8 U. S. BUREAU OF FISHERIES. 



The survey of resources has resulted in opening new fields, and it 

 helps also to furnish the necessary basis of information for estimate 

 of the perpetuity of the resources, for the adoption of intelligent 

 protective measures, and for guidance m the work of propagation. 

 The special investigations and experimental studies have led to im- 

 provements in method of propagation and are pointing the way to 

 further improvements. By investigations in the field and by con- 

 tinued observation of the industries, the Bureau has arrived at a 

 better understanding of the measures necessary for effective con- 

 servation of mussels; and, by propaganda, correspondence, publica- 

 tions, and personal conferences, it has been enabled to stimulate 

 more general interest in the subject and to cooperate with State 

 authorities and others in the framing of suitable protective measures. 



The enactment and enforcement of such measures must be left 

 to the several States, but a commendable interest has been shown 

 in some States, and a beginning has been made, notably in Wisconsin 

 and Minnesota. 



Mussel propagation is carried on by field parties engaged at various 

 places on important rivers. Fishes are seined from the rivers or 

 from overflow waters, are infected with the glochidia (the larval 

 forms of mussels) and then liberated again in the public waters. 



The methods of propagation are based upon a peculiar feature of 

 the normal course of development of fresh-water mussels. The 

 very young fresh-water mussels, with rare exception, when first 

 liberated from the incubation pouches of the parent, must become 

 parasitic upon fish in order to pass through the next stage of their 

 existence. To this end, if the chance offers after liberation, the 

 young mussels, or glochidia as they are called in this stage, attach 

 themselves to the gills, fins, or scales of a fish. The mussels of 

 economic importance attach themselves almost exclusively to the 

 gills. In attaching, or biting on the fish, a very slight wound seems 

 to be caused, which begins at once to heal over; but, in the process 

 of mending, the glochidium is overgrown and thus inclosed within 

 the tissues of the fish. The mussel is now actually an internal 

 parasite, i^ which condition it remains for a period of two weeks, 

 more or less. It is thus conveyed wherever the fish goes, until, 

 when the proper stage of development is reached, it frees itself from 

 the host and falls to the bottom; if, through favorable fortune, it 

 finds suitable lodgment, it continues its growth to form an adult 

 mussel. 



The glochidia are so small that the infection, if not excessive, has 

 no apparent injurious effect upon the fish that serves as host. In- 

 vestigations by the station have shown that mussels do not attach 

 to fish indiscriminately, but that for each species of mussel there is 

 a limited number of species of fish which may serve as hosts. 



The task of propagation is to bring together suitable fish and the 

 glochidia of mussels. Careful studies of natural and artificial in- 

 fections show that a moderate-sized fish may successfully carry in 

 parasitism from 1,000 to 2,000 of the microscopic glochidia, but 

 that under the chance operation of nature few of the glochidia find 

 lodgment upon the proper fish or upon any fish. 



During the fiscal year 1920, in round numbers, 183 million glo- 

 chidia were liberated in parasitic condition. A considerable pro- 

 portion of these glochidia undoubtedly fall upon unfavorable ground 



