CRONE and BOND: LIFE HISTORY OF COHO SALMON 



California (Shapovalov and Taft 1954) and British 

 Columbia (Foerster and Ricker 1953), the mean 

 lengths of coho salmon smolts (mostly age I) 

 usually ranged from 110 to 120 mm. 



In coho salmon, attaining the smolt stage is 

 apparently more a function of size than age. Data 

 on lengths and numbers of juvenile coho salmon in 

 Sashin Creek during September and early summer 

 suggest that most require two summers of fresh- 

 water residence to reach smolt size. Coho salmon 

 can grow much faster; some juveniles in a brackish 

 pond in Oregon grew from about 40 mm (890 to the 

 pound) to about 120 mm and became smolts in only 

 3 mo instead of the usual 1 yr (Garrison 1965). 



The growth of juvenile coho salmon in Sashin 

 Creek varies from year to year. During summer 

 1966, for instance, fry were larger than in 1964, 

 1965, and 1967 (Figure 8). In the summers of 1966 

 and 1968, fingerlings (mainly age I) were larger 

 than in 1965 and 1967 (mainly age I). The number 

 of fingerlings in 1966 and 1968^ (*3,000 on 1 July) 

 was less than in either 1965 (=5,000 on 1 July) or 

 1967 (*3,500 on 1 July), and less competition for 

 food would be expected and could account for the 

 larger size of the fingerlings in 1966. Also, the 

 presence of fewer fingerlings in summer 1966 may 

 have allowed the fry to reach a larger size because 

 of less competition for food or space. Food abun- 

 dance, controlled by factors other than coho salm- 

 on population size, may have an important 

 influence on coho salmon growth in Sashin Creek. 

 We have no information on possible year-to-year 

 differences in food supply independent of fish 

 populations which could result in differences in 

 growth of juvenile coho salmon. 



"•An estimate of 2,960 fingerlings in Sashin Creek was made on 

 2 July 1968. 



Survival from Potential Egg Deposition 

 to Emergence 



The estimated potential egg depositions for 

 brood years 1963, 1964, and 1965 were determined 

 by multiplying the mean fecundity (determined 

 from 1966, 1970, and 1971 samples) by the es- 

 timated number of females (one-half of the es- 

 timated population of spawners). These estimates 

 are considered to be only rough approximations: 

 1,460,000 for 1963; 260,000 for 1964; and 350,000 for 

 1965. 



Estimates of the numbers of preemerged salm- 

 on alevins in the streambed were obtained in the 

 early spring by hydraulic streambed sampling 

 (McNeil 1964). In Sashin Creek this sampling is 

 done to estimate the number of pink salmon 

 alevins, but after relatively large escapements of 

 coho salmon reliable estimates of the number of 

 coho salmon alevins in the streambed also can be 

 made. No coho salmon alevins were found during 

 the hydraulic streambed sampling in the spring of 

 1966, so the estimate of the alevin population was 

 zero. Because many age fry were in the stream in 

 the summer of 1966, we have estimated the 

 number of alevins that were in the gravel that 

 spring by interpolation of the survivorship curves. 



The numbers of preemerged coho salmon 

 alevins for 1964-66 estimated from the results of 

 hydraulic sampling or interpolation of survivor- 

 ship cur\^es are: 214,000 in spring 1964 (1963 brood 

 year), 58,000 in spring 1965 (1964 brood year), and 

 100,000 in spring 1966 (1965 brood year). From 

 these figures and estimates of potential egg depo- 

 sition, we calculated survival from potential egg 

 deposition to just before fry emergence to be 15%, 

 22%, and 26% for the 1963, 1964, and 1965 brood 

 years, respectively. 



Figure &-Mean fork lengths of coho 

 salmon fry measured several times each 

 summer, 1964-67, and resulting 

 fingerlings (*90% age I and 10% age II) 

 the next summer, 1965-68, Sashin 

 Creek. 



JUNE 



913 



