DAUBLE ET AL.: SPATIAL DISTRIBUTION OF JUVENILE SALMONIDS 



0-age Chinook salmon during seining. For some 

 species, active sampling techniques helped sup- 

 port distribution trends observed from fyke net 

 data. For example, the preference of migi-ating 

 sockeye smolts for offshore areas was also indi- 

 cated by the absence of sockeye salmon in near- 

 shore collections using electroshocking and 

 seines. 



In our study, hatchery-reared 0-age chinook 

 salmon (range 75 to 90 mm FL) were less abun- 

 dant in shallow nearshore areas than wild stocks. 

 This spatial segregation was evident for both 

 seining and fyke net collections. Thus, differ- 

 ences in distribution patterns that may be attri- 

 buted to size, season, or physiological condition 

 were also evident. 



Diel movement patterns were consistent with 

 those observed in previous studies of migi-ating 

 juvenile salmonids in the Columbia River. Prin- 

 cipal movement of outmigi-ating juvenile chinook 

 salmon occurred during the night at Priest 

 Rapids Dam (Sims and Miller 1977) and at Byers 

 Landing (Mains and Smith 1964). Smith (1974) 

 collected 91'7f of primarily 1-age juvenile chinook 

 salmon at night in impounded waters on the 

 Snake River. Sockeye salmon have also shown a 

 preference for nocturnal movement in other 

 river systems (Kerns 1961; Dames and Moore 

 1982). In general, natural light intensity appears 

 to be the major environmental factor controlling 

 diel migration patterns of salmonid fry (Godin 

 1982). 



The observed patterns of diel behavior may 

 have affected the cross-sectional distribution of 

 the juvenile salmonids. For example, we ob- 

 served that spring chinook salmon smolts were 

 often abundant just after sunset in shallow near- 

 shore areas (<30 cm deep) of low current veloc- 

 ity. This inshore appearance may have preceded 

 active or passive downstream movement. Night- 

 time movement into the current may result from 

 a loss of visual contact with the surroundings 

 (McDonald 1960) or a reduction of rheotactic 

 response (Hoar 1953). Both of these mechanisms 

 could result in passive downstream displace- 

 ment; however, there were distinct differences 

 in diel timing among the four species collected. 

 These differences suggest that migi'ation is not 

 controlled solely by passive mechanisms. 



Documented migi-ation rates of juvenile sal- 

 monids in the Columbia River are consistent 

 with activity rhythms that include feeding, 

 quiescent behavior, and active migration. At 

 midstream velocities averaging 1 m/s, a pas- 

 sively drifting fish would travel about 29 km in 



an 8 h night. Migration rates would be faster 

 with higher current velocities, as occurs during 

 the spring freshet (2-3 m/s), or for actively mi- 

 gi'ating fish. Most salmonid smolts apparently 

 migrate actively in midchannel for only a few 

 houi's daily since reported mean migration rates 

 of juvenile salmon through the Hanford Reach 

 are about 56 km/d (Weitkamp and McEntee 

 1982). The patterns of distribution that we ob- 

 served probably provide only a partial descrip- 

 tion of the interacting behavioral characteristics 

 that increase species survival and efficient use of 

 energy reserves. 



ACKNOWLEDGMENTS 



Several individuals contributed to the success 

 of this project. We thank E. M. Greager and D. 



A. Neitzel for project guidance; E. W. Lusty for 

 coordinating equipment setup; L. M. Athey, D. 

 W. Carlile, and J. C. Simpson for statistical 

 analysis; J. B. Athern for assistance in obtaining 

 the barges; J. Campbell for engineering specifi- 

 cations; P. McDonald for use of facilities; and M. 



B. Dell for fisheries information. A. J. Scott, C. 

 S. Abernethy, D. C. Klopfer, and W. B. Duke 

 provided principal technical assistance in the 

 field; they and others suffered through the ran- 

 dom days and rotating shifts. C. D. Becker pro- 

 vided valuable review comments and K. A. 

 Borgenson edited the manuscript. Studies were 

 supported by UNC Nuclear Industries, Inc. and 

 the U.S. Department of Energy under Contract 

 No. DE-AC06-76RLO 1830. 



LITERATURE CITED 



Allen, R. 



1977. Status of the Columbia River salmon and steel- 

 head runs. In E. Schwiebert (editor), Columbia 

 River salmon and steelhead, p. 22-32. Am. Fish. 

 Soc. Spec. Publ. No. 10, Bethesda, MD. 

 Becker, C. D. 



1973. Food and growth parameters of juvenile chinook 

 salmon, 0»corhi/nchns tshawytsclm. in central Co- 

 lumbia River. Fi.'ih. Bull., U.S. 71:387-100. 



1985. Anadromous salmonids of the Hanford Reach, 

 Columbia River: 1984 status. PNL-5371. Pacific 

 Northwest Laboratory, Richland, WA, 81 p. 



Cochran, W. G. 



1977. SampHng techniques. 3rd ed. John Wiley and 

 Sons, N.Y.,428p. 

 Coutant, C. C. 



1986. Thermal niches of striped bass. Sci. Am. 

 2.5(2):98-104. 



Dames and Moore. 



1982. Bristol Bay regional power plan: Newhalen 

 River juvenile sockeye salmon studies. Prepared for 



789 



