STABILITY OF THE SPAWNING BED 



Instability of the spawning bed was an impor- 

 tant cause of egg and larval mortality in the study 

 streams. There was no apparent relation between 

 gravel composition and stability during flooding, 

 and high mortality occurred in spawning beds 

 composed of coarse and fine materials. Change 

 in position of temporarily stationary debris caused 

 gravel movement, but the presence of floating 

 debris had little or no effect on gravel movement. 



The influence of temporarily stationary debris 

 on the stability of bottom materials was studied in 

 Maybeso Creek (fig. 4), a stream located near 

 Hollis (Bishop and Shapley, 1963). The results 

 of this study gave further support to relations ob- 

 served between movement of wood debris and 

 mortality of eggs and larvae in Harris River, 

 Indian Creek, and Twelvemile Creek. 



High discharge may be a common cause of mor- 

 tality among salmonid eggs and larvae. In New 

 Zealand, Hobbs (1937) concluded that floods 

 seldom destroyed salmon and trout redds, but at 

 times accounted for a partial failure of a brood 

 year. At Sagehen Creek, Calif., a flood in Decem- 

 ber 1955 destroyed brook trout, Salvelinus fonti- 

 nalis, and brown trout eggs, causing the complete 

 failure of the zero age group of these species in 

 1956 (Needham and Jones, 1959). 



Observations on dislodgment of salmon eggs and 

 larvae from British Columbia and Southeastern 

 Alaska streams during flooding also have been 

 reported. After flooding, Withler (1952) found 

 preemergent sockeye salmon fry washed from a 

 tributary of Babine Lake, British Columbia, and 

 Wickett (1959) reported the observation of coho 

 and chum salmon eggs on banks and in bushes 

 along the Qualicum River, Vancouver Island. 

 Floods at Sashin Creek in 1941 and 1943 were 

 thought to have killed many pink salmon eggs and 

 larvae (Hutchinson and Shuman, 1942; Davidson 

 and Hutchinson, 1943). 



FREEZING OF INTRAGRAVEL WATER 



Although freezing has been considered an im- 

 portant cause of mortality, except for Neave 

 (1947), Smirnov (1947), and Semko (1954), little 

 direct evidence showing freezing to be an im- 

 portant cause of pink and chum salmon egg and 

 larval mortality has been reported. 1 found that 

 freezing caused high mortality of 1956 brood year 

 eggs and larvae, particularly in intertidal Indian 



520 



Creek. There was also an indication that the 

 1958 brood year experienced similar high mortality 

 in Indian Creek. Winters of 1957-58, 1959-60, 

 and 1960-61 were mild by comparison, and there 

 was little or no evidence of mortality from freezing. 

 It was apparent that freezing exerted the great- 

 est influence on mortality in the stream having the 

 lowest minimum discharge. Further study may 

 reveal that certain spawning areas are unproduc- 

 tive because of low discharge during periods of 

 freezing. There was no evidence of high mortality 

 from freezing during winters that maximum day- 

 time air temperatures remained above 0° C. 



SUMMARY 



1. The available evidence indicates that high 

 mortality of eggs and larvae greatly limits the 

 abundance of juvenile pink and chum salmon. 

 Proper management of pink and chum salmon 

 fisheries will depend in part on a thorough under- 

 standing of the factors affecting the potential of 

 spawning beds to produce fry. Field studies of egg 

 and larval mortality are important because they 

 provide information on time and magnitude of 

 mortality that can be associated with causative 

 factors. 



2. In the study streams, egg mortality after the 

 end of spawning seldom exceeded 20 percent in the 

 absence of low levels of dissolved oxygen, freezing, 

 or high water. There was evidence that mortality 

 attributable to oxygen privation was highest where 

 water circulation within the spawning bed was 

 impaired. 



3. Low levels of dissolved oxygen in intragravel 

 water as well as high egg mortality occurred with 

 low flow during and shortly after spawning. The 

 resulting mortality of eggs was estimated to vary 

 between 60 and 90 percent in the spawning areas 

 sampled. 



4. Movement of bottom materials during high 

 water was an important cause of mortality, which 

 was most severe where temporarily stationary 

 debris shifted position within the flood plain. 

 Mortality from movement of bottom materials 

 was estimated to exceed 90 percent in one instance. 



5. Freezing was an important cause of mortality 

 only when maximum daytime temperatures re- 

 mained below 0° C. for at least 2 consecutive days. 

 Mortality from freezing was highest in the stream 

 having the lowest minimum discharge, and was 

 estimated to approach 65 percent on one occasion. 



U.S. FISH AND WILDLIFE SERVICF. 



