l62 



BULLETIN OF THE BUREAU OF FISHERIES- 



Stirred from time to time in order to keep the glochidia in somewhat even suspension, 

 but in most cases the movements of the fish themselves insure a circulation of the water 

 and a general distribution of the glochidia. At inter\'als individual fish are taken by 

 hand or small dip net, and the gills examined with a lens; when, in the opinion of the 

 operator, a sufficient degree of infection has occurred, the fish are placed at once in 

 open waters, or transferred to other containers for conveyance to a place suitable for 

 their liberation. The rapidity with which infection takes place depends upon a variety 

 of conditions, such as temperatures of water, kind and size of fish, and activity of glochi- 

 dia. Ordinarily a period of from 5 to 25 minutes is sufficient to insure an optimum 

 infection. The infection time is usually shorter in warm water than in cold. As basis 

 for approximate computation of the number of glochidia planted, several average-sized 

 specimens of each species of fish infected are killed and the gills removed for subsequent 

 counts of the glochidia attached. The counting is done by the foreman with the aid of a 

 microscope and usually in the evening after the close of the field operations of the day. 

 The number of glochidia per fish of each species having been determined by the count of 

 representative examples, and the numbers of fish of the species being known, the entire 

 number of glochidia planted on a given lot of fish is easily computed. The data in detail 

 are promptly recorded on form cards pro^^ded for the purpose. The count of total 

 glochidia planted is of course only approximate, but the method of count and computation 

 described is as accurate as the conditions of operation permit, and it is as precise as the 

 methods of count generally practiced in fish-cultural operations. In the long run, the 

 actual errors on one side and the other must approximatelv balance. 



That degree of infection which employs the fish to best advantage in mussel propa- 

 gation, without doing appreciable injury to the host, is termed the "optimum infection." 

 It varies with the species of mussel and with the kind and the size of the fish. Table 23 

 gives illustrative instances. 



Table 23. — Optimum Infection for Cert.\in Species of Mussel on Several Species of Fish. 



Incidental to the field work in mussel propagation, valuable results are frequently 

 gained in the reclamation of fish from the overflowed lands bordering the various rivers. 

 All fishes rescued in connection with propagation work, whether suitable or unsuitable 

 for infection, are liberated in the open waters, and under such circumstances the value 

 of the fish thus saved in large measure recompenses for the cost of the mussel propaga- 

 tion work. 



The operations of mussel propagation as just described serve to carry the young 

 mussels through the most critical stage of the life history — to give to thousands the 



