HIGLEY and BOND: CHINOOK SALMON IN A RESERVOIR 



Table 2. — Statistics used in estimating the standing crop in numbers ofthe 1961 and 1962 year classes. 



' Direct-proportion estimate. 



2 Schnobel estimate (Rounsefell and Everhort, 1953). 



•^ Limits set with Poisson distribution (Rounsefell and Everhart, 1953). 



(0.79 g vs. 0.48 g) on 28 April, when the seine 

 was removed. The reduced growth evidently 

 extended the period during which the fry were 

 susceptible to predation, and losses were again 

 high. Population size was estimated at 123,000 

 (82% survival) on 1 May and 23,700 (16% sur- 

 vival) in late November 1962. 



GROWTH 



The salmon grew rapidly, especially under 

 conditions of low population density. During 

 their initial 18-mo residence (January 1959 to 

 June 1960), the 48.000 members of the 1959 

 year class grew to average weights of 32 g 

 (chinook salmon) and 41 g (coho salmon). Chem- 

 ical treatment of the reservoir in June 1960 

 drastically reduced population size, and surviv- 

 ing salmon grew at very rapid rates. By August 

 1961, chinook salmon averaged 215 g and coho 

 salmon 280 g. These salmon were captured 

 less and less frequently during the 1961 sum- 

 mer, apparently because of deaths brought on 

 by sexual maturation and water conditions. By 

 fall 1961, the population was virtually extinct. 



Growth by the 1961 and 1962 year class 

 salmon also appeared density-regulated. About 

 30,000 of the 1961 year class escaped initial 

 predation and other losses. By December 1961, 

 population size was about 10,000 fish, which 

 averaged 17 cm in fork length (Figure 12) and 

 62 g (Figure 13). In December 1962, 5,000 of 

 these salmon remained. They then averaged 

 21 cm and 89 g, which represented relatively 

 small 12-mo increases. Rapid second-year 



growth, such as occurred in the 1959 year 

 class, appears to have been prevented by the 

 introduction of the 1962 year class. Of the 

 150,000 fry planted in March 1962, about 23.000 

 remained in December 1962, when they aver- 

 aged 13 cm and 22 g (Figures 12 and 13). Thus, 

 slower growth occurred among first-year salmon 

 in 1962 than in 1961 due to the higher density 

 in 1962. Other influences may have been the 

 later planting date in 1962 and the protective 

 enclosure of the 1962 year class in a relatively 

 small area for 5 wk. 



Ricker and Foerster (1948) studied compe- 

 tition and population size in juvenile sockeye 

 salmon, 0. )ierka (Walbaum). of Cultus Lake. 



22 



20 

 IS 

 I 6 



E 



o I 4 



z 



I 2 



I 



a: 

 O 8 



6 

 4- 



95% CONFIDENCE 

 ZON 



SAMPLE SIZE 



1962 YEAR CLASS 



FMAMJJASONDJ FMAMJJASOND 

 1961 1962 



Figure 12. — Growth in length of 1961 and 1962 year class 

 salmon. Some confidence zones were omitted because of 

 their small size. 



885 



