336 BULLETIN OF THE BUREAU OF FISHERIES. 
larve as well as the bass, crayfishes as well as the dogfish, small minnows as well as the 
gar. In the open lake the perch’s chief competitors for food are the cisco and the 
white bass, but neither of these fishes excels it in ability to strain plankton from the 
water. In the deeper regions of lakes the perch must contend with the vegetarian 
and bottom-feeding sucker, cottid, and carp, and with the predacious pickerel and lota. 
The sucker, cottid, and carp are real rivals when it comes to bottom feeding, for they 
are especially able to take advantage of the nourishment in the bottom mud.? They 
are also better protected, by reason of their size, from the attacks of the predacious 
deep-water fishes; but their large size, on the other hand, limits their numbers, and 
they can never compare with the perch in this respect. These bottom feeders are 
limited, however, in times of scarcity or when they are driven into shallow water by 
stagnant conditions in the depths. They can not then feed as well as the perch in 
pelagic or littoral regions. 
Perch are, then, more abundant in lakes than other kinds of fishes because they 
are of intermediate size and because they are better able to secure food from all ayail- 
able habitats and at all seasons of the year. 
There are probably a number of factors which cause perch to attain a certain 
characteristic maximum size in different lakes. This is a phenomenon which is not 
confined to perch alone but has been noted in other fishes. It is apparent, for example, 
in the ciscoes in various Wisconsin lakes, and has been observed in other localities 
in various parts of the earth. Petersen and Jensen (rg11) state that the plaice in a 
certain estuary ceased to grow for two-thirds of a year, whereas some which were 
transplanted quadrupled in size during the same period of time. They believe that 
the discrepancy in this instance was due to differences in food. The present authors 
believe that their comparison of the habits of perch and their conditions of life in Lake 
Wingra and in Lake Mendota have shed some light on the causes for such contrasts, 
and they feel that they can, at least in part, give specific reasons why the perch are 
smaller in the former lake. 
The shallowness of Lake Wingra is probably the chief cause for the small size of 
its perch. ‘The limitation of perch to a stratum of water 3 m. in thickness, between the 
air above and soft muddy bottom below, causes many unfavorable conditions. Winds 
stir up the whole body of water; thus movement and feeding are often made difficult 
or impossible. Knauthe (1907) has made the generalization that in two ponds of equal 
capacity in other ways the quieter one will be the more productive for rearing fish. 
On account of the shaliowness of the water in Lake Wingra there are wider and 
more rapid variations in temperature. The water is all warm in summer; there is no 
possibility of retreat into cool, quiet depths; and consequently the perch in this lake 
pass through a period in late summer when little food is eaten (Table 22). In winter, 
the perch in Lake Wingra move about very little and hence feed less than those in Lake 
Mendota (Tables 2 to 5, 22, and 27; figs. 3 to 5). Though oxygen was always present 
in quantities sufficient for respiration, many of the fish caught in gill nets in Lake 
Wingra died during the warmer months, when the water was murky with alge and 
other organic or sedimentary products. 
The perch in Lake Wingra, on account of the earlier warming of the water, breed 
before those in Lake Mendota, when the season is less advanced and food is less 
aT he importance of this deposit as a source of food has been pointed out in a masterly wav by Petersen and Jensen (1911). 
