Abundance Trends for Adult Pacific Lamprey in Western Oregon (USA): Historic Declines, Recent Increases, and Relative Contributions from Coastal Rivers

Abstract

AbstractThe anadromous Pacific Lamprey Entosphenus tridentatus is native to the northern Pacific Ocean and its drainages, returning each spring to spawn in streams along the West Coast of North America. We analyzed abundance trends of adult Pacific Lamprey from dam counts and redd surveys in western Oregon, USA. We compared a series of generalized additive models (GAMs) to describe lamprey abundance trends. Lamprey counts varied considerably among dams but showed steep declines over 70+ years (1949–2019) at Bonneville Dam on the Columbia River, over 50+ years (1965–2019) at Winchester Dam on the North Umpqua River, and over 17 years (1993–2009) at Gold Ray Dam on the Rogue River. By contrast, redd surveys displayed synchronous abundance trends of different magnitudes among geographic management areas on the Oregon coast, with an overall increasing abundance over 13 years (2007–2019). This corroborates pre‐existing notions that Pacific Lamprey spawning in Oregon coast rivers are from a single mixed population. The most supported GAM included a fixed effect for location and allowed abundance to vary over years as a function of location. These results suggest that abundance trends among dam counts are more variable than coastal redd surveys and that dam counts and redd surveys across this large region are not useful proxies for each other. Furthermore, counts across dams may be more variable because they record prespawning lamprey and thus may be recording a mixture of cohorts. Redd surveys, by contrast, provide a metric of a single cohort of spawning lamprey.