Draft: Outmigration Timing and Distribution of Larval Eulachon, Thaleichthys pacificus, in the Lower Columbia River, Spring 2001

The eulachon Thaleichthys pacificus, an anadramous member of the smelt family, spawns along the Pacific coast of North America, from the Pribilof Islands (Bering Sea) to the Klamath River in California (Wydoski and Whitney 1979). The Lower Columbia Basin supports one of the largest spawning runs of eulachon, with the majority of spawning activity occurring in the main stem of the Columbia River and the Cowlitz River. Smaller, periodic runs occur in the Grays, Skamokawa, Elochoman, Kalama, Lewis, and Sandy tributaries. Adult migration in the Columbia River system usually begins in December, peaks in February and continues through May (WDFW 2001).

Spawning eulachon females generally deposit eggs in areas where the substrate consists of coarse sand/fine gravel, and where water flows are �"moderate” in velocity (Hart and McHugh 1944; Smith and Saalfeld 1955). Eggs adhere to the surface of the substrate and incubate over a period of about 30-40 days, depending on temperature. Upon hatching the larvae become part of the drift as (presumably) passive plankters and are rapidly transported out to sea (Hart and McHugh 1944; Hart 1973) where they rear in near-shore marine areas at moderate to shallow depths (Barraclough 1964).

Historically, the commercial catch of eulachon in the Columbia River system has been strong, yet variable. Annual returns, based on commercial landings, were relatively stable until 1994 when a sharp decline occurred - a trend that continued through 1999. Although the 2000 and 2001 spawning runs were apparently stronger in the Lower Columbia, the relative magnitude is difficult to quantify as restrictive fishery management strategies imposed in response to the recent decline in returns severely reduced commercial effort.

Mechanisms controlling eulachon recruitment and survival are poorly understood. Conditions in the freshwater environment, where eulachon spawn, may influence productivity. Of particular concern are the potential effects of dredging associated with the U.S. Army Corps of Engineers (USACE) proposed channel-deepening operations on the Columbia River (USACE 1999). Dredging activity has the potential to impact eulachon through entrainment of spawning adults (Larson and Moehl 1990, McGraw and Armstrong 1990) and possible smothering of developing eggs by increased turbidity and suspended sediment in the vicinity of operations (Morton 1977, Prussian et al. 1999). Entrainment of developing eggs and out-migrating larvae has not been documented but remains a concern. In response to these concerns the USACE contracted the Oregon Department of Fish and Wildlife (ODFW) and the Washington Department of Fish and Wildlife (WDFW) to identify eulachon spawning sites within proposed channeldeepening areas and to characterize the spatial and temporal distribution of eulachon larvae in the mainstem Columbia River during the out-migration period.

Results from the first year of the study (Spring 2000) showed that smelt larvae were widely distributed throughout the river during the out-migration period; however, sampling limitations precluded us from determining the relative importance of the shipping channel as a migration corridor relative to the remainder of the river. Our objectives in 2001 were to quantify the timing of larval outmigration, to determine cross channel larval distribution and to identify the depth distribution of larvae within the shipping channel.