FISHERIES OF THE GEEAT LAKES. 459 



billion fry annually in a body of water as large as Lake Michigan would 

 be equivalent to only one fish to every 600 square feet of lake surface, 

 and when the great natural destruction of fry, by natural enemies and 

 unfavorable physical conditions, is taken into account, the disparity 

 of this proportion is vastly increased. 



2. Assuming that the plants of fry are sufficiently large to compen- 

 sate for the capture of fish now goiug on, the fish-cultural work may 

 not do more than maintain an already diminished supply and may be 

 inadequate, owing to previous depletion and present methods, to pro- 

 duce a substantial increase in the abundance of a given fish. 



3. Artificial propagation may be extensive enough to counteract 

 the influence of previous overfishing and permit the continuance of 

 fisheries of great magnitude under proper conditions, but the methods 

 followed may be so destructive to the adult fish prior to and during the 

 spawning season and so deleterious to the growth of young fish that 

 the natural tendency to multiply maybe made abortive and even enor- 

 mous fish-cultural operations be rendered nugatory. 



