BUREAU OF FISHERIES 325 
the growth rates of the various species which indicate the minimum 
size at which it is desirable to subject these fish to commercial exploi- 
tation. Another study which has now yielded useful information is 
an extensive tagging project carried out in cooperation with a com. 
mercial fishing firm of Port Washington, Wis. Of the 2,902 Lake 
Michigan fish that were tagged and released, 88 percent were recap- 
tured, the results disclosing important information on the extent of 
migration and the intensity of fishing operations. A study of the 
age and growth of the yellow perch of Saginaw Bay indicates that 
the present legal length of 814 inches possibly is too low, inasmuch 
as yellow perch of both sexes attain this length just as they are 
entering on the period of most rapid increase in weight. 
Shellfish investigations.—Research on oysters was conducted during 
the year, in accordance with the established policy of the Bureau, (1) to 
increase technical knowledge of the methods of propagating oysters 
under various conditions in the coastal waters; (2) to find practical 
means of improving the quality of marketable oysters; (3) to improve 
methods of protecting oysters against various pests and parasites; and 
(4) to determine the effects of pollution and devise means of rendering 
certain trade wastes harmless to oysters. 
Studies of the factors controlling the spawning of oysters have re- 
vealed that the old concept of a single critical temperature that induces 
spawning in oysters is no longer tenable. Depending on the physiolog- 
ical state of ripeness which may be influenced by proper chemical 
stimulation, the oyster may spawn at various temperatures ranging 
from 63° to 95° F. These physiological studies have provided a key to 
a practical method of controlling the spawning of oysters under 
natural conditions. 
In compliance with the request of local oystermen, the Bureau’s 
Milford (Conn.) Station has continued its custom of issuing weekly 
bulletins on the physiological condition of the oysters in Long Island 
Sound and the expected time of setting. To this information there 
have now been added reports on the distribution and abundance of 
starfish, the principal oyster enemy in this region. 
Experiments in North Carolina demonstrated the value and prac- 
ticability of growing oysters in improved marsh ponds and canals 
where the depth and velocity of the currents can be controlled. In 
such areas it is possible to prevent the “smothering” of seed obtained 
from the early summer spawning by the spat of the September spawn- 
ing. Even from poor quality seed a growth in volume of over 300 
percent was obtained during the first year. By holding seed in 
special trays it was found possible to cultivate oysters at concentra- 
tions equivalent to over 2,000 bushels per acre. These experiments 
are being continued and developed on a small commercial scale to 
