Coho Salmon and Habitat Response to Restoration in a Small Stream

Abstract

Stream restoration is a common conservation strategy for salmonids, but evidence that it has improved watershed scale habitat conditions or fish population abundance, survival, or spatial distribution is rare. We employed an intensively monitored watershed approach to measure the responses of both the habitat and Coho Salmon Oncorhynchus kisutch to a culvert replacement (2002) and a three-phase large woody debris (LWD) addition (2007, 2009, and 2016) in Little Anderson Creek (LA), a small stream in western Washington. Following the replacement of a fish barrier culvert with a channel-spanning bridge near the mouth of the creek, average smolt abundance for Coho Salmon more than tripled (910 vs. 275), demonstrating a significant increase in production capacity. In evaluating the habitat responses to the LWD placement, a series of mixed effects models indicated a modest, reach-scale increase in LWD density and a statistically divergent trend in spawning gravel and pool frequency, whereby both metrics increased at a slightly lower rate in treated sites than in reference sites. However, substantial interannual habitat variation appeared be driven by regional processes rather than treatment effects, as habitat change among years was consistent between the treatment and reference watersheds. Although we observed more Coho Salmon smolts after the 2009 LWD placement (364 fish), this difference was not statistically significant (P = 0.22) and neither parr abundance nor parr-to-smolt survival increased. Also, we did not detect a change in density-dependent growth dynamics or the spatial distribution of redds. Our results suggest that increasing accessibility through barrier removal can provide an immediate boost to freshwater fish production, but measurably improving population performance by adding LWD requires greater treatment magnitudes or more time for response than was examined in our study. Our ability to detect a fish response also may have been limited by low population abundance, which appeared to be below carrying capacity in most years following the LWD treatment.