SEASONAL VARIATION IN MORTALITY 

 IN SPAWNING BEDS 



The number of live pink salmon eggs of the 1965 

 brood declined from an estimated 12.7 million a.t 

 the beginning of spawning in summer to S.."3 mil- 

 lion at the end. Mortality caused further reduc- 

 tions to fi.5 million in autumn and to -I.^ million 

 in wintei-. 



It is instructive to compare mortality of the 1965 

 brood with that of the 1963 brood. For this com- 

 parison, 1 have calculated values of the mortality 

 coefficient, J/, for the entire stream rather than 

 for the individual areas as given in table 8 of this 

 paper and in table 1 1 of McNeil (1966) . The values 

 for .1/ for brood years 1963 and 1965 for period 1 

 (summer), period 2 (autumn), and period 3 (win- 

 ter) ai'e as follows : 



Although the rat« of mortality both years was 

 highest during spawning, the difference between 

 the two years was appreciable ( 21/2 times) . Mortal- 

 ity in autumn was similar for the two yeai-s, but the 

 difference between the years was pronounced in 

 winter (31/^ times). Differences in mortality coeffi- 

 cients are suggestive of differences in the environ- 

 ment encountered by the two populations, because 

 the number of spawners was similar each year 

 (16,757 in 1963 and 14,833 in 1965). The unusual 

 drought which prevailed throughout September 

 1965 could have produced increased survival dur- 

 ing spawning from increased efficiency of egg dep- 

 osition in spawning beds and decreased survival 

 in autumn and winter from delayed moi-tality of 

 embryos and alevins exposed to low concentrations 

 of dissolved oxygen early in development. 



SUMMARY 



1. The area of streambed in Sashin Creek used 

 by pink salmon for spawning is 13,629 m.= Ob- 

 servations on distribution of spawners and sur- 

 vival of embryos and alevins are made annually in 

 97 percent of the spawning ground which is di- 



vided into upper (2,945 m.= ), middle (4,067 m.-), 

 and lower (6,072 m.-) areas. 



2. Migration of 14,833 pink salmon spawners 

 to Sashin Creek in 1965 was relatively early, and 

 high fresh-water survival of their progeny (18 

 percent of potential egg deposition) resulted in the 

 production of 2,235,000 fry. High survival was 

 predicted at the time the parents entered the 

 stream from a linear relation of survival of prog- 

 eny and the date the parents entered the stream. 



3. Although the density of spawners was rela- 

 tively high and fairly uniform throughout the 

 stream, densities of fry were considerably differ- 

 ent in the upper, middle, and lower areas. The 

 highest density of fry (228 per square meter) was 

 in the upper area; the lowest (104 per square 

 meter) was in the lower area. The relatively high 

 density of fry in the upper area and low density 

 in the lower area resembled the situation in three 

 previous years (1959, 1961, and 1963) when the 

 density of spawners in the three areas was also 

 relatively high. 



4. Spawners do not concentrate in the upper 

 area despite the existence of a favorable environ- 

 ment for embryos and alevins. Failure to concen- 

 trate in that area is most pronoimced when 

 spawnei-s are scarce, but the reasons for this 

 behavior remain obscure. 



5. Delayed hatching of eggs in the autumn of 

 1965 was attributed to low dissolved oxygen in 

 intragravel water, which resulted from a drought 

 throughout September. Detrimental effects of the 

 drought on embryos were least pronounced in the 

 upper area where concentrations of dissolved oxy- 

 gen in intragravel water were highest. 



6. Rates of disappearance of eggs and alevins 

 from spawning beds in 1965-66 were low during 

 spawaiing and high during winter in comparison 

 with previous years. Low waterflow in September 

 1965 may have allowed a better than norm.al 

 recruitment of eggs to spawning beds. The dis- 

 appearance of large numbere of eggs and alevins 

 in winter is unexplained. 



7. The rate of mortality of the 1965 brood year 

 was two times higher during spawning than in 

 autumn. This situation is contrasted with the 1963 

 brood, whose rate of mortality was about 7 times 

 higher during spawning than in autumn and about 

 11 times higher than in winter. 



PINK SALMON SPAWNERS IN SASHIN CREEK 



585 



