FISHERY BULLETIN: VOL. 74, NO. 4 



periods 1 and 2 for the 1963 brood than for the 1964 

 and 1965 broods. We estimated that eggs of the 

 1963 brood were over four times as abundant as 

 those of the 1965 brood and over five times as 

 abundant as those of the 1964 brood. Resulting 

 density-dependent factors such as superimposi- 

 tion of redds, selection of inferior redd sites, and 

 emigration of fry from the stream because of lack 

 of living space could be the cause of the higher 

 initial mortality of the 1963 brood. 



For all three broods the highest instantaneous 

 mortality occurred in period 3-between the first 

 and second population estimates of the first sum- 

 mer of life-during July and the first half of 

 August (Table 20). Predation from fishes (both 

 intraspecific and interspecific) is thought to be a 

 major cause of this high mortality. In period 2 the 

 fry live in the backwater and shallow edges of the 

 stream where larger piscivorous fish do not 

 regularly occur. During period 3 the fry move into 

 deeper parts of the main channel where current is 

 still relatively slow, but here larger fish occur and 

 the fry may be more subject to predation. 



Instantaneous mortality during the winter 

 (period 4) was much less than that of the first 

 summer. Predation probably was less during this 

 period for two reasons: 1) in winter the feeding 

 rate of cold-blooded predators is slowed, and 2) 

 restricted access because of ice and snow and 

 lowered activity and availability of the juvenile 

 coho salmon combine to lessen the hunting success 

 of warm-blooded predators. 



Mortality increased during the second summer 

 (period 5) but only to a third or less of the corres- 

 ponding part of the first summer (period 3). 



Some of the estimated mortality of fry and 

 fingerlings might have been due to undetected 

 emigration from the creek. When the fry weir or 

 fyke nets were fished in summer (periods 3 and 5), 

 however, only a few fry and fingerlings emigrated 

 and the low mortality rate in period 4 also suggests 

 that only a few fry emigrated in fall and winter. 

 Some age I smolts probably migrated from the 

 stream in the spring of period 4 in each of the 3 yr 

 studied. The drop in population of a brood year due 

 to age I smolt emigration is included as part of 

 period 4 mortality. Age composition of smolts in 

 1965, 1966, and 1967 was not determined. The age 

 composition of returning adults in 1966 and 1967 

 (25% and 29% age So, respectively) indicates that 

 some age I smolts emigrated in the spring of 1965 

 and 1966. In 1968 the smolts were sampled for age 

 composition; about 500 yearling smolts migrated. 



920 



Scale samples for age analysis were not collected 

 from adults in 1968. 



SUMMARY 



The number of adult coho salmon that enter 

 Sashin Creek varies from year to year. Coho 

 salmon have been counted at the weir as they 

 enter Sashin Creek each year since 1934, but this 

 count has usually been incomplete. 



Several methods were used to estimate coho 

 salmon escapements to Sashin Creek for the years 

 1963-65 and 1967. These included weir counts, 

 adults on spawning riflfles, mean redd life, and 

 marked-to-unmarked ratios of spawners. The last 

 system produced the most accurate estimates, 

 resulting in 916, 162, 221, and 370 salmon for the 

 respective study years. 



In the 4 yr that spawning ground studies were 

 made, the density of coho salmon on the spawning 

 grounds in Sashin Creek tended to be greater in 

 the middle and lower study areas than in the upper 

 area. 



The effect of coho salmon spawning on the 

 survival of pink salmon embryos was insignificant 

 in 1965 relative to the population ratios of coho and 

 pink salmon present. Significant numbers of pink 

 salmon embryos might be killed if relatively large 

 numbers of coho salmon utilized Sashin Creek for 

 spawning. 



The 4.3 age-group of coho salmon made up 78%, 

 59%, 64%, and 62% of the adults that returned to 

 Sashin Creek in 1965, 1966, 1967, and 1969-higher 

 percentages of this age group than reported for 

 most other streams. In California, Oregon, Wash- 

 ington, and southern British Columbia, adult coho 

 salmon are almost exclusively age 82. Studies of 

 growth and scales of fry, fingerlings, and smolts 

 and estimates of the population sizes of juveniles 

 indicate that most coho salmon remain in Sashin 

 Creek for two summers and winters. 



In some years, substantial numbers of coho 

 salmon fry enter the estuary of Sashin Creek 

 shortly after emergence. These fry were tested 

 and found to be able to survive in salinities 

 encountered in the inner bay of the Little Port 

 Walter estuary. However, analysis of scales of 

 adult coho salmon returning to Sashin Creek 

 revealed none that had migrated to the estuary at 

 the fry stage, suggesting no fry (or at best very 

 few) that migrate to the ocean survive to return as 

 adults. This agrees with studies of other stocks of 

 coho salmon. 



