Natural Recolonization by Pacific Lampreys in a Southern California Coastal Drainage: Implications for Their Biology and Conservation

Abstract

AbstractThe historical range of anadromous Pacific Lamprey Entosphenus tridentatus includes coastal drainages from Alaska south to northern Baja California. However, there has been a widespread loss of distribution inland due to the construction of dams that form impassable barriers as well as a loss of Pacific Lampreys from some historically occupied drainages in the south. These losses have prompted the consideration of suitable approaches to reestablishing Pacific Lamprey populations through either active intervention or natural processes. While Pacific Lampreys apparently do not home to natal streams, they are thought to use cues that are related to such factors as flow patterns or the presence of ammocoete (larval lamprey) pheromones in offshore river plumes to identify suitable rivers for spawning and rearing. In this report, we present documentation of natural recolonization of the San Luis Obispo drainage by Pacific Lampreys on the central California coast, located 130 km south of the nearest freshwater population of Pacific Lampreys and with no existing ammocoete population. We also review recent observations of adult Pacific Lampreys in other southern rivers, recolonization of the Carmel River following dam removal, and indications that Pacific Lampreys are still using oceanic habitat off southern California. The results of our investigation demonstrate that Pacific Lampreys are fully capable of returning to an isolated drainage from which they have been extirpated and then rapidly reestablishing a viable population of ammocoetes, contradicting the hypothesis that they are dependent on existing ammocoete pheromones or adult sex pheromones to attract adults. These results indicate the importance of proactive management for Pacific Lampreys in drainages within their historical range, even if they are not currently present. The results also demonstrate that recolonization does not necessarily require active reintroduction efforts. Rather, conservation efforts should focus on identifying factors that may have led to the loss of Pacific Lamprey and removing potential impediments in order to facilitate natural recolonization.