Current Status and Distribution of the Pacific Lamprey South of Point Conception, Coastal Southern California, USA
Camm C. Swift and Steve R. Howard
Abstract.—Since the 1970s, Pacific lamprey Entosphenus tridentatus (formerly Lampetra tridentata) have been documented in various drainages from coastal Southern California. These collections and observations document a relatively stable population especially in the Santa Clara River from the 1970s to 1990s, followed by a significant decline in numbers since the late 1990s, most abruptly after about 2000. This decline may be steeper than documented since collection efforts have increased in this same period. Pacific lamprey counts taken in downstream migratory steelhead Oncorhynchus mykiss traps were documented by the California Department of Fish and Game in Sespe Creek, a Santa Clara River tributary, in 1983 and 1984. The Highway 126 crossing of Sespe Creek, our sample site, is near this trapping site and was sampled repeatedly with seines in 1981–1988, 1995–1998, and 2005–2007. The most robust Pacific lamprey data set in Southern California is from intermittent annual counts at the Freeman Diversion on the Santa Clara River since 1991. Other localities south of Point Conception have also been sampled intermittently. Since the late 1990s, Pacific lamprey trapped during annual collections has dropped to a few individuals. The past 7 years (2002–2009) of efforts have encountered one ammocoete in lower Sespe Creek and 12 macrothalmia or ammocoetes in the main-stem Santa Clara River through both wet and dry years. Five ammocoetes collected in the lower Ventura River on March 8, 2005 are the only other individuals recorded south of Point Conception in the 2000s. This decline in Pacific lamprey south of Point Conception is not easily explained since many conditions have not changed appreciably in that time. Current and long-standing impacts such as dams and diversions that lack adequate Pacific lamprey passage, extended droughts, frequent high-intensity fires, poor water quality, fluctuating ocean conditions, and predation by nonnative piscivores may be sufficient to explain the decline.