PROGRESS IX BIOLOGICAL IXQUIEIES, 1921. 25 



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 studies 

 of diseases and parasites were also conducted by other investigators 

 principally in temporary employ. 



MORTALITY IN I'IKE-PERCII EGGS. 



The great losses in the hatching of pike-perch eggs have made this 

 phase of fish culture a ground of repeated incjuiry, 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 in(iuired into the 

 practices of handling fish and eggs and investigated the actual 

 changes taking place within the abnormally devek)ping eggs. His 

 observations point to the conchision 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 otliers manifest such ill-etl'ects as hardening 

 of ovaries, wateriness of milt, and low percentage of hatched fry — 

 in short, degeneration of eggs and si)erms. 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 h:ive been held in pens. 



AIIXORMAL CONDITIONS OF FTSH OVARIES. 



Dr. Schrader also conducted inquiries into ovarial conditions in 

 fishes with the purpose of throwing light upon various dilHculties 

 arising in hatcheries, including the loss of spawning fishes, ovarian 

 diseases, and failure of fei'tilized eggs to hatch, troubles which 

 possil^ly have some relation to abnormal conditions of confinement, 

 handling, or stripping. Observations were made upon developing 

 immature eggs of the sfup. the retention of eggs beyond the normal 

 period of spawning, the reid)Sorption of retained eggs, and the 

 hardening of ovaries. Abnormally retained eggs show signs of 

 degeneration in course of one or two weeks. Keabsorption takes 



