LeBRASSEUR and KENNEDY: LAKE FERTILIZATION. II. 



ing stock size were able to utilize the higher rates 

 of primary production. 



It is also apparent that the higher biomass in 

 1970 cannot be entirely attributed to fertiliza- 

 tion since the biomass in May (prefertilization) 

 was also higher than any of the 1969 values. 

 However, the nearly continuous production of 

 eggs by most species and the maintenance of an 

 increased standing stock over a 6-month period 

 are indicative of a direct relation between zoo- 

 plankton and nutrients. It is also noteworthy 

 that there was no change in species diversity. 



The techniques employed for wet weight de- 

 terminations in this study have produced weights 

 which are apparently lighter than would be ob- 

 tained by other investigators. Wet to dry ratios 

 in the literature suggest that the dry weight is 

 5% to 10% of the wet weight. Schindler and 

 Noven (1971) employed a ratio of 6%, although 

 their reason for using this particular value is 

 not given; the present results indicate that the 

 dry weight is 19% of the wet weight. Conse- 

 quently, the present weights could be increased 

 approximately three times for comparison with 

 other studies. Thus in the lakes which range 

 from oligotrophic to eutrophic, listed by the 

 above authors. Great Central Lake has, in terms 

 of its mean summer zooplankton biomass, 

 changed from oligotrophic to oligotrophic-meso- 

 trophic, i.e. 12 mg in 1969 to 120 mg dry 

 weight/m^ in 1970. In lakes producing sockeye 

 salmon the mean abundance of zooplankton 

 ranges from values which are less than 5 mg dry 

 weight/m^ to greater than 1 g dry weight/m^ 

 (Johnson, 1965). The mean concentrations in 

 Great Central Lake have increased from the very 

 low end of the range to values which are com- 

 monly reported for some of the larger sockeye 

 producing lakes, e.g., Babine Lake. 



Johnson (1965) concluded that there was a 

 general relationship between the rate of growth 

 of underyearling sockeye and zooplankton 

 abundance. However, he also suggested that 

 with increasing fish density food abundance was 

 supplanted by a space effect as a limiting factor. 

 In Great Central Lake the underyearling sock- 

 eye in October of 1970 were ca. 30% heavier than 

 fish caught in October of 1969 (Parsons et al., 

 in press; Barraclough and Robinson, 1972). In 



addition to the increase in weight these authors 

 report (on the basis of the number of adult salm- 

 on spawning) that the number of sockeye fry 

 in the lake were from two to five times more 

 numerous than in the previous year. Assuming 

 an initial weight of 120 mg for individual fry 

 of each year the respective rate of growth over 

 their first 200 days of lake residence was 0.9% 

 and 1.2% per day for 1969 and 1970 respec- 

 tively. The increased growth rate of sockeye 

 in 1970 is less than might be anticipated from 

 the 10-fold increase in zooplankton abundance. 

 Johnson's data (1965) indicated that a pop- 

 ulation density of 1 fish per m- might be the 

 point at which space becomes a factor limiting 

 growth. The maximum estimate of 1 x 10'' 

 sockeye in Great Central Lake during 1970 is 

 approximately 1 fish in every 5 m^. Conse- 

 quently it appears unlikely that the density of 

 the fish population in Great Central Lake limited 

 their growth. 



Among other factors which limit growth of 

 sockeye, Foerster (1968, Figure 45) indicates 

 that temperature has a major affect upon growth 

 and the efficiency with which food is utilized. 

 The optimum temperature for food conversion 

 for sockeye lies between 10° and 15°C. At 

 higher or lower temperatures the efficiency of 

 food conversion decreases, especially at temper- 

 atures in excess of 20°C or less than 6°C. The 

 laboratory studies of Brett et al. (1969) with 

 fingerling sockeye support the findings reported 

 above. In their experiments 15°C was found to 

 be the optimum temperature for growth at high 

 rations; however, maximum efficiencies with 

 which a ration was utilized occurred at lower 

 temperatures, e.g. the maximum food conversion 

 efliciency of 40% with a 0.2% increase in fish 

 weight per day occurred at a temperature range 

 of 8° to 10°C and a ration of 1.5% of the fish 

 weight/day. Temperatures between 5° and 

 17°C were found to provide the laboratory fish 

 the optimum conditions for conversion efficien- 

 cies and growth. In Great Central Lake, during 

 their first 200 days of lake residence, the under- 

 yearling sockeye concentrate at depths of 50 m 

 or greater during daylight; with the approach 

 of sunset the fish move to shallower depths and 

 by nightfall the major portion of the population 



35 



