Variation in Juvenile Salmonid Use and Fish Community Structure in Off-Channel Floodplain Habitats, Queets River, Washington, between Two Years with Contrasting Discharge Patterns
Treva Coe, Master's Project

Reduced hydrological connectivity in floodplain habitats relative to other stream habitats may render them disproportionately susceptible to interannual variations in summer streamflows. The objective of this study was to compare availability, accessibility, and connectivity of off-channel floodplain habitats between two consecutive years with contrasting summer low-flow discharge patterns, and to examine the effects of this interannual variability in floodplain habitats to patterns in juvenile salmonid use and fish community structure between the two years.

A total of nine sites were sampled during both 1997 and 1998 summer low-flow periods, and an additional 11 sites were sampled in 1998. Diminished streamflow and precipitation during summer 1998 reduced availability, connectivity and accessibility of floodplain habitats relative to 1997. Of the nine sites sampled both years, 1998 summer low-flow wetted area was estimated to be 21,500 m2, or 40% less than in 1997. Conditions during the drier summer negatively impacted fish populations, causing stranding of outmigrating coho and chinook smolts, increased vulnerability to predation and physiological stress, and restricted access to and ability to disperse among floodplain habitats.

Young-of-the-year coho salmon (Oncorhynchus kisutch) occurred in most isolated patches during 1998, especially in terrace tributaries, while YOY trout (O. mykiss or O. clarki clarki) occurred in most persistent patches, especially on the active and marginal floodplain. Yearling and older cutthroat (O. clarki clarki) and steelhead/rainbow trout (O. mykiss) were more common in terrace tributaries, although steelhead/rainbow trout were also common in sites on the marginal floodplain. Relationships of CPUE of salmonid taxa with habitat were stronger in 1997 relative to 1998, presumably due to greater habitat selectivity during wet years when connectivity and availability of floodplain habitats are enhanced. Fish communities were dominated by Cottus perplexus and young-of-the-year coho salmon and steelhead/rainbow and cutthroat trout.

Fish community composition showed high assemblage persistence and stability between years and among channel and habitat patch types. Variation in summer streamflows and floodplain habitat conditions may differentially affect different species, due to the influence of emergence timing on habitat colonization. Coho salmon may benefit in a wet year when earlier emergence may allow for colonization of optimal feeding territories, while later-emerging trout, which are precluded from colonizing floodplain habitats due to reduced connectivity, may benefit from reduced risk associated with floodplain habitat use in dry years.