WHITE SUCKER 177 
In the streams (23 specimens): Chironomids, 62; Entomostraca, 4; diatoms, 8; 
Protozoa (Difflugia), 6; Rotifera, 7; fragments of arthropods, 7. 
The latter is the truer picture of the diet during this second stage. The dif- 
ferences are due to the lack of current in the pond, increasing the proportion of 
Euglena and Entomostraca (Cyclops and Bosmina), which flourish in still waters. 
It will be shown presently that chironomid larvae are common as floating objects 
in the streams. 
Sucker fry can be distinguished in the field, after a little practice, by their 
form and actions. They are more slender than dace and shiners of the same length, 
and seem always to prefer a slight current, in which they maintian a rather fixed 
position by a very characteristic motion, which deserves a word in passing. Whereas 
many other fry sw r im by swinging the whole body, the sucker fry move only the tail. 
Thus they never show the jerky darting so characteristic of young minnows. Pre- 
fering a moderate current, they are not to be found in the rapids nor in still ponds, 
but in intermediate situations — that is, where the stream enters deep, quiet stretches. 
Knowing this, one can predict with some certainty what portions of a stream will 
harbor the young fry. Also, they swim within 6 inches of the surface. While dozens 
of pools entirely cut off from the streams were examined and were often found to 
be teeming with minnows, sucker fry under 17 millimeters never were discovered in 
them. They are often found associated with the plant Elodea. 
During the stage from 12 to 16 millimeters, the fry, as we have said, maintains 
ts position in the moderate current within 4 to 6 inches of the surface and waits 
until food organisms float to it. Constantly they swim here and there (a matter of 
an inch or so), feeding on particles in suspension. On an average of once in a minute 
and a half they touch (taste?) or take organisms floating by. They are in small 
groups of 2 or 3 to 8 or 10. 
Of the food eaten, it is of interest to show that it may all be obtained in the 
floating condition. To establish this, the following tests were made: 
The surface mud of the bottom taken below a school of 10 fry contained Pro- 
toza (Stylonichia), Gastrotricha, diatoms, and rotifers, but no chironomid larvae. 
Evidently the latter were not obtained from the bottom. Then a handful of Elodea 
collected 4 inches below the surface, where 10 fry were feeding, was violently stirred 
in a shell vial of water, allowed to settle, and the sediment examined. This con- 
tained 20 chironomid larvae per cubic centimeter of sediment. 
To show that these larvae do become detached and float free in the water, a 
Birge net was set for 24 hours a few inches below the surface. By centrifuging 
and examining, two larvae were obtained in addition to Cyclops, rotifers, and desmids. 
The fry may pick these larvae from the weeds, though this was not observed. It is 
well known that Difflugia also floats. 
The sucker begins feeding when 12 millimeters long, approximately 9 days 
after hatching. The mouth is terminal but in process of becoming inferior. For 10 
or 11 days it is a top feeder or feeds close to the surface on floating microorganisms, 
principally chironomid larvae, Entomostraca, diatoms, rotifers, and Protozoa. 
Critical 'period, 16 to 18 millimeters . — As observed in the nursery pond, this 
period covered nine days. On June 7 (20 days old) the fry were seen to make occa- 
sional excursions to the bottom (16 inches) and pick up mouthfuls of the sand, etc. 
