Fall Chinook salmon spawning activity versus daylight and flow in the tailrace of a large hydroelectric Dam

We deployed an acoustic system during the spawning season for fall Chinook salmon Oncorhynchus tshawytscha in 2001 to determine whether fall Chinook salmon spawning activity in a hydroelectric dam tailrace area was affected by daylight or river flow. Our study design allocated sampling effort nearly equally between hours of darkness and hours of daylight throughout each 24-h period. The acoustic system recorded sounds of fall Chinook salmon spawning activity in two index areas downstream of Wanapum Dam on the Columbia River in Washington State. One index area was a deepwater spawning site located in 9-11 m of water. The other index area was a moderate-depth midchannel bar, where water depths ranged from 2.5 to 6 m. We defined the rate of spawning activity in digs per minute. Fall Chinook salmon spawning activity rates in the Wanapum Dam tailrace were influenced by both daylight and river discharge, which had a pronounced nonlinear effect on spawning activity rates. To account for nonlinearity, a generalized additive model was used to characterize the combined effects of river flow and daylight. The final model also suggested that both flow and daylight influenced spawning activity. Spawning activity occurred during both daylight and darkness, significantly more activity occurring during daylight in both index areas. Spawning activity was generally highest at project discharges between 1,700 and 2,266 m(3)/s in both spawning areas; spawning activity diminished as discharge increased from 3,400 to 4,250 m(3)/s. We concluded that fall Chinook salmon spawning activity in this regulated discharge environment was affected more by flow (and velocity) than by daylight.