EWING ET AL.: EFFECTS OF SIZE AND RELEASE TIME ON SALMON 



grate because they did not reach a critical size and/or 

 growth rate by the appropriate photoperiod. Migra- 

 tion from Pelton ladder seemed to occur as fish 

 reached a particular size, since during a 3-wk period 

 of migration, there was no difference in average 

 fork length of the fish recaptured (Table 5). From 

 estimated growth rates (Table 3), fish at the end of 

 the migration period might be expected to be nearly 2 

 cm larger than those at the beginning. This influence 

 of size on migration could be best demonstrated in 

 fish recaptured at the Dalles Dam after a migration 

 distance of 213 km. Apparent growth rates were 

 much higher than that of fish reared at Round Butte 

 Hatchery, suggesting that a selection for larger fish 

 occurs during the long migration distance. 



A major concern in utilizing a closed system for 

 studying seaward migration is the importance of 

 aggressive behavior by resident fish toward newly 

 introduced fish. Chapman (1962) found that aggres- 

 sive behavior of resident fish may be partly respon- 

 sible for emigration of fish introduced into the 

 system. Aggressive behavior may have caused the 

 rapid movement immediately following release for 

 the March and April release groups in both 1977 and 

 1978. Further movement of these fish was not ob- 

 served until May. Alternatively, migration in these 

 fish immediately after release may have been due to 

 disorientation of the fish upon release and a passive 

 drifting downstream with the current. Fish released 

 earliest into Pelton ladder migrated first in both 1977 

 and 1978. 



The importance of determining appropriate times 

 for hatchery releases of spring chinook salmon in 

 order to obtain maximum seaward migration is de- 

 monstrated by the short time during which maximum 

 migration occurred (Tables 1, 2). In both 1977 and 

 1978 peak migration occurred within a period of a few 

 weeks. Releases made on either side of this time 

 period exhibited decreased migratory activity. The 

 use of model systems, such as the Pelton ladder, to 

 determine when peak migration occurs can benefit 

 hatchery programs by suggesting sizes and times for 

 release of salmonids which maximize seaward mi- 

 gration. 



ACKNOWLEDGMENTS 



We thank the members of the Deschutes River 

 Salmon Study, Round Butte Hatchery personnel, 

 and biologists of Portland General Electric Company 

 for their help and cooperation throughout this study. 

 We acknowledge the special assistance of Ray Hill, 

 Jerome Diamond, Zeke Madden, Garet Soules, and 

 Richard Aho. This study was supported by a grant 



from Portland General Electric Company to the 

 Oregon Department of Fish and Wildlife. 



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