid larve, 
1 per cent Corethra larve, 17 per cent plant remains, 2 per cent filamentous alge; in the 
other, of 35.9 per cent silt and débris, 30 per cent plant remains, 3 per cent Corethra 
larve, 1 per cent chironomid larve, o.1 per cent gelatinous alga. Birgeand Juday (1911, 
pp. 108, 171) analyzed the mud from the bottom and the crust from aquatic plants in 
Lake Mendota. The former contained, in percentages of dry weight, 33.21 per cent, 
and the latter, 47 per cent of calcium oxide. As has been previously stated, the perch 
feed largely inshore during February, March, and April, and the two individuals just 
cited, which showed a high percentage of crystals, also contained bottom mud and plant 
remains. ‘The crystals may be accounted for on the supposition that calcium carbonate 
taken in through the mouth is dissolved in the stomach and that crystals form in the 
intestine as water is withdrawn during absorption. 
VARIETY IN FOOD AND ADAPTABILITY IN FEEDING. 
Knauthe (1907) pointed out that certain fishes, such as the trout and the perch, 
changed readily from one type of available food to another; but others, like the pike, 
the smelt, and the lota, made such changes with difficulty and hence died more often 
during scarcity of certain foods. The senior writer (1918) developed this idea still 
further in his studies of the shore fishes of Wisconsin lakes and also demonstrated that 
different species of fishes manifest a rather marked degree of specificity in choosing 
food. Each species, even though it may be versatile, shows preferences for particular 
foods, and some kinds of fishes select from a very limited number of foods. Fishes 
certainly select specific foods from those available, and it is only by examining the 
contents of their alimentary canals that preferences can be determined. Abundant 
foods are often apparently avoided in one lake by a particular fish, but the same food 
is eagerly eaten by it in another. As Knauthe (1907) says: “The value of materials 
as food must be determined biologically even more than phenologically or chemically.” 
One example will illustrate the fact that different species show specific preferences. 
On April 22, 1916, six species of fish were caught at the same time and place in Lake 
Wingra, and an analysis of the food eaten gave the following results: 
Ten breams (Abramis chrysoleucas) had eaten of Chironomus decorus larve, 3.5 per cent; Chironomus 
sp.? larve, 2 per cent; Cricotopus trifasciatus larve, 2.6 per cent; mayfly nymphs, 2 per cent; chi- 
ronomid pup2, 1 per cent; Cricotopus trifasciatus pupe, 1.5 per cent; Hyalella, o.1 per cent; ostracods, 
0.2 per cent; Canthocamptus, 1.5 per cent; Cyclops, 33.8 per cent; Daphnia pulex, 19.3 per cent; 
Chydorus sphericus, 5.1 per cent: Bosmina longirostris cornuta, 1.5 per cent; Physa, 2 per cent; Oscilla- 
toria, 4.7 per cent; alge, o.1 percent; flagellates, o.2 per cent; Volvox, 0.7 per cent; plant remains, 9 
per cent; fine débris, 9.1 per cent. 
110307°—21——21 
