Insights into the spawn timing of Pacific Lamprey in western Oregon (USA) and considerations for adapting surveys

Abstract Objective Monitoring the spawning of native lampreys, including the Pacific Lamprey Entosphenus tridentatus, in western Oregon streams relies on survey protocols for winter-run steelhead Oncorhynchus mykiss monitoring. Pacific Lamprey and steelhead have similar spring spawn timing. Our objectives were to (1) model the effects of date, year, geographic management area (GMA), and predicted stream temperatures during peak redd density for Pacific Lamprey within the steelhead spawning season and (2) use these model results, along with additional insights, to identify how future lamprey redd surveys could be adapted to increase the accuracy of redd counts. Methods We used data collected during randomized, spatially balanced redd surveys for winter steelhead across western Oregon during 2007–2022. We analyzed redd density as a proxy for the spawn timing of Pacific Lamprey by using a series of generalized additive models that included date, year, GMA, and stream temperature as covariates. Results Lamprey redds were documented from February through early July, with most occurring in April and May. The most supported candidate generalized additive models revealed that redd density for Pacific Lamprey could be described using Julian day, year, GMA, and simulated stream temperatures 10 months to 2 weeks prior to spawning. However, these models described only 4.3–25.5% of the overall variation in Pacific Lamprey redd density, which may be partially attributable to the sampling design. Conclusions In the absence of stronger statistical results, our conclusions are necessarily speculative. The results appear to corroborate previous studies on the thermal triggering of Pacific Lamprey spawn timing months in advance. Consideration of the spatial and temporal extents of the surveys and habitat covariates is necessary to identify the desired scope of inference and to improve the explanatory power of statistical models used to make inferences on lamprey spawn timing.