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Research Document - 2002/117

Fraser River Eulachon Biomass Assessments and Spawning Distribution: 1995-2002

By D. E. Hay, P. B. McCarter, R. Joy, M. Thompson, K. West

Abstract

The eulachon (Thaleichthys pacificus) is a small anadromous smelt (Osmeridae) that spawns in the lower reaches of the Fraser River in April and May. In most years, modest catches were taken by First Nations, a small commercial fishery and a recreational fishery. Since the mid-1990's the many participants and observers of these fisheries have expressed concerns about the apparently declining spawning runs. To address some of these concerns we adapted marine ichthyoplankton survey methods and assessed eulachon spawning stock biomass (SSB) based on egg and larval production surveys. Eulachon deposit small adhesive, demersal eggs. At ambient temperatures, these eggs hatch into small (<7 mm) pelagic larvae that are rapidly advected downstream, at the same velocity as the river. We developed survey protocols that estimated larval density (n/m3) at five different locations over a seven-week period. We use the measured river discharge (m3/s) to estimate the total number of larvae discharged over specific periods of time. We convert this estimate of larval numbers into SSB from our measured estimates of relative fecundity (about 400 egg/g). The SSB estimated are presented with bootstrapped confidence limits for each area, and we discuss the sources of variability and error associated with this assessment method. By comparing the estimated spawning biomass among different areas of the river we can also comment on the general spawning locations each year. Since 1995, we estimate that SSB has varied from a minimum of about 100 tonnes (in 1997) to a maximum of 1600 tonnes (in 1996). The SSB for years 2001 and 2002 are between 800 and 1000 tonnes, but the 2002 samples analyses is not complete, because of budget constraints.

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