THE DOUBLE-CRESTED CORMORANT. 23 



fish he captures, probably the highest possible efficiency of 

 the streams under present food conditions will be reached. If 

 this is done shelldrakes and kingfishers can be neglected 

 except to see that they do not increase to an abnormal extent. 



As the smolt go to sea they pass the gauntlet of the cor- 

 morants, but these, I think, we see, have little or no effect upon 

 their number and can be neglected. 



What happens at sea to the smolt, grilse, and salmon at pre- 

 sent none can tell. In the teeming abundance of marine life 

 their food can be assumed to be plentiful. The rapidity with 

 which a fingerling smolt grows to a five pound grilse is sufficient 

 evidence of this. The most serious limitation to numbers 

 at sea must come from the salmon's many enemies. In the 

 final stage of the salmon's existence before maturity lies pro- 

 bably the factor that determines how many will re-ascend the 

 streams to procreate their species and incidentally become 

 available for human use. As said before, a food species is 

 consumed by its enemies until it becomes too scarce to be profit- 

 ably hunted. A notable increase of food supply attracts new 

 consumers and the resultant population is apt to be little, if any, 

 greater than before. With the enemy factor controlling the situ- 

 ation the number of resultant food fish seems to become a matter 

 of population per unit area of ground occupied. 



For example, assume that ten salmon per acre is scarce, 

 i.e. that population is too scattered to be profitably hunted, and 

 the expended energy in finding and capturing a meal of salmon 

 is considerably greater than would be expended in pursuit of 

 other species or in other quarters; the salmon under these 

 conditions and assumptions will cease to be systematically 

 hunted by its enemies and, except for occasional and accidental 

 encounters, will enjoy comparative immunity. Should the 

 population be suddenly increased to fifty or a hundred per acre, 

 it will be salmon season for their enemies who will abandon 

 other usual prey for the new abundance. Should the resident 

 enemies find more than they can consume neighbouring com- 

 petitors will be attracted, and it will not be long before the 

 population is reduced again to the old ten per acre and com- 

 parative peace will be resumed. 



