|Forest Investment Account|
|Abstract of FII Project R02-10|
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Stream temperature, invertebrate standing crop, and fish biogenetic responses
|Author(s): Hinch, Scott (University of British Columbia)||Subject: Fishery||Series: Forest Investment Account (FIA) - Forest Science Program|
Although the future timber supply in the northern hemisphere is expected to come from boreal and sub-boreal forests (areas characterized by an abundance of small lakes), little research has been conducted in these regions that examines the temperature responses of small, lake-headed streams and their salmonid populations to streamside timber harvesting. Our prior research results showed only modest changes (averaging 0.05 - 1.1 °C) with respect to summer daily maximum and minimum temperatures, diurnal fluctuations, and stream cooling in two small, lake-headed sub-boreal outlet streams in north-central British Columbia. These streams were subjected to a novel streamside harvesting treatment, whereby only commercial timber within the riparian zones was harvested, leaving behind non-commercial vegetation that would continue to provide shade and future supplies of large organic debris. Furthermore, a multi-stream comparative survey conducted in the same geographic region revealed that streams headed by small lakes or swamps tended to cool as they flowed downstream, and headwater streams warmed, regardless of whether or not timber harvesting took place. Stream cooling was attributed to a combination of warm outlet temperatures (promoted by the presence of the lakes) and cold groundwater inflows. A predictive temperature model revealed that summertime downstream warming or cooling in headwater and outlet streams could be predicted with relative accuracy by upstream maximum summer temperatures and canopy cover. In this current project, we assessed post-logging summertime temperature changes in an additional, lake-headed stream during the first summer after it was subjected to the same novel harvesting prescriptions described above. This stream also exhibited downstream cooling both prior to and following logging, and only relatively modest temperature changes (with average summertime increases of < 0.7 °C with respect to daily maximum temperatures and diurnal fluctuations, and decreases of < 0.1 °C in terms of downstream cooling) were observed following the removal of ~ 25% of the streamside canopy. Furthermore, in our lake-headed streams harvested five years ago as part of our previous research, stream temperatures appeared to be returning to pre-harvest levels, partly as a result of increased canopy closure. These results suggest that lentic water bodies and groundwater inflows are important determinants of stream temperature patterns in sub-boreal forests, and may subsequently moderate their responses to streamside harvesting. A limited temporal test of our predictive temperature model (using streams that formed part of its development data set) revealed that it predicted with relative accuracy the average summer cooling observed in three lake-headed study streams during 2002, with an overall average deviation from observed values of ~ 0.5 °C. However, further extensive testing (both temporal and spatial) is required before the model can be confidently applied to management issues. Lastly, juvenile rainbow trout from a relatively cold control stream ate more invertebrates than trout from warmer, lake-headed streams subjected to streamside logging, and feeding was found to decrease during the night and to peak during early-late afternoon. The condition of these trout was also higher than those from our treatment streams, a finding that contrasts our previous results which showed that the warmer treatment streams were beneficial for young-of-the-year trout in terms of emergence and growth. We speculate that the naturally warmer temperatures in our treatment streams, combined with higher stream-wide densities and a more aggregated distribution, placed greater bioenergetic demands on juvenile fish and resulted in less energy being available for growth. The lower condition of juvenile trout from our treatment streams relative to our control stream was therefore likely a reflection of natural differences among our study streams, and not a result of our novel streamside harvesting prescriptions. Further research into the long-term patterns of invertebrate abundance, temperature effects on fish growth, and patterns of fish distribution is clearly needed and will assist in our understanding of the dynamics of stream communities in such ecosystems.
Updated September 08, 2005
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