FISHERY BULLETIN: VOL 76, NO. 3 



Table l. — Number of adult pink salmon, potential egg deposi- 

 tion, number of fry produced, and freshwater survival in Sashin 

 Creek, 1934-67. (Modified from McNeil 1968.1 



'Based on 2,000 eggs female except when actual fecundity was calculated 



^An attempt was made to destroy the spawners or their progeny. 



^Natural returning adults (327) were supplemented by the introduction of 

 1.866 adults taken from Bear Harbor. Kuiu Island 



"Fry weir not operated: figures are estimates of live alevins in the gravel |ust 

 before start of emergence 



study the effects of a large spawning population on 

 freshwater survival. For the 1967 escapement I 

 studied 1 ) timing of entry into the stream and the 

 distribution and density of pink salmon on the 

 spawning grounds, and 2) survival of progeny by 

 time periods in the three ecological areas of the 

 stream and the overwinter disappearance of eggs 

 and alevins from streambed gravels. In this paper 

 I present all the available data on escapament size 

 and production of fry in Sashin Creek and develop 

 the hypothesis that streambed overseeding occur- 

 red in 1967. As Ricker (1962:186) pointed out, 

 detailed knowledge on the effects of overseeding is 

 important in understanding why pink salmon 

 populations fluctuate. He stated, "Because it 

 [overseeding] happens rarely nowadays, no 

 chance should be lost to make such a study if one 

 occurs." Simply stated, overseeding can be defined 

 as an egg density in spawning bed gravels that 

 leads to a significantly greater freshwater mortal- 

 ity than a lesser density would cause. As discussed 



more fully later, it is a complex and dynamic in- 

 teraction between egg density, streambed ecology, 

 and specific climatic conditions. 



TIMING OF ENTRY AND 



DISTRIBUTION AND DENSITY 



OF SPAWNERS 



The timing of stream entry was determined 

 from daily counts of the adults at the Sashin Creek 

 weir. This timing apparently influences the 

 freshwater survival of the progeny. An inverse 

 relation between time of stream entry of spawners 

 and survival of progeny is usual in Sashin Creek 

 (Skud 1958; Merrell 1962; McNeil 1968; Ellis 

 1969): high survival has been associated with 

 early spawning and low survival with late spawn- 

 ing. Merrell ( 1962) further pointed out that pink 

 salmon spawn in Sashin Creek an average of 12 

 days earlier during odd years than even years. ^ As 

 a result, freshwater survival is usually higher 

 among progeny of spawners from odd years than 

 among those from even years (Table ll. 



In 1967, 507f of the spawners had entered 

 Sashin Creek by 20 August, the second earliest 

 date on record. The early entry indicated that sur- 

 vival of eggs and alevins would be high, but this 

 did not prove to be the case. 



Throughout the run, random lots of females 

 were tagged at the weir, and the distribution and 

 density of spawners were determined from daily 

 counts of both tagged and untagged females on the 

 spawning grounds. This technique, described by 

 McNeil (1968) and used by Ellis (1969), provides 

 two methods of estimating the numbers of females 

 spawning in the upper, middle, and lower areas. 

 One method assumes that tagged females distrib- 

 ute themselves among the three sections the same 

 as untagged females. In the other method, the 

 summed daily count of all females in each area is 

 divided by the average longevity on the spawning 

 grounds. The results from the two methods were 

 generally in agreement, except for the upper area 

 where estimates based on distribution of tagged 

 females were considerably higher than those 

 based on total females. The difference may reflect 

 the difficulty in making accurate counts on spawn- 

 ing riffles where densities of spawners are high; in 

 such a situation an observer might count small 



^The date when 50% of the escapement to Sashin Creek had 

 entered the stream has been commonly used as an index of time 

 of spawning. 



570 



