Disease-induced juvenile mortality and transient and asymptotic population dynamics of Chinook salmon
November 18, 2010
2:30 p.m.
Masami Fujiwara
Abstract
The effects of increased juvenile mortality on transient and asymptotic dynamics of Chinook salmon (Oncorhynchus tshawytscha) were investigated. Disease-induced mortality of juvenile salmon has become a serious concern in recent years. Studies in the Klamath River basin (California, USA) have suggested that the mortality rate of juvenile salmon passing through a parasite infection zone can be as high as 80%. However, the extent to which an increased mortality rate among juveniles translates into adult abundance is still largely unknown. This uncertainty stems from a lack of information on where the density dependent compensation process takes place and how mortality in juveniles as a result of other factors is distributed over their age. We used stage-structured models to explore various scenarios with regard to the density dependent process, the distribution of juvenile mortality, and the magnitude of increased disease-induced juvenile mortality. Our results suggest that an increase in the disease-induced juvenile mortality rate may or may not affect the abundance of adult salmon because it depends on where the density dependence occurs relative to the timing of increased mortality, and the survival rate on which the density dependence acts. For example, if density dependent compensation occurs after an increase in disease-induced mortality, the adult abundance may be affected very little. However, if the survival rate following the disease-induced mortality is already high, the compensation process cannot fully compensate for the reduced abundance, resulting in reduced adult abundance. Regardless of the timing of the density dependence and disease-induced mortality, the resilience of the salmon population is always reduced by increased juvenile mortality. Reduced resilience results in the population taking longer to recover from any additional natural or anthropogenic disturbance.