444 U. S. BUREAU OP FISHERIES 



tlie average catch per lioat was 3.6 fish. The average length of the 

 rental period was slightly over 4 hours. Of the 2,014 marked rain- 

 bows, 349 or 17 percent of the total plant were caught. By using 

 the proportion of marked to unmarked rainbows caught, it is pos- 

 sible to estimate roughly the total rainbow population of Convict, 

 Lake. Catches of shore fisliermen were likewise recorded ; and trout 

 caught both by this group of fishermen and persons fishing from, 

 boats totaled 848 fish. Of this number 25 percent were loch leven, 

 33 percent unmarked rainboAv, and 41 percent marked rainbow of 

 the 1935 plant. Approximately 76 percent of all fish taken were 

 caught by shore fishermen while 24 percent were taken from boats. 

 Boat fishermen caught larger fish hence more pounds than shore 

 fishermen obtained, but they caught fewer fish. By determining the 

 catch per unit of effort annually a definite measure of the trend of 

 the fishery in the lake will be obtained. Thus, the effects of any 

 system of management, stocking, or restrictions put into effect by 

 fish and game authorities can be ascertained. 



FISH DISEASES 



As in previous years, the study of fish diseases was largely con- 

 fined to investigations of epidemics at hatcheries and the develop- 

 ment of better methods of control. The establishment of a patho- 

 logical laboratory at Seattle, Wash., in charge of Dr. Frederic F. 

 Fish has made it possible to devote more attention than formerly 

 to disease problems in the intermoimtain region and the Pacific 

 Coast States. 



The major investigations of the West Coast Pathology Laboratory 

 during the past year have been directed towards a reduction in the 

 toll of salmon and trout taken by diseases in the hatcheries. As the 

 presence or absence of disease largely determines the advantage of 

 artificial over natural propagation, the work of this laboratory has 

 been directed chiefly toward preventing diseases rather than attempt- 

 ing to develop new methods for treating them after they have ap- 

 peared. The prophylaxis of fish diseases represents a practically 

 unknown field, and many preliminary problems had to be satisfac- 

 torily solved before a suitable technique could be evolved. The 

 method chosen for experimental disease prevention work w'as the 

 routine application of a constant and weak concentration of a dis- 

 infectant to tlie running water supply of a hatchery trough for a 

 uniform period of 1 hour at vreekly intervals. This method, com- 

 monly known as "prolonged dipping", was favored because it in- 

 volves a minimum amount of labor for its application, and does not 

 necessitate the handling of the fish in any way. Consequently it 

 solves the disease-control problem for the one-man hatchery. 



Although it has been definitely demonstrated that at least certain 

 parasitic diseases of trout may be prevented by routine prophylaxis, 

 disease-prevention work is still in the experimental stage. The losses 

 attending prophylaxis are materially lower than those occasioned 

 by therapeutics. Attempts to cure epidemics of parasitic protozoa 

 have invariably involved considerable losses of fish even though the 

 treatments were carefully administered at the first indication of 

 trouble. The disinfectants available for disease-prevention work are 



