Importance of Small Streams as Rearing Habitat for Coastal Cutthroat Trout

Abstract

Previous research indicates that the abundance of small coastal streams is often underestimated on topographic maps, and their relative contribution to total salmonid habitat within coastal drainages is unknown. To document the extent and distribution of streams in different channel-width classes and to estimate the proportion missing on topographic maps, we walked and surveyed entire stream networks in representative high- and low-gradient coastal topographies on the west coast of Vancouver Island, British Columbia. The amount of wetted stream area in different channel-width classes within a drainage was greatest in larger channels, but most of the linear length of stream was in smaller channels, especially in low-gradient topographies. Fish-bearing streams with a bank-full channel width less than 1.5 m and 1.5–2 m represented 16% and 27%, respectively, of total wetted stream length at summer low flow in a single representative low-gradient third-order drainage but averaged only 3% and 7%, respectively, of total wetted stream length in high-gradient basins. Streams with a channel width less than 1.5 m have the potential to contribute even more to overwintering habitat, in that their proportion of total channel length increased to 23–57% in low-gradient drainages surveyed during winter high flows. In low-gradient topographies, from 31% to 100% of fish-bearing stream length was missing on both 1:20,000 and 1:50,000 topographic maps. Given that small streams contribute disproportionately to the rearing habitat of juvenile cutthroat trout Oncorhynchus clarki, development plans and riparian management practices that identify and protect small streams are critical for the long-term conservation of coastal cutthroat trout populations.