PROBABLE CASE OF STREAMBED OVERSEEDING— 1967 PINK SALMON, 

 ONCORHYNCHUS GORBUSCHA, SPAWNERS AND SURVIVAL OF 

 THEIR PROGENY IN SASHIN CREEK, SOUTHEASTERN ALASKA 



William R. Heard' 



ABSTRACT 



The 1967 escapement of 38,067 pink salmon, Oncorhynchus gorhuscha. to Sashin Creek, southeastern 

 Alaska, was the largest since 1942. Studies on distribution and density of spawners and freshwater 

 survival of their progeny indicated that deposition of excessive numbers of eggs caused a severe 

 compensatory mortality of alevins during winter. Spa wner density was 1.7, 1.6, and 1.2 females/m^ in 

 upper, middle, and lower study areas respectively. The greater density of spawners in the upper area in 

 the odd-numbered years may be determined by genetic factors like timing of escapements and by 

 greater marine survival of fry from the upper area. Based on the previously consistent relation between 

 timing of adult entry and resulting freshwater survival, 1967 spawners should have produced 8 million 

 fry rather than the 3 million that were produced. 



Mortality of eggs and alevins was high during spawning, low between spawning and hatching, and 

 high between hatching and emergence. Between 1 December 1967 and 25 March 1968, 11.1 million 

 eggs or alevins, 10.7 million of which were alive on 1 December, disappeared within the streambed. 

 Initial mortality of these progeny probably occurred in the early alevin stage from oxygen privation, 

 whereas disappearance was probably related to rapid decomposition and invertebrate scavenging. A 

 "snowball effect" is postulated whereby alevins that die shortly after hatching place increasing 

 demands on available oxygen, causing accelerated mortality. A review of historical patterns of fry 

 production in Sashin Creek indicates that streambed overseeding occurred in 1967. 



Studies of pink salmon, Oncorhynchus gorbuscha, 

 in Sashin Creek, Baranof Island, southeastern 

 Alaska, have shown that certain factors markedly 

 affect freshwater survival. These factors include: 

 1) seasonal timing of spawning (Skud 1958); 2) 

 density and distribution of adults on the spawning 

 grounds relative to ecological characteristics of 

 the stream, especially gradient (Merrell 1962); 

 and 3) quality of the intragravel environment, 

 including oxygen content of intragravel water and 

 amount of silt and fine particulate material in 

 streambed gravels (McNeil 1966, 1968). Other fac- 

 tors of significance, but believed to be of less 

 influence on freshwater survival in Sashin Creek, 

 include predation on eggs and alevins (McLarney 

 1967), stream discharge during spawning (Ellis 

 1969), and incubation (McNeil 1968). 



The spawning ground of Sashin Creek extends 

 from the head of tidewater to an impassable falls 

 1,200 m upstream and includes 13,629 m^ of 

 streambed. Ninety-six percent (13,084 m^) of this 

 ground comprises three distinct ecological areas 



'Northwest and Alaska Fisheries Center Auke Bay Labora- 

 tory, National Marine Fisheries Service, NOAA, P.O. Box 155, 

 Auke Bay, AK 99821. 



Manuscript accepted February 1978. 

 FISHERY BULLETIN: VOL. 76. NO. 3. 1978. 



that differ in gradient and size of particles in the 

 substrate. McNeil (1966) called the areas upper, 

 middle, and lower and described them briefly as 

 follows: upper (2,945 m^) — relatively steep gra- 

 dient (0.79'f ) and coarse streambed gravel; middle 

 (4,067 m^) — intermediate gradient (0.3*^) and 

 medium-sized streambed gravel; and lower (6,072 

 m^) — low gradient (0.1%) and relatively fine 

 streambed gravel. The remaining 4% (545 m^) of 

 spawning ground is located in a short section of 

 stream between the counting weir and the lower 

 area and is not treated in this paper. 



Pink salmon spawners entering Sashin Creek 

 (the escapement) have been counted at a weir at 

 the mouth of the creek since 1934, and the result- 

 ing numbers of fry from these escapements have 

 been determined since 1940. During this time, the 

 number of spawners varied from as few as 8 to 

 more than 90,000 and the number of fry produced 

 varied from 50 to almost 6 million. The percentage 

 of freshwater survival, based on the estimated po- 

 tential egg deposition, ranged from 0.06 to 21.75% 

 (Table 1). 



The high escapement of 38,067 pink salmon 

 spawners in 1967, following a long series of rela- 

 tively low escapements, gave me an opportunity to 



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