The Kootenai River white sturgeon Acipenser transmontanus has been isolated from other white sturgeon populations for over 10,000 years, Bonnington Falls in British Columbia, Canada, creating the isolation. Libby Dam, constructed in 1972, modified the flow and temperature regime of the river, which affected spawning and recruitment of white sturgeon. Kootenai River white sturgeon are only known to spawn in a reach near Bonners Ferry, Idaho, 100 km downstream from the dam. In 2 of 8 years of study, only 15 white sturgeon eggs were collected over gravel–cobble substrate. However, in the other 6 years, 1,193 eggs were collected over sand substrate; these areas usually exceeded 5 m in depth, were within the main channel, and had water velocities of 0.2–1.0 m/s and temperatures of 8.5–12°C. In general, these characteristics differed from optimum white sturgeon spawning habitat in the Columbia River, where velocities average 0.8 m/s, water temperatures are 12–17°C, and gravel–cobble substrate is available. Spawning over sand substrate might contradict survival strategies because white sturgeon have an adhesive egg to which sand adheres. Differences in spawning habitat may be an outcome of behavioral divergence or disruption to environmental cues but was probably caused by preferred habitat no longer being available. Recovery of Kootenai River white sturgeon will depend primarily on continuation of mitigated flows for spawning migrations, suitable spawning habitat, and ultimately survival of eggs and larvae. It is unknown at this time whether recent spawning alone will lead to sufficient recruitment to help recovery of the population, but if it does not recruit substantial year‐classes, we believe consideration must be given to measures that would provide coarser spawning substrates and warmer water temperatures.

Each autumn and spring, adult white sturgeon Acipenser transmontanus migrate from the lower Kootenai River and Kootenay Lake, British Columbia, to prespawn staging reaches in Idaho. In spring, they migrate further upriver to a spawning reach near Bonners Ferry, Idaho. We monitored movement and behavior of 49 reproductively mature white sturgeon with radio and sonic telemetry from 1991 through 1997. White sturgeon responded to mitigated flows from 1994 through 1997, migrating substantial distances to a spawning reach soon after the onset of local runoff and rising water temperatures. Males migrated at temperatures of 5.5–12.1°C, 2 weeks before spawning; females followed about a week later, at slightly warmer temperatures. Females stayed in the spawning reach 1–28 d, averaging 10.5 d. Males spent 7 d to 2 months in the spawning reach, averaging 30 d. After spawning, 63% of the females moved immediately to Kootenay Lake; the remainder spent a longer time in the river downstream of the spawning reach. Some (52%) males remained in the river, and the remainder migrated to Kootenay Lake. Female behavior and migration was more attuned to environmental conditions than was male behavior. Several environmental variables were examined to determine their effect on female white sturgeon migration to the spawning reach. Changes in temperature and river stage were the best predictors of the probability that females would migrate to the spawning reach. A logistic regression model, when applied to a subset of our original observations, correctly predicted movement to the spawning area 93% of the time. Our model can be used as a tool for risk assessment of white sturgeon spawning migration during various snow pack or temperature forecasts. It will be helpful in determining approximate migration or spawning times, making water management decisions, and assessing effects of temperature fluctuations. The model will be useful to continued study of white sturgeon by predicting spawning migration and improving efficiency in deploying sampling gear.

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