Upper Chehalis Instream Fish Study 2015

Although knowledge of a fish's spatial distribution is a key element both for evaluating impacts to it and developing a restoration plan, little is known about the distributions of non-salmonid fishes in the Chehalis River Basin. This study is designed to address that data gap and more fully inform the analyses of a potential flood control dam in the Upper Chehalis River. Primary objectives for the study were to identify and describe the fish assemblage (not abundance) within the proposed dam inundation footprint.

A total of 59 reaches were surveyed during July, August, and September of 2015 - 25 in the inundation footprint for the Upper Chehalis, 24 in the lower extent of several tributaries within the inundation footprint, and 10 located upstream and downstream of the inundation footprint. Characterization of fishes in the reaches surveyed used a combination of upstream and downstream snorkeling, electrofishing, seining, and dip netting.

Overall, 14 species of fishes were identified in the study area�"one species of catostomids, two species of cottidae, four species of cyprinids, two species of petromyzontids, and five species of salmonids. The most widely-distributed species were torrent sculpin (93% of surveyed reaches), juvenile steelhead or rainbow trout (92% of reaches), coho salmon (83%), speckled (59%) and longnose dace (44%). Lamprey ammocoetes not identified to species occupied 49% of surveyed reaches, with Pacific lamprey ammocoetes identified in 41% of the reaches. In addition to torrent sculpin, reticulate sculpin (41%) were also detected, and their distribution was limited to the mainstem portion of the inundation footprint. Largescale sucker, redside shiner, and mountain whitefish were detected in the downstream portion of the inundation footprint, and northern pikeminnow were only detected in supplemental survey reaches downstream of the proposed dam site.

Salmonids were distributed throughout the inundation footprint. The majority of trout identified to in the mainstem reaches were rainbow trout, while cutthroat trout were positively identified in surveyed reaches within the tributary portion of the inundation footprint. Juvenile coho salmon were found throughout the inundation footprint, while juvenile Chinook salmon were detected in only two reaches of the downstream portion of the mainstem inundation footprint.

Non-salmonid species richness was highest in the downstream, mainstem portion of the inundation footprint. Detections in the tributary portions of the inundation footprint were limited to torrent sculpin, speckled dace, and, to a lesser extent, longnose dace. Distributions of the detected fishes are likely related to a variety of factors including habitat preferences, water temperature, physical and biological barriers, and migration timing. Given their relatively limited migratory behavior, many of the native non-salmonids are present year-round. With respect to the dam, inundation, either permanent or during the winter months during flood-stage events, would impact the fish species present in the Upper Chehalis.