Low carrying capacity a risk for threatened Chinook Salmon

Since they were listed under the U. S. Endangered Species Act of 1973, Snake River spring-summer Chinook Salmon (Oncorhynchus tshawytscha) have been the subject of numerous population viability analyses (PVAs). In all of the previous PVAs that we are aware of, management actions to improve the species’ status have focused on increasing survival rates of juveniles and adults. These PVAs did not explicitly treat carry capacity; instead, they assumed that survival rate improvements increased intrinsic productivity, and implicitly assumed that this would also increase carrying capacity. In a novel alternative approach, we instead examined how carrying capacity itself was related to extinction risk. We estimated three alternative multi-population PVAs using maximum likelihood estimation and chose the model with the best fit to the spawner-recruit data from 26 populations in the Snake River Spring/Summer Chinook Salmon evolutionarily significant unit. We then estimated carrying capacity and 24-year extinction probability for each of these populations using alternative quasi-extinction thresholds. We found that carrying capacities estimates were low in several of the populations and that extinction probability increases sharply with decreasing carrying capacity. A sensitivity analysis with fixed carrying capacities and increasing intrinsic productivity illustrated that unless actions increase carrying capacity, little change in extinction risk can occur.