Shifts in sauger spawning locations after 40 years of reservoir ageing: influence of a novel delta ecosystem in the Missouri River, USA

To determine how reservoir ageing affects sauger Sander canadense spawning locations in Lewis and Clark Reservoir, we quantified riverine habitat and determined sauger spawning locations during recent years (2003–2004) and compared these results to historic (1950–1966) habitat conditions and spawning locations. The riverine habitat above the reservoir, which was historically homogeneous, has developed into two distinct areas: the upstream remnant river reach, characterized by a distinct main channel, and cold and clear water from the hypolimnetic discharge from the upstream reservoir, and the downstream delta section, which has abundant side channels and backwater areas and is warmer and more turbid water than the remnant reach. Sauger spawning locations have shifted markedly in the last 50 years. Sauger historically spawned in the upper reaches of this system, near Fort Randall Dam. However, in 2003, we found that sauger spawned in the lower delta reach and avoided the remnant reach. This shift likely occurred concomitantly with the development of the novel delta habitat. The delta likely functions more similarly to the historic remnant reach of this system (warmer temperature, turbidity, active meandering, complex habitats, etc.), indicating that sauger prefer to spawn in areas with historic riverine function. Thus, future management activities intended to enhance sauger populations should focus on the restoration of riverine function, such as that provided by emerging reservoir deltas, which may mimic pre‐impoundment conditions on the Missouri River. Copyright © 2008 John Wiley & Sons, Ltd.