White sturgeon spawning and discharge augmentation

  Observed spawning events of endangered Kootenai River white sturgeon, Acipenser transmontanus (Richardson), during 1994–2002 were examined to evaluate the effects of discharge from Libby Dam on spawning. Discharge from Libby Dam was manipulated in most years to provide enhanced flows (hereafter, augmented discharge) for white sturgeon spawning. The annual onset of spawning appeared independent of augmented discharge because initial spawning occurred prior to augmented discharge in 4 of 6 years. Spawning began 2–11 days after the highest river discharge in 4 of 9 years. Linear regression analysis indicated the onset of spawning was positively related to mean daily discharge from the dam but not mean daily temperature, 1‐day change in discharge (Δ discharge) or 3‐day Δ discharge. Logistic regression analysis suggested the probability of a spawning event was influenced by Julian date, mean daily water temperature, mean daily discharge and the 7‐day Δ discharge, but the predictive indices were small. Minor fluctuations in temperature and discharge characteristics had little additional predictive benefit for spawning after the river reached spawning thresholds. The highest probability of spawning (0.48) was for the temperature interval 9.5–9.9 °C, and 93% of the estimated spawning events occurred above 8 °C. Sixty percent of the estimated spawning events occurred at discharges ≥600 m 3  s −1 , which comprised 45% of the range of discharge values.