Rapid Out‐Migration of Lost River and Shortnose Sucker Larvae from In‐River Spawning Beds to In‐Lake Rearing Grounds

We examined the process of larval out‐migration of endangered Lost River suckers Deltistes luxatus and shortnose suckers Chasmistes brevirostris from spawning grounds in the Williamson and Sprague rivers to rearing grounds in Upper Klamath Lake. Most downstream movement occurred at night, when larvae drifted in surface currents at the center of the channel. During daylight, larvae were absent from the drift and abundant near the periphery of the wetted channel, in areas that lacked current. The ages, sizes, and developmental stages of larvae from spawning grounds and the river mouth were similar, indicating that larval suckers transit from spawning grounds to the lake as soon as 1 d after the beginning of the larval period and that in‐river rearing is rare. The percent of larvae with empty guts declined from upriver towards the river mouth, suggesting that first feeding is an important driver of the out‐migration process. Sucker larvae greater than 13.99 mm standard length were rare in the rivers but abundant in the lake, suggesting out‐migration to Upper Klamath Lake is advantageous. Warmer temperatures and greater food availability may be important attributes of lake nursery grounds that are not found in the river. Most larvae entered the system on the falling river hydrograph, a strategy that does not promote floodplain access. Also, larvae of related species do not use floodplains. Therefore, suggestions that channelization and dredging of the lower Williamson River and severance of river–floodplain connectivity by levee construction have negatively affected larval suckers by slowing the out‐migration process or eliminating preferred habitats were not supported.