The lower estimated percentage loss in 1963 

 than in 1962 of chinook salmon in the Elokomin 

 River through predation by sculpins (table 9) 

 could conceivably be attributed in part to the 

 change from a "wet" diet in 1962 to the Oregon 

 moist pellet in 1963. It has been shown that 

 the return of adult hatchery salmon is con- 

 sistently higher from fish reared on the Ore- 

 gon moist pellets than from those fed a "wet" 

 diet (Hublou, 1963). The energy reserves of 

 rainbow trout fed another fortified pellet in 

 Canada were also superior to those of fish 

 reared on a wet diet (Miller, Sinclair, and 

 Hochachka, 1959). Because wet diets seem to 

 be less desirable than some fortified pellets or 

 the diets of wild fish, hatchery salmon fed the 

 wet diet may be weakened by the requirements 

 of stream life and have such small energy re- 

 serves that predators can catch them more 

 easily. Many factors apparently contribute to 

 differences in loss of chinook salmon to scul- 

 pins; data for the 1962 and 1963 salmon re- 

 leases, however, suggest that reduced losses in 

 1963 resulted in part from the dietary change 

 from "wet" diets to the Oregon moist pellet. 



The specific reason for the apparent relation 

 between losses from predation and the size of 

 chinook salmon at release is not known. It is 

 clear, however, that at certain sizes salmon are 

 too large for certain predators; possibly more 

 important is the fact that larger individuals 

 of a release may be salmon of higher quality. 

 That is, the larger salmon in a release may be 

 more healthy, agile, and alert than smaller fish 

 and therefore be less vulnerable to predators. 



Factors Related to Release 



Techniques 



Although the physical condition of cultured 

 salmon is important in reduction of mortality, 

 the manner in which they are released can 

 also be significant — especially the use of 

 methods that reduce exposure to intensive pre- 

 dation. The three statements which Neave 

 (1953) used to describe predation on fry of 

 wild pink salmon, O. gorbuscha, and chum 

 salmon apply to the release of hatchery-reared 

 chinook salmon fry: (1) percentage mortality 

 increases with the distance the fry travel; (2) 

 percentage mortality decreases with increasing 



number of migrants; and (3) percentage mor- 

 tality increases during the progress of the run. 



Neave's first point indicates that location 

 of the release point is important because it 

 determines the length of the route the salmon 

 must travel and the duration of exposure to 

 predation. Salmon released from the Oxbow 

 Hatchery reached the Columbia River with 

 fewer mortalities than those released from the 

 Elokomin River Hatchery. Ellis and Noble 

 (1960) reported that losses were less for 

 juvenile chinook salmon transported 65 km. 

 downstream from the Klickitat River Hatchery 

 than for fish released at the hatchery. The 

 transporting of juvenile salmon past concen- 

 trations of predators, however, did not seem to 

 affect the percentage of adult fish that returned 

 to the hatchery; the proportion of transported 

 fish that returned to the hatchery was smaller 

 than the proportion of returning fish that were 

 not transported. In the Elokomin River, chi- 

 nook salmon traveled 13 km. near numerous 

 predators and were generally subjected to pre- 

 dation for about 2 days. If Elokomin River 

 salmon were transported and released below 

 km. 2, they would be, initially at least, sub- 

 jected to fewer predators for a shorter period. 

 Semko (1954) also theorized that more hatch- 

 ery salmon fry would be saved if they were 

 released below concentrations of predators. 



Neave's second point indicates that per- 

 centage mortality from predation will be least 

 if large numbers of migrants are released at 

 the same time. Results from the 1963A and 

 1963B releases were consistent with this prin- 

 ciple. The 1963A release of chinook salmon 

 into the Elokomin River was larger than the 

 1963B liberation. The calculated percentage 

 of the first release consumed by sculpins was 

 about one-third that of the smaller second re- 

 lease and suggests that the percentage of a 

 release eaten by sculpins increases as the num- 

 ber of fish liberated decreases. On the other 

 hand, the percentage of salmon eaten by scul- 

 pins was similar for the 1962 and 1963B re- 

 leases despite a much greater number of salmon 

 released in 1962. Fish were cultured differ- 

 ently in these 2 years, however, and I attribute 

 much of the loss of salmon in 1962 to their diet. 



The third point by Neave suggests that the 

 duration of exposure to predation should be 



12 



