FISHERY BULLETIN: VOL. 74, NO. 4 



in Sashin Creek before coho salmon spawning 

 begins, spawning by coho salmon could be det- 

 rimental to pink salmon embryos. In 1965, we 

 tried in each of the three study areas to assess the 

 effect of coho salmon superimposing their redds on 

 those of pink salmon. The densities of live pink 

 salmon embryos, which were estimated from 

 routine sampling of the spawning riffles with a 

 hydraulic sampler prior to coho salmon spawning 

 (McNeil 1964), were used in conjunction with the 

 average size of a coho salmon redd to estimate the 

 total number of pink salmon embryos that could 

 have been destroyed in gravel disturbed by 

 spawning coho salmon. At 13 redds throughout the 

 stream the average area of gravel disturbed by 

 spawning coho salmon was 2.6 m- per redd. 



The possible effect of coho salmon spawning on 

 pink salmon embryos in October 1965 is shown in 

 Table 6. The estimated spawning population of 110 

 female coho salmon would have disturbed a total of 

 286 m- of spawning gravel. Hydraulic sampling of 

 the spawning grounds in late September before 

 the coho salmon spawned indicated an average 

 density of 680 live pink salmon embryos per square 

 meter (see footnote 1, Table 6). About 200,000 live 

 pink salmon embryos resided in areas disturbed by 

 coho salmon spawners. 



In years when the numbers of coho and pink 

 salmon spawners are similar to those of 1965, it is 

 doubtful that coho salmon spawning has a 

 significant detrimental effect on the survival of 

 pink salmon embryos. Even assuming complete 

 mortality of pink salmon embryos in gravels 

 utilized by spawning coho salmon, the impact on 

 survival of pink salmon in 1965 would have been 

 slight-about 2% of the viable pink salmon em- 

 bryos present. Mortality of pink salmon eggs from 

 redd superimposition by coho salmon could be 

 significant if the number of coho salmon spawners 

 were to greatly increase by natural or artificial 

 processes. 



Age Determination 



We determined the age structure of samples of 

 adult coho salmon in Sashin Creek in 1965-67 and 

 1969 and in Nakvassin Creek in 1966-72 by scale 

 analysis (Table 7). Most of the salmon had spent 

 two summers and two winters in fresh water after 

 emergence from the gravel, had migrated to sea in 

 the beginning of their third year, and had then 

 spent two summers and one winter in the ocean 

 (designated in the Gilbert-Rich system as age 4-i). 

 A smaller portion of those sampled had spent 1 yr 

 in fresh water after emergence, had entered the 

 sea at the beginning of their second year, and had 

 then remained two summers in the ocean (age 82). 

 Adults that had migrated to sea at the beginning 

 of their fourth year of life and spent two summers 

 in salt water (age 54) usually constituted the 

 smallest fraction of each year's run. 



The presence of a large and dominant brood 

 year of coho salmon in Nakvassin Creek is in- 

 dicated by the percentage age distribution of 

 returning adults. In 1967, 40% of the adults sam- 

 pled for scales were age 30—1964 brood coho 

 salmon that had spent 1 yr in fresh water before 

 migrating as smolts; in 1968, 94% of the adults 



Table 7.- Age structure as determined from samples of scales of 

 adult coho salmon from Sashin Creek, 1965-67 and 1969, and 

 Nakvassin Creek, 1966-72. 



Source and year 

 of sample 



No. of fish 

 sampled 



Percentage age distribution 



Table 6.-Possible effect of coho salmon spawning on pink salmon embryos in Sashin Creek steambed in October 1965. 



Area 



Percentage of 



observed coho 



salmon spawning 



effort 



Live pink salmon Estimated 



Area of gravel embryos/m^ viable pink Estimated 



Estimated disturbed by before salmon embryos viable pink 



coho salmon coho salmon coho salmon disturbed by salmon embryos 



females (m^) spawned' coho salmon in study areas 



Percentage of 



total pink 



salmon embryos 



disturbed by 



coho salmon 



Upper 



Middle 



Lower 



Total 



6 



51 

 43 



100 



7 

 56 

 47 



110 



18 

 146 

 122 



286 



750 



1,200 



300 



2680 



14,000 



175,000 



37,000 



226,000 



2,209,000 

 4,880,000 

 1,822,000 



8,911,000 



0.6 

 3.6 

 2.0 



32.2 



'W. J. f^cNeil, Auke Bay Fisheries Laboratory, (pers. commun.). 

 ^Mean density, weighted according to area size. 

 3|^ean percentage, weighted according to area size. 



904 



