cording to Sims (1970), nearly 100 percent of the 

 sockeye salmon migration had passed through 

 Brownlee Reservoir. 



SUMMARY 



The distribution and movement of juvenile salm- 

 on that migrate through Brownlee Reservoir, a 

 large impoundment on the Snake River, was 

 studied from 1962 to 1965. The study included na- 

 tive spring and fall chinook salmon and kokanee 

 and hatchery-reared fall chinook, coho, and sock- 

 eye salmon. 



Each native salmon population had a character- 

 istic age, size, and time of entry into the reservoir. 

 Most spring chinook salmon from Eagle Creek 

 entered the reservoir in the late fall as age-group 

 O, 53 to 125 mm. long. These fish overwintered in 

 the reservoir, primarily in the Powder River Arm. 

 They resumed their seaward migration in the 

 spring just before the peak of a second outmigra- 

 tion from Eagle Creek in March and April of age- 

 group I fish, 65 to 138 mm. long. The migration 

 of juvenile spring chinook salmon from the Weiser 

 River entered the reservoir in peak numbers in late 

 April or early May; it consisted of age-group I 

 fish, 106 to 176 mm. long. This movement was 

 closely followed by that of age-group O fall 

 chinook salmon from the Snake River, 33 to 105 

 mm. long, which entered the reservoir in peak num- 

 bei-s in mid-May. Kokanee migrants entered the 

 reservoir latest in the season ; age-group I fish, 70 

 to 155 mm. long, arrived in mid-June. 



Hatchery-reared groups of salmon migrated into 

 the reservoir as follows: fall chinook salmon of 

 age-group O (46-135 mm.) in mid-May of 1964 

 and 1965; coho salmon of age-group I (71-166 

 mm.) in mid-May 1964; and sockeye salmon of 

 age-group I (86-175 mm.) in early April 1965. 



Migrants from all populations were near the 

 surface as they entered. As the season progressed 

 and the juvenile fish moved downreservoir, the}' 

 tended to move into deeper water. 



Migration peaks from the reservoir varied from 

 year to year, depending on reservoir conditions, 

 but were sequential by stock. Fish (mainly fall 

 chinook salmon) that remained in the reservoir 

 from the previous year's migration left the reser- 

 voir early (late January or February) at the ap- 

 proach of a cold water mass that moved through 



the reservoir. This migration was followed in late 

 February and early March by fish (mainly fall 

 chinook salmon) that had overwintered in the 

 Powder River Arm. Spring migrants from Eagle 

 Creek and Weiser River arrived at the dam in 

 large numbers in April and May. Fall chinook 

 salmon from the Snake River arrived in late May 

 to early July and kokanee, in July or August. 

 Hatchery-reared fall chinook salmon left in Ma}' 

 1964 and in April 1965, coho salmon in late May 

 1964, and sockeye salmon in April 1965. 



Juvenile fish that did not leave the reser- 

 voir by late June or July were confined to re- 

 stricted areas of the reservoir by high epilimnion 

 temperatures and low concentrations of dissolved 

 oxygen, which extended into the epilimnion from 

 the hypolimnion. Wlien this process began, juve- 

 nile salmon in the upper end of the reservoir usu- 

 ally reentered the slightly cooler waters of the 

 Snake River. They returned to the reservoir when 

 temperatures in the river began to approach 20° C. 

 The survival of holdover salmon through the sum- 

 mer and early fall was extremely poor. 



The differences in success of passage through the 

 reservoir were more closely related to the physical 

 conditions of the resei-voir than to behavioral dif- 

 ferences between species of salmon stocks. Success 

 of passage for all populations was poorest in 1963 

 when the reservoir was nearly full throughout the 

 migration. Under this condition, the reservoir was 

 92 km. long and surface currents were either weak 

 or nonexistent and often moved upreservoir. Pas- 

 sage through the reservoir was intermediate in 



1962 and 1964 when drawdown was 6 to 14 m. 

 through May and the reservoir averaged 70 km. 

 long. The most successful passage was in 1965 

 when the drawdown was large (26-28 m. through 

 May), the reservoir was relatively small (45-50 

 km. long) , and currents were consistently oriented 

 downstream. Loss of orientation and upreservoir 

 movement of the juvenile salmon were correlated 

 with conditions in the reservoir most prevalent in 



1963 and least prevalent in 1965. 



Early entrance into the reservoir appeared to 

 improve the chances of successful passage. In gen- 

 eral, early fish encountered the best combination 

 of reservoir length, current conditions, and envi- 

 ronment. Late migrants, such as the kokanee, en- 

 tered a rapidly deteriorating environment, and 

 their success of passage was extremely poor. 



242 



U.S. FISH AND WILDLIFE SERVICE 



