Refuge or Risk: Impacts of Irrigation Diversions on Larval/Juvenile Lamprey in the Yakima Basin

Irrigation diversions provide deceptive refuge to many thousands of larval/juvenile lampreys each year and pose a potentially serious threat to Pacific Lamprey (Entosphenus tridentatus). Within the Yakima Basin, large portions of entrained lamprey are Western Brook Lamprey (Lampetra richardsoni), with a smaller portion being Pacific Lamprey (~ 7%). After dewatering in 2014, the number of lampreys residing downstream of the fish screens at Sunnyside Diversion (river km 171.0 in Yakima River) and upstream of the fish screens at Wapato Diversion (river km 175.5) was estimated to be 11,667 and 7,200, respectfully, based on VIE mark-recapture estimates. Of the larval and juvenile Pacific Lamprey PIT-tagged and released in Chandler Diversion (river km 74.3), only 2.9% and 12.9%, respectfully, were detected in the fish bypass a year after release, raising concerns about the fate of these entrained fish. In the short term, periodic repair of damaged seals, slower dewatering rates, and installation of small structures to reduce fine sediment collection in front of fish screens could potentially reduce lamprey losses in irrigation diversions. Long term benefits include introduction of flow deflectors, upgrading headgate structures, and/or installation of new fish screens to reduce entrainment of fine sediment and larval lamprey into irrigation canals.