Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

13accounted for much of the variability of cutthroat abundance in small coho-cutthroatstreams, using 119 sites from Bella Coola and south (r 2 = 0.55; Rosenfeld et al. 2000).Anadromous Cutthroat Only ReachesAnadromous + Resident Cutthroat ReachesResident Cutthroat Only Reaches700Cutthroat Trout Biomass g/100 sq m60050040030020010000 1 2 3 4 5 6 7 8Average Summer Wetted Width m9Figure 1. Juvenile cutthroat trout standing crops (total cutthroat biomass per 100 m 2 ) inRegion 2 stream reaches versus wetted summer stream width (adapted from Deleeuw andStuart 1982: widths averaged from 1-2, 2-3, 3-5, 5-8 and 8+ m).Biomass data for rearing coastal cutthroat trout were also plotted by DeLeeuw and Stuart(1982) against average percent stream gradient. This clearly showed that streams of zerogradient supported zero rearing cutthroat, and streams near 0 to 0.5 % gradient supportedfew cutthroat. Typically, zero gradient reaches are sloughs or ditches, particularly withinthe lower Fraser Valley. In striking contrast, higher gradients from 0.5 % to >7 %supported high biomasses per unit area (Figure 2). Of note, resident trout reachessupporting higher biomasses per unit area were within the 4-8 m range of summer wettedwidths with moderate gradients of 2 %, the latter likely associated with boulderysubstrates and lower competition with sympatric juvenile coho salmon.As for the lentic waters of sloughs, ponds and lakes, cutthroat use is related to postglacialaccess, and to availability of nursery streams as well as the quality (cover) andproductivity (food availability) of the main water body. Further, some sloughs andditches are frequently only migratory corridors. Velocities can be low, and summer tofall conditions (particularly water temperature and/or dissolved oxygen) can be unsuitablein summer (unless groundwater-fed). Conditions may improve in fall to spring (Slaneyand Northcote 2003). For example, although velocities and depths are suitable in somesections of Mountain Slough and lower McCallum Ditch near Agassiz, they are