In pink salmon (Oncorhynchus gorbuscha) the survival during the freshwater phases of the life-cycle has been found to vary from about 1 to 24 per cent of available eggs. Natural survival during marine existence is considered to average about 5 per cent. In the central region of the British Columbia coast the annual catch averages about 60 per cent of the adult fish. Populations maturing in "even" and "odd" years vary in size independently. Fluctuations in level of stock originate mainly in fresh water.Population levels and changes are determined by the combined effects of three types of mortality: (a) mortality which becomes relatively heavier as populations increase in density (compensatory); (b) mortality which becomes relatively heavier as populations decrease in density (depensatory); (c) mortality which is independent of density (extrapensatory). Compensatory mortality is especially identified with the, period of spawning and incubation. Depensatory mortality is considered to occur mainly during the period of fry migration and to be due to predation. Extrapensatory mortality may occur at any stage; it is most variable during the period between entrance of the adults into fresh water and emergence of the free-swimming fry.Populations of chum salmon (O.keta) are controlled by similar influences. Effects are modified by higher egg-production and a less rigid life-span than in the pink salmon.