EFFECTS OF SIZE AND TIME OF RELEASE ON 



SEAWARD MIGRATION OF SPRING 



CHINOOK SALMON, ONCORHYNCHUS TSHAWYTSCHA 



R. D. Ewing,' C. E. Hart, 2 C. A. Fustish, 3 and 

 Greg Concannon' 



ABSTRACT 



Juvenile spring chinook salmon, Oncorhynchus tshawytscha, from Round Butte Hatchery on the Deschutes 

 River, Oregon, were released monthly into a 3.7 km fish ladder. Fish released into the ladder from February 

 to May migrated through the ladder in mid-May in both 1977 and 1978. Fish released after mid-May 

 migrated through the ladder within 2 weeks after release. The extent of migration decreased progressively in 

 fish released after 15 June. The migration was presumably photoperiod dependent, although temperature 

 may have acted both as a releasing factor for migration and as a stimulus for growth. In the fish ladder, size of 

 the fish remained constant over a 3-week migration period, suggesting that larger fish migrated before 

 smaller fish. After a migration of 213 km, fish captured at the Dalles Dam had very large apparent growth 

 rates, suggesting that larger fish were faster migrants. 



Maximum survival of juvenile salmonids after 

 release from hatcheries is dependent upon their 

 rapid migration to the sea (Raymond 1979). Delays in 

 this seaward migration may subject the juveniles to 

 starvation and stress which rapidly deplete their 

 numbers (Miller 1952, 1958). Residual hatchery 

 juveniles in a river often have an impact on wild 

 stocks of fish through piscivory (Sholes and Hallock 

 1979) and competition for food (Chapman 1966). 

 Rapid migration of hatchery juveniles ensures max- 

 imum survival to adulthood with minimal interaction 

 with wild stocks. 



Timing and duration of the physiological conditions 

 which result in migratory behavior are still relatively 

 unknown. Timing of seaward migration in juvenile 

 salmonids depends upon a number of environmental 

 factors, including photoperiod (Wagner 1974), tem- 

 perature (Solomon 1978), water flow (Mains and 

 Smith 1964), and fish size (Wagner 1974). The 

 interrelationships between these are not well 

 understood, but the available data suggest that these 

 relationships may be complex. Hoar (1958) and 

 Baggerman (1960) have postulated that these 

 environmental factors act as "releasers" which, in 

 conjunction with a physiological readiness to 

 migrate, trigger overt migrational behavior. 



'Corvallis Fish Research Laboratory, Oregon State University, 

 Corvallis, OR 97331. 



department of Zoology, Oregon State University, Corvallis, OR 

 97331. 



'Oregon Department of Fish and Wildlife, Research and Develop- 

 ment Section, Corvallis, OR 97331. 



Manuscript accepted August 198:!. 



FISHERY BCLLETIN: VOL. 82. NO. 1. 1984. 



In most river systems, the relative influence of such 

 factors is estimated by extensive sampling programs 

 which use multivariate analysis of the data. Control 

 of environmental variables in such a system is not 

 possible. Furthermore, the size of many river sys- 

 tems prevents an unbiased sampling of juveniles dur- 

 ing migration. It is difficult, therefore, to obtain 

 reliable estimates of the size of fish at migration, the 

 timing of migration, and the influence of the environ- 

 ment on that timing. 



In the present study, an unused fish ladder provided 

 a relatively constant environment for migration of 

 juvenile spring chinook salmon, Oncorhynchus 

 tshawytscha, over a 3.7 km distance. Serial releases 

 of hatchery-reared juveniles into this system permit- 

 ted an investigation of the timing of seaward migra- 

 tion, the duration of the migration tendency of the 

 juveniles, and the relationship of several environ- 

 mental variables to seaward migration. 



METHODS 

 Study Area 



The study area included the lower 175 km of the 

 Deschutes River, Oreg., and the lower Columbia 

 River from its confluence with the Deschutes River to 

 the Dalles Dam (Fig. 1). 



Rearing Conditions 



Progeny from spring chinook salmon spawned at 



157 



