Constable, R.J. Jr., Suring, E.

Publication Date

29 May 2023

Publication Name

Northwest Science

Tuesday December 12, 2023

We attempted to determine whether electrofishing removal estimates or single-pass snorkeling was a more reliable method for Oregon Department of Fish and Wildlife (ODFW) monitoring of juvenile coho salmon (Oncorhynchus kisutch) and steelhead (O. mykiss) abundance and occupancy trends. Based on 1997 to 2000 data, we assumed abundance estimates from the method that tracked more closely with parental abundance would better approximate true juvenile abundance. Parental abundance from spawning ground surveys and juvenile abundance metrics unique to each method were estimated from 2000 to 2004 and 2007 to 2008. Parental abundance did not explain the variation in juvenile abundance from either method (r2 < 0.22), invalidating our assumption, but results had relevance for snorkel surveys used in ODFW monitoring. For both species, correlations between density (fish m2 -1) and abundance (quantity, based on fish km-1) estimates from snorkeling were weak (r < 0.379), but correlations between abundance estimates from both methods were strong (r > 0.846), implying abundance was more appropriate than density for ODFW monitoring. Neither method could sample all habitats, and annually variable proportions of coho salmon (15 to 47%) and steelhead (0 to 24%) abundance estimates obtained by electrofishing were in pools too shallow to meet the ODFW depth criterion for snorkeling. This resulted in lowering the criterion to ≥ 20 cm in 2010. The lower criterion, relative to original, has not shown differences in trends, but 30% more pools have been sampled, resulting in 23% higher abundance estimates with 10% proportionately smaller confidence intervals. These changes improved ODFW monitoring and related management decisions.

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