330 BULLETIN OF THE BUREAU OF FISHERIES. 
One other possibility remained to be tested, however. During the period of stag- 
nation in the lower water the perch might remain on the bottom in the region of the 
thermocline or spread out over the whole lake to feed on the plankton organisms in the 
water containing oxygen. ‘The latter alternative seemed improbable from the fact that 
it is easiest to catch perch near the bottom at any season, but it was decided to perform 
an experiment to find out. Accordingly, on August 10, 1916, four 1-inch mesh, 3 by 60 
feet, gill nets were set north of the University of Wisconsin in Lake Mendota. At this 
time the thermocline was well established at a depth of 9 meters, and the gaseous content 
of the water (according to titrations by the Winkler method; Birge and Juday, 1911) at 
certain depths was as follows: At 18 m.—Oxygen, o.o1 c. c. per liter; carbon dioxide, 
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Fic. 34.—Perch caught in 3 by 60 feet, 1-inch bar mesh gill nets, Lake Mendota, 1916. Nets were left in the water for various 
periods of time, but those set on any particular day were left for the same length of time, and the catches for that day at 
different depths are therefore comparable. The curve represents the thermocline. ‘The ice left the lake on April 8; the fall 
overturn occurred October 5 to 10, f indicates 10 perch caught at 16 m. in about 2 minutes while washing net. 
10.31 c. c. At 14.5 m.—Oxygen, o.o1 c. c.; carbon dioxide, 4.1 ec. c. At 13 m.— 
Oxygen, 0.02 c. c.; carbon dioxide, 4.17 c.c. At 6.6 m—Oxygen, 4.49 c. c.; carbon 
dioxide, oc. c. One net was set at 19.2 m. on the bottom; another was set where the 
water was 19 m. deep, but the net was fastened to eight weighted 11 m. lines, so that 
it floated just above the thermocline; a third was set on the bottom where the water 
was 8 to 9.2 m. deep; the fourth was set on the bottom at a depth of 3m. All of these 
nets were set at right angles to the shore line and were placed in a straight line from deep 
to shallow water. ‘They were left in the water three hours (9.45 to 10.30 a. m. to 12.45 
to 1.30 p. m.). Nothing but perch was taken in the nets, and the catches were as fol- 
lows: On bottom at 19.2 m., 0; at a depth of 8 m. above bottom 19 m. deep, 0; on 
bottom at 8 to 9.2 m., 118 (49 alive, 69 dead; males, 42 dead, 17 alive; females, 27 dead, 
