Water Stewardship Information Sources

Citation Hyatt, K.D., Stiff, H.W., Stockwell, M.M., Luedke, W., Rankin, D.P., Dobson, D., and Till, J. 2015. A synthesis of adult Sockeye salmon migration and environmental observations for the Somass watershed, 1974-2012. Can. Tech. Rep. Fish. Aquat. Sci. 3115: vii + 199 p.
Organization DFO
URL http://publications.gc.ca/collections/collection_2015/mpo-dfo/Fs97-6-3115-eng.pdf
Abstract/Description or Keywords Daily mean air and water temperature, stream discharge, and adult Sockeye
migration data were assembled for Sproat Lake and Great Central Lake Sockeye
stocks returning to the Somass watershed, British Columbia. A continuous regional
air temperature index was statistically related to intermittent water temperature
observations to hind-cast daily water temperature trends at several sites (lower
Somass River, Sproat Falls, Stamp River Falls) from 1918-2012. Statistical relations
between discharge observations were used to extend discontinued discharge
records at the Somass and Stamp Falls sites based on the continuous series (1913-
2012) available from the Sproat River site. Peak-over-threshold analyses were
applied to reconstructed time-series to review trends in extreme temperature and
flow. Daily mean water temperature and discharge were correlated with Sockeye
migration rates at the Sproat Falls and Stamp Falls fishways.
Daily mean water temperatures exceeding 19-21°C at two sites (Stamp and Sproat
Falls) were associated with stopped or reduced migration, especially during
extended low flow periods. At Stamp Falls, a high proportion (70%) of upstream
migration activity occurred at river temperatures of 18-19°C and discharge levels of
20-50 cms. However, peak daily migration rates occurred at combinations of lower
water temperatures (16-18°C) and higher discharge levels (60-90 cms).
Fifty to eighty-five percent of Sproat migration activity (days) occurred when Sproat
River daily mean temperatures were above 18-20°C. Above average flows (>30
cms) may enable Sproat Sockeye to withstand stressful water temperatures during
the 3-km migration interval in the Sproat River, which is typically 1-2°C warmer than
the relatively cool waters at the confluence of the Somass-Sproat rivers (<21°C) and
at depth in Sproat Lake (<15°C hypolimnetic water). Highest daily migration rates
were associated with temperatures of 17-22°C and flows of 32-36 cms. At Sproat
discharge levels <22 cms, maximum migration rates occurred at no more than 18°C.
The frequency and duration of “warm” weather episodes (daily mean temperature
>20°C) have steadily increased in the Somass area since the 1950’s, with
corresponding increases in the frequency and duration of equivalent “warm” water
periods observed to be stressful (reduced migration rates) or lethal (mass mortalities
in 1990 and 2004) to fish in the Stamp and Sproat rivers. Given results in this report,
climate change projections do not auger well for sustainable production of Somass
system Sockeye salmon in decades beyond 2050 in the absence of human
interventions (e.g. creation of additional water storage, engineering of “cold-water”
release structures for the Somass, Stamp and Sproat rivers) to mitigate for trends in
environmental conditions that, left unaddressed, will most certainly decrease future
migration success of adult Sockeye salmon.
Information Type report
Regional Watershed Vancouver Island North
Sub-watershed if known Somass River
Aquifer #
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Project status complete
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