FISHERY BULLETIN: VOL. 71. NO. 3 



this reduced gill-net catches. The lower lethal 

 temperature was estimated by Brett (1956) as 

 0.8°C for Chinook salmon fry acclimated to 

 10°C. Cold tolerance, Brett indicated, is gained 

 slowly by fish. Winter temperatures of 7°C and 

 below were reached at Happy Valley only after 

 about a 2-mo period of decreasing temperatures 

 (Figure 2). This period seems sufficient for the 

 salmon to have acclimated to cold. 



SURVIVAL 



Mortality of 0-age salmon exceeded 80% for 

 both the 1961 and 1962 year classes. This con- 

 trasted with the light losses among 1959 0-age 

 salmon, which did not face predation by older 

 salmon at planting. 



Predation rate in 1962 was estimated in two 

 ways. In the first method, stomach contents 

 from 1961 year class salmon were studied. A 

 total of 29 0-age salmon were found in the stom- 

 achs of 299 salmon collected between 26 May 

 and 29 June 1962. There were about 10,000 of 

 the 1961 year class present during this period 

 (Figure U). Assuming that the stomach con- 

 tents represented daily consumption, the daily 

 consumption rate was on the order of 1 prey 

 per 10 predators, and mortality was about 1,000 

 fry per day. Predation rate by the 1961 year 

 class was also estimated using growth and food 

 habits data (see sections on Growth and Food 

 Habits). Relative growth of the 1961 year class 

 was 1.4 mg/g per day from 15 May to 15 June 

 1962. Experimental evidence, provided by 

 Davis and Warren (1968) for chinook salmon 

 fry held at 10°C, indicated that at 1.4 mg/g per 

 day, food consumption was about 40 mg/g per 

 day. The total 1961 year class biomass was 

 about 700,000 g (10,000 fish times 70 g average 

 weight). Thus, consumption was 28,000 g per 

 day. According to food studies on the 1961 year 

 class, 21% of this consumption, or 5,880 g per 

 day, was fry. At 4 g per fry, 1,470 fry per day 

 were consumed by the older salmon. A similar 

 calculation for 18 June to 20 July gave a mor- 

 tality rate estimate of 1,125 fry per day. 



The two estimates agree on a predation rate 

 sufficiently high to account for most of the 

 losses in 1962 and, by inference, in 1961. The 

 heaviest losses actuallv would have occurred 



when the fry were smallest and most abundant, 

 i.e., close to planting time as shown by the 

 survival curves in Figure 11. More specific in- 

 formation about the fates of the 1961 and 1962 

 year classes is given below. 



The 75,300 fry planted on 8 February 1961 

 (1961 year class) met severe predation by 1959 

 year class salmon and rainbow trout surviving 

 chemical treatment. Additional losses occurred 

 when a flash flood on 11 February carried many 

 of the fry over the spillway. A mark-and- 

 recapture operation indicated population size 

 was 28.600 fish (38% survival) on 22 March 

 1961 (Table 2). According to a second estimate, 

 the population contained 6,800 fish (9% survival) 

 in late August 1962. Continued losses were 

 caused by predation, harmful summer water 

 conditions, and a transplantation program 

 which removed 2,500 salmon to Beaver Creek, 

 a tributary of the Warm Springs River. 



The second plant of chinook salmon (1962 

 year class) was made on 22 March 1962. Be- 

 cause predation was expected, a large, fine-mesh 

 seine was used to impound and protect the 

 fry in a cove of about 0.05 hectare, while gill 

 nets were set to deter predator movement. 

 These measures reduced predation at planting, 

 but also produced a high fry density in the cove 

 and this caused the fry to grow relatively slowly. 

 Some fry escaped from the cove; these fish were 

 substantially larger than fry still in the cove 



F M A M 



sonOjfmamjjasond 



 3,S00 salmon of the 1961 year cloit 

 Irantplanted m July and Awguit 



Figure 11. — Survival curves for 1961 and 1962 year class 

 salmon. Curves were fit by eye according to population 

 estimates and information on predation rates described 

 in text. 



884 



