An investigation of rainbow trout populations in the Sumallo River, a major upstream tributary of the Skagit River, British Columbia, was conducted to assess the survival, size and abundance of two hatchery strains of rainbow trout (Skagit and Blackwater, stocked in 1988 and 1989, respectively) and to examine life history patterns of wild rainbow trout (Oncorhynchus mykiss) and Dolly Varden (Salvelinus malma). Few hatchery fish of either strain were captured in the Sumallo and Skagit rivers. Estimated numbers of Blackwater trout remaining in the Sumallo River during September, 1990, were very low (162 fish; 0.3% of all rainbow trout or 2.3% of those stocked). The downstream migration of wild rainbow trout was comprised of four age classes including 1+ (mean length, 97 mm), 2+ (126 mm), 3+ (149 mm) and 4+ (187 mm). Based on scale analysis, length-frequency distributions, and a comparison of age and growth with a near-by non-migratory fluvial population of rainbow trout (upper Skagit), most adult rainbow trout (> 200 mm) in the Sumallo were migratory and had undergone a period of lake growth before returning to their natal stream to spawn or feed, or were adfluvial-lacustrine. Back-calculations from scale samples indicated there was a strong positive relationship between size and survival, thereby the larger and older juveniles disproportionately contributed to the adult trout population.The density and biomass of juvenile rainbow trout and Dolly Varden charr during late September were 0.032 fish/m2 and 0.53 g/m2, and 0.040 fish/m2 and 0.62 g/m2, respectively, which are low compared to most trout streams. Extreme phosphorous deficiency (< 1-2 ppb concentration in summer) and, secondarily, a relatively low summer temperature regime (ca. 10 C) are the main limitations to production of trout and charr of the Sumallo River. Management implications and options are summarized.
Rosenau, Marvin L., Slaney, Patrick A.. 1991. A Population Assessment and Stocking Evaluation of Rainbow Trout in the Sumallo River. Ministry of Environment, Fisheries Branch. Fisheries Project Report. RD-26