In this study, the feasibility of using Large organic debris (LOD) to improve 'damaged' stream habitats was assessed over a two-year period in a debris-torrented stream on the Queen Charlotte Islands, British Columbia. The main objectives of the study were:
1. to determine if LOD could be re-introduced into debris-torrented streams to increase localized;
2. to determine whether or not the increases in pool habitat would also increase coho overwinter survival and thus coho smolt production;
3. to compare the benefits of increased coho smolt production with the costs of replacing LOD in debris-torrented streams.
Large organic debris (LOD) was introduced into a small debris-torrented stream in 1982 in an attempt to increase the amount of pool habitat available for overwintering coho. After one winter, the number of pools present in an experimental section of the stream increased from four to fourteen, the number of pool types increased from three to six, pool area increased from 2.5 to 18.4%, and LOD cover increased from 2.2% of the wetted stream area to 11.2%. After a torrent-like summer flood and a second winter, the number of pools present had increased again to 21, while the amount of LOD cover and pool area was 17.7 and 15.4% of the total stream area available, respectively. By comparison, pool number, pool diversity, pool area, and LOD cover all remained relatively constant in a control section over the same time period. The stream was torrented again in spring 1985, at which time the only structures still in place and functioning, were those that had been cabled to trees and stumps alongside the stream.
Compared to a control section, coho overwinter survival in the experimental section increased from 13 to 52% in the 1st year, while smolt production increased from 290 fish per kilometre of stream to 1050 fish per kilometre. The second year, coho survival declined to 10% and smolt production to 140 fish per kilometre of stream, but both were still considerably higher than those in nearby torrented streams without new LOD. Benefit/cost analysis based on the first year's results indicated that the benefits derived from increases in coho production could pay for the cost of new LOD emplacements after five or six years, as long as there is no more mass wasting. Because three LOD emplacements were still creating new rearing habitat after a debris torrent, restoring LOD in streams is considered to be a realistic and feasible way of offsetting some of the impacts of mass wasting in productive fish habitats. Further studies are now needed to determine what LOD configurations can be used to manipulate stream flows more precisely and improve fish spawning and rearing habitats. Such structures must be able to accommodate recurrent large scale sediment movements without blocking the stream and without being destroyed.
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Updated December 16, 2008