Barriers and flow as limiting factors in the spread of an invasive crayfish (Procambarus clarkii) in southern California streams

Invasive crayfish are a major threat to stream ecosystems, yet research has seldom identified successful ways of preventing their spread. Thirty-two stream sections were surveyed during 2000 and 2001 in the Santa Monica Mountains of southern California to determine the distribution of the invasive crayfish Procambarus clarkii. Streams with large barriers (waterfalls, culverts) often did not have crayfish present upstream of barriers. A mark-recapture study indicated that P. clarkii moved both up and downstream between pools, but that barriers significantly reduced movement between pools. Seasonal high flow velocities likely increase passive movement downstream and reduce movement upstream. Results indicate that crayfish mainly spread downstream from a point of colonization and are restricted in their movement to adjacent upstream sections by both natural and artificial barriers. We suggest management strategies for removing invasive crayfish and reducing their spread by focusing on smaller stream segments that are bounded by a downstream barrier and by timing removal efforts to follow large flow events.