Figure 1. — Study area showing locations of sampling 

 equipment used to assess migrations of Juvenile salmon 

 and rainbow trout to Brownlee Reservoir. 



SAMPLING SITES, EQUIPMENT, AND 

 PROCEDURES 



The plan of study was to sample populations of 

 migrating fish before they entered the reservoir, to 

 determine their characteristics, and to estimate 

 their abundance. This plan required the installa- 

 tion of floating fingerling traps in the Snake River 

 above the reservoir and a fixed louver in Eagle 

 Creek, a tributary of the Powder River near the 

 lower end of Brownlee Reser\'oir (fig. 1) . 



Sample catches from the populations of juvenile 

 Chinook salmon in the Snake and Weiser Rivers 

 early in the study established that juvenile fall 

 Chinook salmon migrated from the Snake River 

 at age 0, whereas juvenile spring chinook salmon 

 migrated from the Weiser River at age 1. Thus, it 

 was possible to separate these two populations 

 on the basis of age or size and to sample them from 

 a single location below the confluence of the two 

 rivers. 



SNAKE RIVER 



The sampling site for the Snake and Weiser 

 Rivers was about 8 km. below their confluence and 

 about 4 km. above the reservoir. Flows during the 

 spring were 383 to 1,372 c.m.s. (cubic meters per 

 second) and averaged 744 c.m.s. At average flow, 

 the Snake River channel at this location was about 

 152 m. wide and had a maximum depth of 7.6 m. 



The sampling device in the Snake River was a 

 modified "migrant dipper" (Mason, 1966) — a self- 

 cleaning, floating fingerling trap with louvered 

 leads (fig. 2). The basic unit consisted of a trap 

 section, 12.2 m. long by 7.6 m. wide by 1.8 m. deep, 

 with fixed louver leads that extended 9.8 m. up- 

 stream at a 10° angle to the flow. A self-cleaning 

 traveling screen formed the rear of the trap, and 

 a metal screen floor extended upstream to the two 

 fixed louver sections. The louvers guided the fish 

 into the trap area where a continuously rotating 

 scoop dipped the fish and deposited them into a 

 trough. The fish were then flushed into a holding 

 pen at the side of the trap. 



Fish were captured in traps from 1962 through 

 1965. We carried out feasibility tests with one trap 

 in 1962 and examined the horizontal distribution 

 of downstream migrants at the same time. In the 

 spring of 1963, two migrant dipper traps were 

 attached to an overhead cable and positioned in 

 the main current. Floating louver extensions were 

 added to the fixed louver sections to increase the 

 sampling capability. In attempts to increase the 

 catch, the louver angle and lengths were altered 

 each year (table 1). In 1962, the traps were oper- 

 ated continuously from mid-April until early July 

 and then intermittently until December. On the 

 basis of tliese early experiments, the operations in 

 1963-65 began in mid-March and ended in July. 



Table 1. — Traps used in the Snake River and configuration 

 of attached louver arrays 



Fish captured in the traps and subsequent esti- 

 mates of the magnitude of migration were classi- 



204 



U.S. FISH AND WILDLIFE SERVICE 



