Coho salmon (Oncorhynchus kisutch) in the Southern Oregon/Northern California Coast (SONCC) Evolutionary Significant Unit (ESU) were listed as threatened under the federal Endangered Species Act in 1997 (62 FR 24588); and their listing was reaffirmed in 2005 (70 FR 37160). As a component of the California Department of Fish and Wildlife’s (CDFW) Coastal Salmonid Monitoring Program (CMP) monitoring of coho salmon population spatial structure was conducted in the lower Eel River and its tributaries, inclusive of the Van Duzen River, in 2013, 2014, 2015, and 2016. In the CMP’s Northern California area, monitoring of juvenile coho salmon is consistent with the salmonid population spatial structure data needs and protocols of the CDFW’s Coastal Monitoring Program.

Survey protocol and data analysis methods were implemented as described by Garwood & Larson (2014) and Garwood and Ricker (2013, Revised 2016). The Project’s coho salmon population study area is a CMP-modeled and team-defined survey sample frame of 204 lower Eel River and Van Duzen River sample reaches between 1 km and 3 km long and subreaches less than 1 km long. Generalized Random Tessellation Stratified (GRTS) was used to randomly order sample reaches for survey frame, and one third of the reaches are surveyed annually.

Personnel were trained in underwater identification and counts of fish species that occurred in the lower Eel River and Van Duzen River. Counts of salmonids were identified to categories based on size and physical appearance. Using mask and snorkel, two surveyors conducted independent dive observations in each pool and analysis of the independent passes are used to estimate fish detection probability. Coho salmon were the target species of the survey; however, all observed fish were counted and identified, if possible, in each pool.

A total of 163 unique reaches of the 204 available in the sample frame were surveyed at least once from 2013 to 2016. A total of 211 surveys were completed during the summer over the four years; 23 reaches were not surveyed due to lack of landowner access permission and 42 reaches had no pools which met the suitability protocol. Analysis was conducted on data obtained from the 211 reach surveys.

Observations from the 2013, 2014, 2015, and 2016 surveys confirm the distribution of coho in the lower Eel River watershed is limited to 3% to 7% of potential available habitat and occurred in small patches within the frame through all years of the survey. Coho salmon juveniles were not observed in many of the suitable cold water streams, however, on-going drought conditions that occurred throughout the survey period may have played a role in the limited distribution of coho salmon within the study area. Observations of juvenile coho salmon occurred in a few reach locations where they had not previously or recently been documented such as Price Creek, Monument Creek, and Bridge Creek. Coho salmon were observed in the majority of pools at reach locations in Lawrence Creek watershed, Shively Creek, upper Bear Creek, and Chadd Creek, and these streams are potential source populations for coho recovery in the lower Eel River and Van Duzen River watersheds.

Coho salmon and Chinook salmon were often observed in the same reaches and streams. Trout were found in nearly every stream surveyed except for the mainstem Eel River and a few small tributaries. Since the boundaries of this survey were partly defined by coho anadromy, some trout habitat was not surveyed. Sacramento pikeminnow and California roach, which are not native to the Eel River, were observed in all the mainstem Eel and Van Duzen survey reaches and larger tributary reaches with warm water temperatures (>20° C). They were found in smaller, cold water streams as well but in fewer numbers. Sacramento pikeminnow and California roach often occupied habitat that was deemed unsuitable for coho salmon, and overall survey results indicated that coho salmon and these invasive fish species do not overlap.

In addition to the CMP reports, the CDFW's Coastal Watershed Planning and Assessment Program’s Lower Eel River Watershed Assessment report (2008) and the Van Duzen River Watershed Assessment Report (2013) also provide additional information concerning monitoring efforts and salmonid resources in the Lower Eel and Van Duzen Rivers.

Garwood, J., S. Ricker 2013. 2013 (Revised 2016) Juvenile coho salmon spatial structure monitoring protocol: Summer survey methods. California Department of Fish and Wildlife. Arcata, CA.

Garwood, J. and M. Larson. 2014. Reconnaissance of salmonid redd abundance and juvenile salmonid spatial structure in the Smith River with emphasis on coho salmon (Oncorhynchus kisutch). Final Report. California Department of Fish and Wildlife. Arcata, CA. Retrieved from Smith River Alliance