Abstract

In 2007 and 2008, drift samples were collected from three sites on the lower Sprague and Williamson Rivers to assess emigration patterns of larval Lost River suckers (LRS) and shortnose suckers (SNS), to include characterization of the drift timing, relative abundance, and growth stage frequencies of larval suckers emigrating from the Sprague watershed prior to the removal of Chiloquin Dam. Chiloquin Dam was built on the lower Sprague River in 1914 to serve as a diversion structure to supply irrigation water for the Modoc Point Irrigation District. The dam was identified as a barrier that potentially inhibited or prevented the upstream spawning migrations and other movements of endangered LRS, SNS, and other fish in the Sprague River. These data will be used to evaluate changes in spawning distribution of LRS and SNS in the Sprague River following the dam’s removal, which occurred in the fall of 2008. Drift samples were collected at two sites on the Sprague River, one upstream of the dam at rkm (river kilometer) 9.5 near the power substation at the intersection of Chiloquin Ridge Road and Sprague River Highway (Power Station) and the other downstream of the dam at river rkm 0.7 near the town of Chiloquin, Oregon (Chiloquin). Samples also were collected on the Williamson River at rkm 7.4 at the Modoc Point Road Bridge (Williamson), which is also downstream of the dam.

Data presented in this report were collected as part of a continuation of a research project that began in 2004. Larval drift parameters measured in 2007 and 2008 were similar to those measured from 2004 to 2006. Most larvae and eggs were collected at the two drift sample sites below Chiloquin Dam (Chiloquin at rkm 0.7 on the Sprague River and Williamson at rkm 7.4 on the Williamson River). Mean and peak sample densities increased with proximity to Upper Klamath Lake. Peak larval densities continued to be collected between one and three hours after sunset at Chiloquin, which is the drift site nearest a known spawning area. Catch distribution of larvae and eggs in the lower Sprague and Williamson Rivers continues to indicate that most of the spawning of SNS and LRS occurs below Chiloquin Dam. The sizes and growth stages indicate that larval emigration from spawning areas via drift occurs within a few days after their emergence from the spawning gravels. Larval suckers appear to move downstream quickly until they reach suitable rearing habitat.

First posted May 2, 2011