Monitoring the Entrainment of Juvenile Pacific Lamprey at Irrigation Canals of the Umatilla River

Outmigrating juvenile Pacific Lamprey are inadvertently diverted from rivers and streams into irrigation diversions common to the arid interior of the Columbia River basin (entrainment), where fish can be trapped and killed. Lamprey may be particularly susceptible to entrainment and harm because fish screens associated with these structures were originally designed to rescue juvenile salmonids from canals, not lamprey that are morphologically and behaviorally different from salmonids. As a result, the entrainment of lamprey into intermittently operating irrigation canals where fish may interact with screen infrastructure is seen as a potential factor that limits lamprey in some environments. Entrainment of PIT (Passive Integrated Transponder)-tagged juvenile Pacific Lamprey was primarily monitored at Feed Diversion Canal on the Umatilla River using stationary and mobile PIT tag arrays to determine what factors (river flow, fish size) influence entrainment, and if fish leave the canal unharmed through rotary drum screen and bypass infrastructure common to the Columbia River basin. A large proportion of PIT-tagged juvenile Pacific Lamprey released upstream of the canal headgate was estimated as entrained into the canal in 2020 (54%, CI95 39–78%), but no fish were released in 2023 and no fish tagged in previous years were detected at the fixed array within the canal in 2023. No juvenile Pacific Lamprey were detected as stranded or killed within the irrigation canal during mobile PIT tag detection surveys. Mobile PIT detection surveys were highly efficient at detecting PIT tags seeded within the canal during water diversion and after dewatering, suggesting if significant numbers of PIT tags were liberated from dead juvenile lamprey they would be detected by these surveys. Juvenile bypass antenna detections of dead lamprey that were PIT tagged and released into the water column of the canal suggests that tagged fish that die outside of their burrows or on fish screens could be mistaken for live lamprey that are bypassed. Few PIT-tagged lamprey were redetected downstream in the Umatilla and Columbia Rivers after their entrainment, obscuring the fate of these fish. However, the PIT antenna at the bypass outfall detected most entrained salmonids in 2023, suggesting that the bypass of entrained juvenile lamprey is well documented. The number of fish detected as entrained inside of Feed Diversion Canal in previous years was negatively related to Umatilla River flows at their time of release upstream of the canal, and this lamprey entrainment did not appear to be size selective based on the size distributions of detected and undetected fish. Continued annual monitoring of lamprey entrainment may provide information managers can use to evaluate how river flows may be used to avoid short windows of potential entrainment during the outmigration of juvenile Pacific Lamprey and how successful entrained juvenile lamprey are at using screen and bypass infrastructure originally made to rescue salmonids from irrigation canals.