BCs Inland Rainforest Conservation and Community

Conference Proceedings

Harvesting effects on fish food resources in headwater streams of the interior rainforest

Brian A. Heise 1, Cathy Mackay 2 and Jacqueline J. Sorensen 3

We evaluated the effects of clearcut harvesting on invertebrate communities in three ESSF forests of Interior British Columbia; Sicamous Creek, Thompson-Okanagan and the Southern Cariboo. Although typically fishless, headwater streams within these high elevation forests are increasingly recognized for supplying important nutrients and food subsidies for downstream fish habitat. In the three studies described here, our objective was to examine the effectiveness of riparian management practices on both benthic (Sicamous Creek) and drifting invertebrates (Thompson-Okanagan and Southern Cariboo).

In the first study near Sicamous, artificial substrate baskets containing gravel were placed in three cutblocks and two control streams for five years to monitor aquatic invertebrate communities. Logging seriously impacted the streams in two of the three cutblocks. These streams contained significantly fewer shredding stoneflies, lower numbers of EPT taxa, lower EPT genus richness, and altered proportions of shredders and scrapers, compared to their respective controls. These differences can be attributed to the differing amount of logging slash (needles and branches) left in each stream following winter logging.

Invertebrate drift was examined using two different design approaches. In the Thompson-Okanagan study area, drift was measured within clearcuts 1-3 years and 5-7 years post-harvest, and compared to un-harvested controls (22 streams in total). Invertebrate drift density and biomass increased in both the 1-3 year and 5-7 year treatments; however, shifts in community structure occurred predominantly in older clearcuts. In the Southern Cariboo we applied a BACI design on 6 treatment and 4 control streams to look at the effects of harvesting on drifting invertebrates before and after clearcut treatment. The results of this study also showed positive responses in both drift density and biomass in the logged streams.

Overall, clearcut logging in which the stream channel has been highly compromised from the deposition of slash, as in the case of Sicamous Creek, will adversely affect aquatic invertebrates. In terms of drifting food resources for fish, we observed no deleterious effects in short-term responses after clearcut logging. Our estimate of total export of invertebrates from headwater streams in both the Thompson-Okanagan and Southern Cariboo areas ranged from 370-1870 mg day. One small stream could potentially provide food for up to 100 juvenile salmonids per day. This contribution, compounded by the sheer number of headwater streams in a watershed, provides evidence of the importance of these high elevation streams in providing food for fish. The main management recommendation to come out of this research is to keep logging slash out of stream channels even in high elevation, headwater streams to preserve the integrity of invertebrate communities.

Contact Information

1 Natural Resource Sciences, Thompson Rivers University, Kamloops, BC. Email: bheise@tru.ca

2 UNBC, now at Environmental Dynamics Inc., Prince George, BC. Email: cmackay@edynamics.com

3 University of British Columbia Okanagan and Thompson Rivers University, Kamloops, BC. Email: jsorensen@tru.ca

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