Debris Structure Placements and Whole-River Fertilization for Salmonids in a Large Regulated Stream in British Columbia
Abstract:A 1987 Settlement Agreement between Alcan Aluminum Ltd., the Government of Canada and the Province of British Columbia, established a program (Nechako Fisheries Conservation Program) of “measures” to conserve salmon stocks in the Nechako River. Large-scale applications of several habitat measures are planned to offset the risks to chinook salmon caused by reductions in habitat quality associated with reduced flows and mid-summer temperatures. Experimental placement of debris refugia and introduction of inorganic nutrients were conducted from 1988 to 1991 in the upper river. Juvenile chinook salmon highly colonized stream-side debris structures, including rootwad-tree sweepers, floating cribs, pseudo-beaver lodges and debris catchers. Fry density was similar to that in natural woody debris cover, which is sparse in the river. Durability of the structures was low to moderate, except for debris catchers which readily withstood peak flows of 250–270 m3·s−1. Debris catchers were also more extensively utilized by young chinook during the rearing and migratory period in June. Mean numbers of chinook fry per structure during May, June and July (1991) were: debris catchers; 277, 522 and 40; debris bundles; 209, 85 and 17, respectively. By 1991 during June to July, most chinook fry counted along the margins of the 20-km study reach, inhabited the added debris structures. Adult rainbow trout also more extensively colonized debris structures than nonstructure sites along the river margins. An in-situ nutrient bioassay in the upper river demonstrated marked nitrogen deficiency and phosphorus co-limitation, with oligotrophic background levels at detection limits. Subsequent whole-river additions of 5-fold increases in inorganic N and P from May to July in 1989 (flow, 52 m3·s−1) indicated that peak periphyton accrual, dominated by diatoms, increased an order of magnitude, and nutrient additions elevated periphyton accrual over large distances (50 km). High emigration of chinook fry confounded detection of a growth response. Debris structure placements were effective as salmonid cover, but it was unconfirmed if fertilization will augment the growth of young chinook in the upper Nechako River, as was confirmed earlier for juvenile coho salmon and steelhead trout at an oligotrophic coastal river.
Document Type: Research Article
Publication date: September 1, 1994
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