FISHERY BULLETIN: VOL. 80, NO. 4 



ber of returning adults, which suggests that if 

 the timing of returns is size dependent, then den- 

 sity-dependent growth may occur during the 

 ocean residence of chum salmon. Pink salmon re- 

 turn to the Fraser River in abundance in odd 

 years, but return in negligible amounts in even 

 years. The mean age at maturity and number of 

 returning chum salmon adults tended to be 

 higher in even brood years than in odd ones (Fig. 

 5), so that pink salmon may indirectly influence 

 mean age at maturity of chum salmon through 

 an effect on survival of chum salmon. 



The proportion of a brood year returning at 

 age 3, 4, and 5 is frequently of importance in pre- 

 dicting annual returns. For Fraser River chum 

 salmon, the percentage age composition of the re- 

 turns from a brood year is related to the mean 

 age of return of the brood year as follows: 



% age 3 = 321.56 - 78.26 mean age 



(r 2 = 0.93) (5) 



% age 4 = -141.68 + 56.20 mean age 



(r 2 = 0.63) (6) 



% age 5 = -79.88 + 22.06 mean age 



(r 2 = 0.50). (7) 



If the returns from a brood year can be predicted, 

 then the mean age of return of a brood year can 

 be obtained from Equation (4) and applied to 

 Equations (5)-(7) in order to obtain numbers re- 

 turning at each age. 



Return to Escapement 



The ratio of total returns for a brood year to 

 escapement (R/S) has varied from 0.8 to 4.0 for 

 the 1961 through 1974 brood years. The available 

 evidence suggests that for escapements below 

 850,000 adults, egg-to-fry and fry-to-adult survi- 

 vals will generally be large enough to allow the 

 number of returning chum salmon to remain 

 above replacement levels (Fig. 6). However, only 

 the 1968 escapement was above 600,000 adults, 

 so further information on the R/S ratio for escape- 

 ments >500,000 adults is required in order to 

 evaluate the effect of varying escapements on 

 chum salmon survival. There was no evidence to 

 indicate a decline in recruits per spawner at 

 escapements >500,000 fish, and thus the optimal 

 escapement is uncertain. Optimal escapements 

 will not be established with confidence until de- 

 clines in recruits per spawner (or density-depen- 

 dent mortality) is observed at high spawning 

 stock sizes. 



100 200 300 400 500 600 700 800 900 

 Spowners ( * 10 3 ) 



Figure 6.— Total return versus numbers of spawners for the 

 1961-75 brood years of Fraser River chum salmon. 



Fry-to-adult survival has been variable, rang- 

 ing from 0.3% to 2.4% (Table 4). The average sur- 

 vival for the even brood years was 1.53%, whereas 

 survival for the odd brood years was 0.85%. The 

 effect of fry abundance on variability in fry-to- 

 adult survival was investigated for odd brood 

 years by summing the estimated numbers of 

 chum salmon and pink salmon fry, and for even 

 brood years by assuming that the number of pink 

 salmon fry produced was negligible. The data 

 suggested that chum salmon fry survival tended 

 to increase when the abundance of chum and 

 pink salmon fry decreased (Fig. 7). This relation- 

 ship can be expressed by: 



% survival = 0.73 + 



46.53 



Fry abundance (millions) 



(r = 0.52) (8) 



for the 1961-75 brood years. All of the fry-to- 

 adult survivals for the odd brood years, except 

 for the 1965 and 1969 brood years, were below 

 1.0%, whereas all of the fry-to-adult survivals for 

 the even brood years were above 1.0%, with the 

 1964 and 1968 brood year fry survival being 

 higher than the rest. 



Some of the variability in fry-to-adult survival 

 was due to the timing of the downstream fry mi- 

 gration. The higher survival of the 1965 and 1969 

 brood year fry when compared with other odd 



820 



