BECKER: PARAMETERS OF JUVENILE CHINOOK SALMON 



Invertebrate drift is important to the feeding 

 of stream fish, and particularly so at Hanford. 

 The significance of the drift phenomenon is 

 that of increasing the availability of food and 

 supplementing possible site limitations on insect 

 production; moreover, under conditions of high 

 discharge, the quantity of drift organisms pass- 

 ing downriver per unit of time is higher than 

 under low flow conditions (Waters, 1969). If 

 this is true for the central Columbia River, the 

 annual spring spate increases the availability 

 of food organisms to juvenile chinook salmon 

 during their period of maximum abundance. 



Changes in river water levels, both seasonal 

 and regulated, appear to have unique significance 

 at Hanford by exerting an influence on popula- 

 tions of aquatic insects and juvenile chinook 

 salmon in inshore areas. The influence is ap- 

 parent in at least four theoretical ways. 



First, the annual increase in river discharge 

 in April and May (Figure 2) inundates barren 

 shoreline areas that are exposed to air during 

 the preceding winter. Recolonization of flooded 

 inshore areas by aquatic insects depends upon 

 larvae in the drift, which may occur rapidly, 

 or upon the deposition of eggs by adults. There 

 are no available data on recolonization rates of 

 recently inundated areas at Hanford. But 

 detached insect larvae usually spend only a 

 short time in the drift and re-attach as soon 

 as possible (Elliott, 1967). 



Second, weekly and daily variations in water 

 level resulting from flow regulation at Priest 

 Rapids Dam (Figure 3), which periodically 

 floods and exposes vast stretches of shoreline 

 areas, restricts insect recolonization and incor- 

 porates marginal dwellers into the river drift. 

 On this basis, it is not surprising that the diet 

 of juvenile chinook salmon includes food 

 organisms that normally live along the shore- 

 line such as NotoHecta nymphs, adult spring- 

 tails, and terrestrial Arachnida (spiders). 



Third, station occupation by juvenile chinook 

 salmon appears to be temporally limited. Young 

 salmonids commonly occupy relatively small 

 home areas (ecological niches) for a period of 

 feeding and growth prior to seaward migration 

 (Chapman, 1966; Edmondson, Everest, and 

 Chapman, 1968; Chapman and Bjornn, 1969). 

 Analysis of data from fish collected at the 



primary stations reveals considerable variation 

 in sizes from week to week after mid-May. At 

 least part of this variation must result from 

 weekly changes in regulated water level at 

 Priest Rapids Dam (up to 2 m in 24 hr on 

 weekends) that implements population turnover. 

 Fourth, the eminent decline in river discharge 

 volume from the annual spring spate (> 300,000 

 cfs) to the summer period of low flow (=^40,000 

 cfs) suggests that the falling water level is one 

 factor involved in prompting seaward juvenile 

 chinook salmon still lingering at Hanford. At 

 any rate, the seasonal increase and then decrease 

 in river flows accompanied by rising tempera- 

 tures (Figure 2) are the main environmental 

 factors correlated with seaward migration. 

 These phenomena, which have occurred an- 

 nually throughout recorded histoiy, may well 

 have played an evolutionary role in the develop- 

 ment of the spring migration characteristic for 

 young fall chinook salmon produced in the 

 Columbia River ecosystem. 



ACKNOWLEDGMENTS 



Numerous individuals provided assistance 

 in various capacities. L. R. Heaton, E. F. 

 Prentice, E. W. Lusty, 0. L. Jackson, T. M. 

 Clement, and E. G. Tangen aided in field 

 collections. R. T. Jaske, Manager of Water 

 Resources Systems, Battelle Northwest Labora- 

 tories, provided temperature and discharge 

 data for the central Columbia River. C. E. 

 Cushing and K. R. Price reviewed this 

 manuscript. 



LITERATURE CITED 



Allen, K. R. 



1942. Comparison of bottom faunas as sources of 



available fish food. Trans. Am. Fish. Soc. 71: 



275-283. 

 Becker, C. D. 



1970a. Feeding bionomics of juvenile chinook salmon 



in the central Columbia River. Northwest Sci. 



44:75-81. 

 1970b. Temperature, timing and seaward migration 



of juvenile chinook salmon from the central 



Columbia River. U.S. AEC. Res. Dev. Rep. (BNWL- 



1472), Battelle Northwest, Richland, Wash., 21 p. 



399 



