Fraser River Eulachon Egg/larval Abundance Surveys
Annual surveys began in 1995 and are based on methodology described in CSAS 2002/117. View time-series of spawning stock biomass (gif) or in-river progeny densities (gif) and abundance (gif) estimates.
Figure 1. Bongo nets deployed to capture eulachon larvae are equipped with a flow meter to measure the volume of water filtered. Progeny densities (or numbers of embryos and larvae per cubic metre of water filtered) are calculated for each plankton net tow.
Figure 2. Bongo nets filter the surface waters of the lower Fraser River. Samples are collected at variable depths to a maximum of 10 metres. Fraser River photos: Nev Venables
Figure 3. Embryonic eggs and newly hatched larvae are retrieved and preserved in 1-litre jars for subsequent laboratory identification and counting at the Pacific Biological Station, Nanaimo, BC.
Figure 4. An eulachon embryonic egg captured and counted from a Fraser River plankton net tow. Lab photos: Matt Thompson
Figure 5. A recently hatched, eulachon larva captured and counted from a plankton net tow. Microscopic counts typically range from zero to over 3,000 larvae captured during a 6-minute plankton net tow. Densities (or numbers of embryos and larvae per cubic metre of water filtered) are calculated for each plankton net tow. Fish larval Identification assistance is provided courtesy of the Alaska Fisheries Science Center. Select the smelt family: Osmeridae (Eulachon = Thaleichthys pacificus) or other similar or common forage fish family to learn distinguishing characteristics (i.e. Ammodytidae = sand lance, or Clupeidae = herring). See photo identification guide of Fraser River specimens.
Figure 6. Bootstrap estimated, mean densities and confidence intervals were applied (N=1000) to each annual eulachon biomass (t) estimate. Densities of embryonic eggs and recently hatched larvae, moving down river were calculated twice weekly, over a 7 week period from late April to early June. Total Fraser River eulachon spawning stock biomass (SSB) was derived by applying daily river flow data (m3/sec) and relative fecundity estimates (the number of eggs produced per gram of eulachon) to a time-series of measured egg and larval densities using a 1:1 spawner, sex ratio assumption. An area-under-the-curve calculation was applied over the survey period. Spawner biomass box-plots based on progeny exiting the South (main) Arm of the lower Fraser River near Deas Island and progeny density and river flow plots of the combined South (main) and North Arms of the lower Fraser River are updated annually. Samples were collected near Kilometre #20 on each arm of the lower Fraser River shown in Figure 8. Several hundred, 1-litre jar samples were collected over each season at different river depths and transect positions across the river. Cumulative out-migration is assumed, as mixing or "blending" of embryonic eggs and larvae occurs during a brief, 20-100 km journey down-river. Daily Fraser River discharge data was measured by the Water Survey of Canada at Hope, B.C. See CSAS 2002/117 for methods and results or a detailed sampling manual.
|Year||South Arm (t)||North Arm (t)||Combined Index (t)|
Note: SSB index is subject to minor changes due to Water Survey of Canada adjustments.
Figure 7. Length frequency histograms of adult eulachon captured in the lower Fraser River from 2001 to 2004. Eulachon spawners were comprised mostly of age-3, some age-4 and older adults (size range from 140 to 205 mm in standard length). Notice a significantly greater proportion of large, age-3 (or possibly age-4) eulachon spawners captured in 2003. This L-F distribution appears to correspond to a relatively high proportion of large, age 2+ or perhaps age-3 or older eulachon observed in the same year among samples collected from juvenile, feeding grounds located off the west coast of Vancouver Island (SA's 124 + 125). Eulachon spawners captured in the Fraser River were sampled randomly each year from gill net catches, that were selective for large size (truncated distributions). Standard lengths (tip of snout to hypural plate) were measured to the nearest millimetre (mm) and pooled by 2 mm groupings on the histograms. Relative fecundity estimates (the number of eggs produced per gram of eulachon) were determined from several hundred, female spawners collected from the lower Fraser River between 1995 and 1998 and subsequently applied to SSB determinations described in Figure 6.
Figure 8. Map of the lower Fraser River showing floodplain areas and kilometre distances from the mouth, courtesy of the UBC Department of Geography. Most eulachon spawning areas are confined to the lower 110 kilometres of the river (limit of tidal influence). This section of the river overlaps with a significant portion of the rearing habitat of a white sturgeon population, known to feed intensively on eulachon. Ichthyoplankton sampling is conducted annually at Kilometre #20 on the South (main arm) and North Arm of the lower river to assess the size of the eulachon run.
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