Elwha River Weir Project: 2013 Operations and Final Summary Report

The Washington Department of Fish and Wildlife (WDFW) seasonally operated a floating resistance board weir on the Elwha River (river kilometer 5.9) from 2010-2013. Over the course of the project, the weir was adaptively managed, as both the timing of operation and equipment design evolved in response to project results and the dynamic conditions encountered in the river. The Elwha weir's initial goal was to capture, enumerate, and sample the majority of adult salmonids, with a particular focus on Chinook salmon and steelhead trout, which are listed as threatened under the Endangered Species Act. However, in reality, the Elwha weir was unable to estimate either abundance or migration timing of Chinook salmon and steelhead due to low catch of upstream migrating fish. The river flows at which the weir could be safely installed and operated were lower than envisioned at project inception, especially when sediment accumulation following dam removal substantially reconfigured the channel at the weir site, concentrating discharge in a high velocity slot. As a result, the weir was operated for a narrow range of dates in the spring and summer, and even during these periods, often fished at low efficiency due to panels continually sunk by debris accumulation. As the project progressed, it increasingly focused on Chinook salmon because this species' summer migration timing coincided with the period of lowest flow in the Elwha River. However, Chinook salmon appeared reluctant to enter the upstream traps, raising concerns that the weir impeded upstream migration and thus delayed natural colonization following dam removal. The weir did capture large numbers of Chinook salmon moving downstream, both live and dead. These downstream moving fish were difficult to interpret in the context of abundance and migration timing, but provided a large collection of biological data including body size, scale, otolith and DNA samples, and hence important information on age structure, life history diversity and the proportion of hatchery marked fish. Ultimately, the restricted operational season and low capture efficiency of upstream migrating fish limited the weir's effectiveness as a monitoring tool. We recommend that the activities supported by the weir be replaced by other actions already approved for use in the Elwha River under NOAA and USFWS Biological Opinions and permits. Specifically, we recommend that the existing Elwha SONAR operation and its associated species composition net sampling program, which has proven successful at enumerating upstream migrating adult fish, be used as the primary tool for future estimates of adult abundance and run-timing for ESA listed Chinook salmon and steelhead. Stock composition (e.g., hatchery vs. natural origin, age structure, life history diversity) can be informed by intensive carcass surveys for Chinook salmon, and by opportunistically sampling during fish capture events such as the netting efforts for SONAR species composition and implanting radio telemetry transmitters.