Lower Duncan River Habitat Use (DDMMON-2). Year 3 Final Report

Author: J.L. Thorley, R.L. Irvine, J.T.A. Baxter, L. Porto, C. Lawrence

The DDMMON-2 report describes life history and habitat use for three target fish species that may be affected by water level fluctuations resulting from daily and seasonal operation of Duncan Dam. Snorkel surveys were conducted to assess rainbow trout and mountain whitefish spawning by timing, abundance, and habitat. Juvenile rainbow trout and mountain whitefish rearing habitats were also assessed by juvenile abundance and their use of microhabitat.

– The water use plan assumed that juvenile rainbow reared in the LDR from April to October. However, the results of this and previous studies (Hagen and Decker 2006, Decker and Hagen 2009) indicate that juvenile rainbow trout rear in the LDR year-round although the abundance of particular age-classes changes throughout the year.
– The water use plan did not explicitly consider the timing of mountain whitefish rearing (BC Hydro 2008). Nonetheless the mountain whitefish spawning component of the current program has estimated that mountain whitefish eggs in the LDR hatch sometime between February and May, depending on location within the study area and which ATU values from the literature are applied. Although mountain whitefish numbers are high in the fall they apparently decline precipitously during the winter.
– The hierarchical Bayesian analyses of the snorkel count data estimated that there were 44,000 rainbow trout fry, 5,500 rainbow trout parr and 38,000 mountain whitefish fry in the LDR below the confluence with the Lardeau River during Fall 2010. The same analyses estimated that during Spring 2012 there were 10,000 rainbow trout fry, 4,200 rainbow trout parr and 126 mountain whitefish fry.
– Preliminary data analysis indicated that, in general, juvenile rainbow trout and mountain whitefish do not exhibit a significant preference for main or side channel habitat with one exception: during high overwinter flows the number of rainbow trout parr in side channel 2.7L increased. Whether or not the densities of rainbow trout and mountain whitefish fry also increased in side channel habitats is unclear as any patterns are confounded by the large apparent changes in overall abundance of this age class during this period, as described previously.
– Key findings of this and other programs include the presence of significant numbers of juvenile rainbow trout in the LDR year-round; the almost complete absence of juvenile mountain whitefish in late winter; the almost exclusive use of slower, shallower, margin habitat; and the absence of a significant difference in the lineal densities of juveniles in main versus side channel habitats (except perhaps during high discharge).

– The water use plan assumed that juvenile rainbow reared in the LDR from April to October. However, the results of this and previous studies (Hagen and Decker 2006, Decker and Hagen 2009) indicate that juvenile rainbow trout rear in the LDR year-round although the abundance of particular age-classes changes throughout the year.
– The water use plan did not explicitly consider the timing of mountain whitefish rearing (BC Hydro 2008). Nonetheless the mountain whitefish spawning component of the current program has estimated that mountain whitefish eggs in the LDR hatch sometime between February and May, depending on location within the study area and which ATU values from the literature are applied. Although mountain whitefish numbers are high in the fall they apparently decline precipitously during the winter.
– The hierarchical Bayesian analyses of the snorkel count data estimated that there were 44,000 rainbow trout fry, 5,500 rainbow trout parr and 38,000 mountain whitefish fry in the LDR below the confluence with the Lardeau River during Fall 2010. The same analyses estimated that during Spring 2012 there were 10,000 rainbow trout fry, 4,200 rainbow trout parr and 126 mountain whitefish fry.
– Preliminary data analysis indicated that, in general, juvenile rainbow trout and mountain whitefish do not exhibit a significant preference for main or side channel habitat with one exception: during high overwinter flows the number of rainbow trout parr in side channel 2.7L increased. Whether or not the densities of rainbow trout and mountain whitefish fry also increased in side channel habitats is unclear as any patterns are confounded by the large apparent changes in overall abundance of this age class during this period, as described previously.
– Key findings of this and other programs include the presence of significant numbers of juvenile rainbow trout in the LDR year-round; the almost complete absence of juvenile mountain whitefish in late winter; the almost exclusive use of slower, shallower, margin habitat; and the absence of a significant difference in the lineal densities of juveniles in main versus side channel habitats (except perhaps during high discharge).