The Effectiveness of Predator Control in Decreasing the Mortality of Young Sockeye Salmon (Oncorhynchus nerka Walbaum)

SUMMARY

1.	Because of the great commercial value of its fishery, much attention has been given to conservation of the sockeye salmon of British Columbia, and particularly those of the Fraser river. Active measures taken include (1) restriction of the commercial catch, and (2) the artificial fertilization of the eggs and their retention in hatcheries until hatched and the fry are free-swimming. While the former measure has provided sufficient escapement of spawners, in recent years, to check the decline of the fishery, it has been shown that the effects of artificial propagation have always been negligible, and this has recently been discontinued.
2.	Experiments at Cultus lake, a Fraser river tributary, showed that during their year of lacustrine life the young sockeye suffer a very heavy mortality, amounting to 96 percent of the fry released. Netting of the lake disclosed the presence of several predatory fish, whose depredations were believed to be responsible for a large share of this destruction. The more important of these are squawfish (Ptychocheilus oregonensis), trout (Salmo clarkii) and char (Salvelinus malma).
3.	From 1935 to 1937 intensive netting of the lake resulted in the destruction of many of the predators. In two years the decline in the population of medium to large squawfish was of the order of 88 percent, while the total reduction of char has been about 95 percent, the greater decrease being among the larger fish as a rule. The decline was evident from a decrease in the catch per unit fishing effort over the period, and from a decrease in the average length of fish taken in gill-nets of a given size of mesh. The reduction of trout was much less complete, if indeed it was perceptible, and they are the principal predators remaining in the lake.
4.	The destruction of predators has resulted immediately in an increased rate of survival of sockeye in the lake, the smolt migration of 1937 being at least four times what might be expected from returns in earner years. In two years the operations have increased smolt migrations by about 2,700,000, and the resulting increase in adults may be estimated at 270,000. The immediate increase in value of the commercial catch is about 75,000 dollars.
5.	In addition, the Cultus lake run of sockeye may be expected to increase rapidly, if predators are kept under control, until some other factor appears to set a limit to their numbers, and establishes the productive capacity of the lake for sockeye, under the new conditions. It is suggested that optimum utilization of the lake's productive resources would be attained by having the numbers of sockeye regulated, as closely to this productive capacity as possible, by the predatory activity of a valuable sport fish.
6.	The reduction of predators at Cultus lake has proved a much more efficient method of increasing the commercial catch of sockeye than the practice of hatchery propagation, and less costly. Whether it will prove equally feasible on other lakes is a matter which demands attention. The great promise shown by the method at Cultus lake warrants experimentation in other areas.