20 REPORT OF THE COMMISSIONER OF FISHERIES. 



known, notwithstanding the work of a number of investigators. A. 

 number of young fish are now being reared at the laboratory, but 

 they have not yet reached the spawning age. 



Tlie hackleback sturgeon is another highly prized fish likewise 

 imperiled by the fishery as now conducted, and drastic control will 

 be necessary if it is not to be economically exterminated. Unlike 

 most fishes, the hackleback does not spawn annually. The rock 

 sturgeon is in the same category with the hackleback in respect to 

 its individual value and waning numbers, and both species have been 

 the subject of study and experiment for the development of prac- 

 tical regulatory and fish-cultutal measures for their conservation. 



The bureau's fish pathologist has been engaged principally in the 

 investigation of certain diseases that have produced serious losses of 

 fish at various hatcheries and during distribution of the young. 

 One of the most widespread and destructive of these maladies affect- 

 ing trout w^as found to be due to an intestinal parasite {Octomihis 

 salmonis) which causes fatal enteritis. No means for directly com- 

 bating this disease have been developed, but palliative measures, 

 such as the avoidance of overcrowding and the maintenance of i 

 lower water temperature during transportation, have decreased the 



mortality. 



The brood stoclc of bass and. to a minor degi'ee, other pondfishes 

 at Neosho (Mo.) station has developed from time to time numerous 

 cases of sterility, which have seriously curtailed the output of the 

 station. An investigation during the past year has shown this to be 

 due to an ovarian infection by the larvse of a tapeworm. 



During the latter part of the year a series of experiments for 

 developing a more satisfactory diet for trout was inaugurated at 

 the hatchery at Manchester, Iowa, and the laboratory at Fairport, 

 Iowa, under the direction of the fish pathologist. In the light of 

 present knowledge of mammalian nutrition, to which much atten- 

 tion has been directed in recent years, it appears that the foods now 

 used in hatcheries may be deficient in certain essential constituents, 

 particularly vitamines and salts. It is not improbable that these 

 deficiencies may contribute to the troubles encountered by fish- 

 culturists, and it is hoped that a more fully adequate ration can be 

 devised as a result of the current investigation. 



The pathologist and several members of the staff of Fairport 

 laboratory have made a number of investigations of epidemics oc- 

 curring both in wild waters and in State and private hatcheries. 



FRESH-WATER MUSSELS. 



Investigations and experiments relating to the pearl-button 

 mussels of the Mississippi Kiver drainage system have been con- 

 tinued in the interests of improvement in the methods of mussel 

 culture and other essentials for the conservation of these economi- 

 cally important mollusks. 



Studies of the factors affecting the survival and growth of juvenile 

 mussels after they have passed the parasitic larval stage have been- 

 attended with results warranting the establishment of a small rear- 

 ing system, consisting of 150 metal troughs with a capacity of about 

 750,000 mussels, an important feature of which is the exclusion 

 of light. The indications are that more tangible and better- 



