THE FERTILIZATION OF GREAT CENTRAL LAKE 

 III. EFFECT ON JUVENILE SOCKEYE SALMON 



W. E. Barraclough and D. Robinson^ 



ABSTRACT 



Nutrient levels and rates of primary production in nursery lakes are factors which may limit production 

 of sockeye salmon. This paper describes the effect of artificial fertilization on feeding behavior and 

 growth of juvenile sockeye salmon in Great Central Lake, Vancouver Island, British Columbia. Under- 

 yearling sockeye salmon grew 30% larger in 1970 than in 1969 as a result of adding 100 tons of fertilizer 

 to Great Central Lake. The growth pattern for the whole population was complex, however, and the 

 increase in size of juvenile sockeye was not as much as had been expected from the increase in quantity 

 of their food organisms. The fact that the sockeye did not appear to appreciably crop the high epilem- 

 netic concentrations of zooplankton during July and August 1970 may have been partly due to avoid- 

 ance of high temperatures by the fish. 



Decomposing carcasses of anadromous fish, such 

 as the sockeye salmon {Oncorhynchiis nerka) , 

 contribute to the fertilization of nursery lakes 

 following spawning in the lake. In most instan- 

 ces the extent of this fertilization is not known 

 but the removal of maturing sockeye by a com- 

 mercial fishery may deny lake waters of their 

 essential nutrients and contribute to lowered 

 productivity. Particular attention has been fo- 

 cussed on the imbalance of phosphate in the na- 

 tural fertilization of lakes from decomposing 

 salmon carcasses (Krokhin, 1959) and the sug- 

 gestion has been made (Krokhin, 1967) that a 

 positive balance should be maintained by the ar- 

 tificial replacement of the phosphate with inor- 

 ganic fertilizers. 



Early studies carried out in a small unstrati- 

 fied lake in Alaska (Nelson and Edmondson, 

 1955; Nelson, 1958) showed that the addition of 

 phosphate and nitrate fertilizer resulted in in- 

 creased length and weight of sockeye smolts 

 leaving the lake. The potential role of a natural 

 imbalance of phosphate in nursery lakes on sock- 

 eye salmon is emphasized in the following quo- 

 tation from Foerster (1968): 



One wonders whether sufficient significance has been 

 given to this feature of the phosphate balance. With 



^ Fisheries Research Board of Canada, Biological 

 Station, Nanaimo, B.C., Canada. 



Manuscript accepted September 1971. 



FISHERY BULLETIN: VOL. 70, NO. 1, 1972. 



sockeye populations in all areas showing such evident 

 declines, despite legislation on regulation and limita- 

 tion of fishing, it might well be that some basic factor 

 such as this may be having a much more limiting 

 effect on productivity than seems apparent. In addi- 

 tion to the smaller amounts of phosphorus introduced 

 into a lake in the carcasses of fewer sockeye spawners, 

 there may also be occurring a steady decline in the 

 phosphate content of the runoff waters as the phos- 

 phates of the soil and rock become leached out over 

 the years. Future studies of the phosphate balance 

 of sockeye-producing waters and the direction of its 

 trend may prove most enlightening. Addition of suit- 

 able fertilizers may be found advantageous. 



In recent years it has become more evident 

 that suitable fertilizers should not only include 

 phosphates but also other nutrients, including 

 trace elements, in order to increase aquatic pro- 

 ductivity (Goldman, 1960, 1964). 



The theory and application of adding natural 

 fertilizers to aquatic environments has been 

 practiced in fish farming for many centuries. 

 Parsons et al. (in press; 1972) have presented 

 data on various aspects of lake fertilization stud- 

 ies carried out by others. In summary of these 

 findings, there is much evidence to show that the 

 larger the sockeye smolts at the time of seaward 

 migration, the higher the percentage return from 

 the sea (Burgner, 1962; Ricker, 1962). Since 

 food supply is one of the important factors 



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