Quantifying Pacific lamprey (Entosphenous tridentatus) ammocoete habitat availability and the risk associated with the summer hydrograph recession limb in coastal northern California

Pacific lamprey (Entosphenus tridentatus) are an anadromous fish that evolved before dinosaurs and are critical to the Pacific coastal stream ecosystems and Native American cultures. Pacific lamprey are threatened by past natural resource exploitation (logging, mining, dams, and streamflow diversion) and climate change (warming temperature and changing precipitation regime). The lamprey larva, known as ammocoetes, live in fine sediment deposits in coastal streams for three to seven years. The objective of this research was to predict ammocoete habitat based on channel morphology in coastal Northern California, USA and explore the impact of streamflow diversions on their habitat. I surveyed stream reaches for geomorphological features including; bed elevation longitudinal profile, river terraces, grain size distribution, and occurrence of ammocoete habitat throughout the Klamath and North Coast regions. I developed a binomial prediction model for the prevalence of ammocoete habitat and a habitat density model in reaches where ammocoete habitat was measured. I also measured streamflow in Redwood Creek, a tributary to the South Fork Eel River near Redway, CA; constructed a three-dimensional model of ammocoete habitat deposits; and modeled impaired, with water diversions and landscape alterations, and unimpaired scenarios for the 1989-2019 dry seasons. Slope was the strongest predictor for the presence of ammocoete habitat, but was not useful for spatial modeling. Ammocoete habitat was often associated with the downstream end of an instream obstruction. Models of streamflow and ammocoete habitat indicated that an unimpaired stream might not have much risk to ammocoete habitat quality, but an impaired stream with surface water diversions can have extreme risk by dewatering ammocoete habitat annually. These results can help guide stream restoration by knowing where to focus restoration efforts, diversion management plans, and streamflow enhancement projects in coastal Northern California