PROGRESS IN BIOLOGICAL INQUIRIES, 1921. 95 
level continuously maintained above the stubble for a week or 10 
days, when the cat-tails will have been killed out. It is essential to 
flood the stubble immediately after mowing in order to get ahead of 
the rapid growth of the cat-tails. If in consequence of stored oxygen 
in the severed roots a few weakling shoots should appear above the 
water surface, they should be pulled out; when this is done a few 
times the plants will not reappear. 
STUDIES OF FISH DISEASE. 
The position of fish pathologist in the Bureau was vacant during 
the last half of the fiscal year, the last incumbent having left the 
service at the end of December after a year’s service. Some stucles 
of diseases and parasites were also conducted by other investigators 
principally in temporary employ. 
MORTALITY IN PIKE-PERCH EGGS. 
The great losses in the hatching of pike-perch eggs have made this 
phase of fish culture a ground of repeated inquiry, although as yet 
no investigation has been carried to a conclusion revealing the exact 
causes of the high death rate. Further observations during the past 
fiscal year were made by Dr. Franz Schrader, who inquired into the 
practices of handling fish and eggs and investigated the actual 
changes taking place within the abnormally developing eggs. His 
observations point to the conclusion that the original cause of ab- 
normal development and mortality of eggs is to be sought in physio- 
logical injuries sustained by the parent fish while held in captivity . 
before spawn can be taken. The penning of fishes prior to spawning 
is a practice of long standing in the artificial propagation of pike 
perch and other species of fish. Some species withstand the confine- 
ment very well, while others manifest such ill-effects as hardening 
of ovaries, wateriness of milt, and low percentage of hatched fry— 
in short, degeneration of eggs and sperms. The general inference of 
the investigator finds some confirmation in the investigations re- 
ported in the next paragraph and also in the experiences of several 
superintendents who have observed that the mortality of pike-perch 
eggs is proportional to the length of time the fish have been retained 
in pens, and conversely, that the percentage of hatch is greater in 
the case of eggs from fish stripped as taken from the nets than in the 
case of eggs taken from fish that have been held in pens. 
ABNORMAL CONDITIONS OF FISH OVARIES. 
Dr. Schrader also conducted inquiries into ovarial conditions in 
fishes with the purpose of throwing light upon various difficulties 
arising in hatcheries, including the loss of spawning fishes, ovarian 
diseases, and failure of fertilized eggs to hatch, troubles which 
possibly have some relation to abnormal conditions of confinement, 
handling, or stripping. Observations were made upon developing 
immature eggs of the scup, the retention of eggs beyond the normal 
eriod of spawning, the reabsorption of retained eggs, and the 
Rididteninie of ovaries. Abnormally retained eggs show signs of 
degeneration in course of one or two weeks. Reabsorption takes 
