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

Abstract

The 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 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)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(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.

More flow in a regulated river correlates with more and earlier adult lamprey passage, but peak passage occurs at annual low flows

“…However, dam counts may not correlate with the abundance of lamprey that never pass and spawn downstream of the dam (Clemens, Weeber, et al, 2021;Lucas et al, 2009). Indeed, Pacific lamprey is often observed spawning downstream of Leaburg Dam (Michael Hogansen, Oregon Department of Fish and Wildlife, pers.…”

“…Dam counts are used to monitor trends in abundance and ecology of adult lampreys migrating upstream (Clemens et al, 2017;Clemens, Weeber, et al, 2021;Keefer et al, 2009b;Kynard & Horgan, 2019).…”

“…Thus, artificial barriers are commonly recognised as a primary limiting factor or threat to anadromous lampreys of the world, including Pacific lamprey (Clemens et al, 2017; Clemens, Arakawa, et al, 2021; Maitland et al, 2015). Historical dam counts of adult Pacific lamprey enable monitoring trends in relative abundance (Clemens, Weeber, et al, 2021) and assessments of migration ecology relative to river flow, water temperature and year (e.g. Keefer et al, 2009b).…”

Broad‐scale eDNA sampling for describing aquatic species distributions in running waters: Pacific lamprey Entosphenus tridentatus in the upper Snake River, USA

“…In western North America, this species was once so abundant as to be occasionally regarded as a nuisance: the mass die‐off of adults following spawning forced Indigenous peoples to temporarily rely on alternative sources for clean water (Petersen‐Lewis, 2009), and even fisheries biologists late in the 20th century mentioned removing 1000s of immature specimens trapped on irrigation structures (Virgil Moore, Idaho Department of Fish and Game, personal communication). Like many anadromous species in the region, however, Pacific lamprey have markedly declined in recent decades (Clemens et al ., 2021) and were petitioned for listing under the U.S. Endangered Species Act (U.S.…”

“…In response to the repeated calls for better information on the distribution of Pacific lamprey (Clemens et al ., 2017; Luzier et al ., 2011; Reid & Goodman, 2015), methods have been developed to target particular habitats and life stages ( e.g ., larval forms in wadeable streams (Dunham et al ., 2013) or their deep‐water mouths (Harris & Jolley, 2017)). These have been supplemented by seasonal counts at index sites originally chosen for monitoring other species [ e.g ., screw traps for capturing outmigrating salmon smolts in road‐accessible streams (Hayes et al ., 2013) or redd counts in reaches chosen for monitoring steelhead O. mykiss spawning (Clemens et al ., 2021)]. Although these techniques can be effective for determining the presence of Pacific lamprey (Mayfield et al ., 2014; Reid & Goodman, 2015, 2016; Harris et al ., 2016), index sites or reaches are uninformative for determining the upstream extent of occupied habitat, whereas the labour‐intensive sampling required to describe larval habitat occupancy (Harris et al ., 2020) precludes its broad‐scale application.…”