Warner Sucker (Catostomus warnerensis), endemic to the lakes and tributaries of the Warner basin, southeastern Oregon, USA, was listed as threatened by the US Fish and Wildlife Service in 1985 because of habitat fragmentation and threats from introduced nonnative fish. To determine progress towards recovery and document factors that may prevent recovery, we evaluated the distribution, abundance, and movements of this species between 2006 and 2013. The species exhibits 2 distinct life-history types, a lake morph and a stream morph. The abundance of suckers in the Warner Valley lakes has decreased since the mid-1990s, and the lake fish assemblages were dominated by nonnative predatory fishes. The basin experienced drought conditions from 2007 through 2010, and the lakes partially dried which affected both sucker distribution and movement. During the drought, when lake levels were low and receding, we found little evidence of sucker recruitment in the lake populations. In 2011, stream discharge was high and the lakes refilled, and in 2012 we documented successful sucker recruitment. Warner Suckers in the tributaries were widely distributed and relatively abundant, and the stream habitat was in good condition. We did not document substantial downstream movement of suckers during the spring and summer months and found little evidence of mixing between the stream and lake populations. We documented the upstream movement of large numbers of suckers in 2 tributary streams during the spawning season. We also described associations between sucker distribution and habitat characteristics in the tributaries. Our analysis indicated support for the threatened listing and suggests that recovery efforts would be most effective by focusing on improvements to both upstream and downstream passage at irrigation diversion dams and on irrigation efficiencies.
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