332 BULLETIN OF THE BUREAU OF FISHERIES. 
3. That the only time when perch were caught in the surface net was at 5 a. m. 
These results indicate that perch migrate from the region of the thermocline toward 
the surface during the night, but the number of observations is small and should be 
extended. In the experiment summarized in Table 30, where the deep net was exactly 
on the thermocline, not a single perch was caught during the early morning hours. 
All catches in Tables 28 and 30 marked with an e were taken ashore; all others 
were thrown back as soon as they were removed from the net. It will be noted that, 
when the fish were not put back, the next catch was not appreciably smaller. The 
first catch in each experiment should have been larger, if all other conditions were the 
same, for the nets remained in the water exactly 4 hours. Pulling a net and the removal 
of the fish occupied from 2 to 25 minutes, which would make the periods of time for 
the various catches after the first somewhat less than 4 hours. The fact that as many 
fish were caught during the next 4 hours, when an entire catch was removed from that 
region of the lake, as when they were put back indicates that, though perch keep to a 
particular depth, which varies somewhat with the time of day, they do not remain in 
one locality, but continually swim along the shore.* 
One other aspect of the migratory activities of perch remains to be considered. 
This is their habit of swimming in schools. Meek (1916) states that soon after hatching 
certain species of marine fishes form schools which retain their unity for several years. 
He also says that schools of fresh-water fishes are much more likely to mix; fishes of 
different ages, and even of different species, may keep together. Schools of perch have 
been observed at various times in shallow water in the two lakes under consideration 
in this paper. These usually consisted of fish of about equal size—large, medium, or 
small. They have been seen alongshore, among aquatic plants, and in the open lake 
both at the surface and at a depth of 3 or 4m. For example, during the latter part of 
August, 1916, a school of about a thousand young perch remained alongshore in one 
locality, just north of the University of Wisconsin, for more than a week. Hankinson 
(1908, 1916) and Reighard (1915) report similar schools of young perch in Michigan 
lakes. Catches in gill nets also indicate that perch swim in schools when in deep water. 
A net set in one spot and examined at intervals might catch nothing for several hours 
and then be filled in a few minutes (Table 30). A similar thing often happened when 
fishing with a hook and line. Furthermore, a 60-foot gill net might have 50 or 75 perch 
in one end and not a single individual in the other. All these observations signify that 
perch swim in schools throughout life. 
ENEMIES AND PARASITES. 
PREDATORY ENEMIES. 
In the Wisconsin lakes perch pay the penalty for exceeding other fishes in abundance 
by being preyed upon by a number of predacious animals. Among the fishes the pickerel 
(Esox lucius) appears to be the species which most commonly feeds upon perch. During 
the year 1916 the following records were secured from Lake Mendota: 
@ During the summer of 1917 additional evidence was secured which supports this view. Nine hundred and sixty-six perch 
were caught in gill nets at three stations in Lake Mendota. An aluminum tag was fastened to the dorsal fin of each, and they 
were then returned to the Jake. Although fishing with nets was continued for a total of 33 days at the three places, only one of 
the tagged fishes was caught a second time. 
