Exploring the Historical, Current and Future Distribution of Pacific Lamprey

Pacific Lamprey, Entosphenus tridentatus, were once widely distributed in western North America. Currently, their distribution has contracted northwards to Big Sur, just south of Monterey Bay, and they are absent in many smaller coastal streams. Similarly, their elevational limit has contracted, with lampreys no longer reaching previously occupied upper reaches, even in unimpeded streams. Historical records and vouchered specimens are uncommon and scattered. Unvouchered records of sub-adults have little value due to frequent misidentifications. Similarly, absence data is frequently unreliable, due to survey focus on other taxa and techniques that do not effectively sample lampreys. We explore the historical distribution of Pacific Lamprey through a combination of approaches, including museum collections, historical accounts, consideration of natural barriers and application of elevational and hydrologic criteria based on known localities. We also assess salmonids as surrogates, but find them limited in utility due to considerable differences in drainage selection, habitat use and passage capabilities. In resolving the current distribution of Pacific Lamprey we used these criteria, included artificial barriers and incorporated our own field surveys for lampreys at over 900 sites region-wide. Understanding historical and current constraints on distribution lets us assess potential threats and opportunities in the stewardship of Pacific Lamprey.