Detection of population trends in threatened coho salmon (Oncorhynchus kisutch)

Populations of coho salmon (Oncorhynchus kisutch) in California are listed as threatened under the U.S. Endangered Species Act. Such listings refer to adult populations, but often, juvenile life history stages are censused, s o it is important to understand what affects the relationship between true adult and observed juvenile numbers. We present models to address how obser vational uncertainty, census length, and autocorrelation in vital rates aff ect our ability to observe trends. We ask two questions about our ability t o detect declines in one life history stage from censuses of another. First , given an observed decline in parr numbers, what is the chance that this r eflects a decline in adults? Second, given that adult numbers are declining , what is the chance that we see that decline in parr? Our results indicate that statistical power decreases with increasing observational uncertainty and decreasing census lengths and demonstrate how these two parameters int eract. Power increases as the level of autocorrelation in mortality rates i ncreases. Management recommendations include obtaining more accurate estima tes of autocorrelation in mortality and of observational uncertainty.