Pacific Lamprey in Coastal Drainages of California: Occupancy Patterns and Contraction of the Southern Range

Pacific Lamprey Entosphenus tridentatus have been overlooked historically in fish surveys using standard salmonid methods along the Pacific coast of North America. As a result, little is known of even their broader distribution patterns at the drainage level. Over the last 2 decades, the species has shown a northward range contraction of over 900 km, and at this time the southernmost population is the Big Sur River in California. This study evaluates the hypothesis that Pacific Lampreys do not typically utilize relatively small coastal drainages (<100 km 2 ). We examined historical Pacific Lamprey records throughout California and surveyed 349 current sites in 102 coastal drainages along the California coastline and into Baja California. Within their currently occupied range (north of Big Sur), Pacific Lampreys were found in all drainages >100 km 2 , but in only 3 of 29 drainages <50 km 2 , and they were absent in all drainages <25 km 2 . All historical records are from drainages >50 km 2 , and primarily from larger drainages in southern California. Our results will support greater exploration of the ecological roles of lampreys in coastal streams, where they act as importers of marine nutrients, substrate engineers and filter feeders, as well as providing a rich prey base for many species. This paper also provides resource managers with an understanding of where Pacific Lampreys are likely to be found and where they were probably not a natural component of the fish fauna, allowing prioritization of conservation actions and more effective use of resources. Received December 3, 2015; accepted February 22, 2016 Published online June 15, 2016