178 
BULLETIN OF THE BUREAU OF FISHERIES 
For a day or so preceding this they had fed farther from the surface, but not at the 
bottom. On June 12 (25 days old) three-fourths of the time was spent at the bot- 
tom. On June 16 (29 days old), none was feeding above the bottom. While micro- 
scopic examinations of the contents of the alimentary canal were made almost daily, 
the following average percentages of the more important constituents of the diet, 
derived from nine examinations, will suffice to show the facts : Chironomid larvse and 
pupae, 19 per cent; Entomostraca, 15 per cent; diatoms and desmids, 10 per cent; 
Protozoa, 2 per cent; Rotifera, 20 per cent; sand, 27 per cent; unicellular algae, 2 per 
cent. Total — animals, 56 per cent; plants, 12 per cent; sand, 27 per cent. 
The presence of sand is noticeable. Some of this is found in every specimen (even 
as much as 70 or 80 per cent of the volume in some cases), but it is also noticeable 
that diatoms and desmids are the only other items constantly present. These 
three items together form 37 per cent of the food. 
With the change in feeding habit there appears also a new instinctive reaction. 
Whereas the early fry never enter the deeper water when alarmed, these later fry 
retreat to the bottom when one approaches them. Quite a few specimens of this 
age show that the alimentary canal is entirely empty, but when the bottom-feeding 
habit is established it is regularly well filled, if only by sand and diatoms. 
Fingerling period . — This stage covers the time during which the young fish 
(25 days to 2 years old, 18 to 75 millimeters long) feeds upon bottom forms but 
can not separate them from the sand, which, therefore, constitutes nearly 25 per cent 
of its intestinal contents. It differs from the critical period in the fact that the 
bottom-feeding habit is firmly established. It is separated from the* following or 
adult period by the fact that the food includes only microorganisms and no nymphs 
and larvse of the larger insects. 
The feeding habits are now quite different from those of the previous periods. 
The bottom selected for feeding is usually a stretch of uniformly fine sand or silt, 
covered with a thin film of such organic matter as diatoms, desmids, Protozoa, etc. 
The feeding grounds for fish of the first summer (up to a length of about 40 milli- 
meters) is in water 6 to 18 inches in depth along the shores of lagoons, eddies, or 
ponds connected with the stream. For those 40 to 75 millimeters in length the 
feeding grounds are in the swifter-flowing water, as in the case of the adults, but in 
smaller, shallower streams. 
New instincts are manifested and others are emphasized. In this stage a strong 
gregarious habit is apparent, especially during the first summer. The fingerlings 
of the first summer invariably are found in schools. These contain from a dozen 
or so to four or five hundred individuals. When feeding they usually spread out 
like sheep, advancing slowly, all facing the shore and keeping side by side. They 
retreat into deeper water when alarmed, but return within a few minutes to continue 
their almost constant plucking at the bottom ooze. 
They are very protectively colored at this stage, being a mottled, sandy gray 
above and quite translucent. The sides are yellowish and greenish and flash when 
turned upward. Occasionally in very still water a large school (200 to 500) will be 
seen resting within an inch of the surface. They then face in all directions and are 
motionless. At the least cause for alarm the whole school disappears and remains 
in deeper water. 
