FISHERY BULLETIN: VOL. 70, NO. 1 



is at depths between 10 and 20 m while some 

 fraction of the population occur in the upper 

 10 m. This pattern of vertical migrations ap- 

 pears to be repeated daily (Barraclough and 

 Robinson, 1972). Narver (1970) has reported 

 similar vertical movements for sockeye popula- 

 tions in Babine Lake. For the greater part of 

 the day the salmon in Great Central Lake are 

 at temperatures of 4° to 5°C, a somewhat shorter 

 period (ca. 6 hr) is spent at temperatures 

 of 6° to 12°C (10 to 20 m depth) while a rel- 

 atively brief period (ca. 1 hr) may be spent 

 at temperatures ranging from 14° to 23 °C (0- 

 10 m depth) . Details of the time actually spent 

 at different depths by the sockeye are reported 

 by Barraclough and Robinson (1972). It is 

 apparent that the fish are utilizing the maximum 

 concentrations of prey which occur at above op- 

 timum temperatures in the upper 10 m for very 

 short intervals. Consequently in assessing the 

 relationship between the increased abundance 

 of prey brought about through fertilization and 

 the sockeye it should be noted that possibly 60 Sr 

 of the total biomass, i.e. the portion in the upper 

 10 m, may be only partially available to the fish 

 (Table 6) . Furthermore, some prey species be- 

 cause of their size (rotifers) or structure (Holo- 

 pedium) may not be a particularly useful food 

 source for the salmon. Holopedium, for ex- 

 ample, was among the largest and most numer- 

 ous species of crustaceans in the lake; however, 

 a large fraction of their biomass is comprised 

 of a gelatinous material of dubious food value. 

 The difference in the wet to dry weight ratio 

 between Holopedium and other zooplankton 

 (14% to ca. 26^/ -respectively) attests to the 

 water composition of Holopedium. The quality 

 of prey together with the observations of Foers- 

 ter (1968) and Brett et al. (1969) empha- 

 size the need for caution in interpreting preda- 

 tor-prey relations. In the present instance, the 

 benefits of the fertilization appear to have been 

 only partially transferred to the sockeye salmon. 

 Since the thermal structure of the lake is a 

 factor beyond immediate control, it would be in- 

 teresting to consider possible benefits from the 

 addition or deletion of some prey species and to 

 attempt to shift the level of primary and sec- 



ondary production to depths and temperatures 

 favoring sockeye salmon growth. 



LITERATURE CITED 



Barraclough, W. E., and D. G. Robinson. 



1972. The fertilization of Great Central Lake. 



III. Effect on sockeye salmon. Fish. Bull., U.S. 70: 



37-48. 

 Brett, J. R., J. E. Shelbourn, and C. T. Shoop. 



1969. Growth rate and body composition of finger- 

 ling sockeye salmon, Oncorhynchiis nerka, in re- 

 lation to temperature and ration size. J. Fish. 

 Res. Board Can., 26: 2363-2394. 



Brocksen, R. W., G. E. Davis, and C. E. Warren. 



1970. Analysis of trophic processes on the basis 

 of density-dependent functions. In J. A. Steele 

 (editor), Marine food chains, p. 468-498. Oliver 

 and Boyd, Edinburgh. 



Foerster, R. E. 



1968. The sockeye salmon, Ovcorhynchus nerka. 

 Fish. Res. Board Can. Bull. 162, 422 p. 

 Ivlev, V. S. 



1961. Experimental ecology of feeding of fishes 

 (Transl. by D. Scott). Yale Univ. Press, New 

 Haven, 302 p. 

 Johnson, W. E. 



1965. On mechanisms of self-regulation of popula- 

 tion abundance in Oncorhynchiis nerka. Mitt. 

 Int. Ver. Limnol. 13: 66-87. 

 Miller, D. 



1961. A modification of the small Hardy Plankton 

 Indicator for simultaneous high speed plankton 

 hauls. Bull. Mar. Ecol. 5: 165-172, 



Narver, D. 



1970. Diel vertical movements and feeding of 

 underyearling sockeye salmon and the limnetic 

 zooplankton in Babine Lake, British Columbia. 

 J. Fish. Res. Board Can. 27: 281-316. 



Parsons, T. R., C. D. McAllister, R. J.LeBrasseur, 

 AND W. E. Barraclough. 



In press. The use of nutrients in the enrichment 

 of sockeye salmon nursery lakes — a preliminary 

 report. FAO Technical Conference on Marine 

 Pollution, Rome, 9-18 December 1970. 

 Parsons, T. R., K. Stephens, and M. Takahashi. 



1972. The fertilization of Great Central Lake. 

 I. Effect on primary production. Fish. Bull. U.S. 

 70: 13-23. 

 Richer, W. E. 



1962. Comparison of ocean growth and mortality 

 of sockeye salmon during their last two years. 

 J. Fish. Res. Board Can. 19: 531-560. 



Schindler, D. W., and B. Noven. 



1971. Vertical distribution and seasonal abundance 

 of zooplankton in two shallow lakes of the exper- 

 imental lakes area, northwestern Ontario. J. Fish. 

 Res. Board Can. 28: 245-256. 



36 



