Population viability of coho salmon, Oncorhynchus kisutch, in Oregon coastal basins: application of a habitat-based life cycle model

To assess extinction risk for Oregon coastal coho salmon, Oncorhynchus kisu tch, we developed a life cycle model based on habitat quality of individual stream reaches estimated from survey data. Reach-specific smelt output was a function of spawner abundance, demographic stochasticity, genetic effect s, and density- and habitat-driven survival rates. After natural mortality and ocean harvest, spawners returned to their natal reaches. Populations in reaches with poor habitat became extinct during periods of low marine surv ival. With favorable marine survival, high productivity reaches served as s ources for recolonization of lower quality reaches through straying of spaw ners. Consequently, both population size and distribution expanded and cont racted through time. Within a reach, populations lost resilience at low num bers when demographic risk factors became more important than density-depen dent compensation. Population viability was modeled for three coastal basin s having good, moderate, and poor habitat. With constant habitat conditions , extinction risk in 99 years was negligible in basins with good and modera te habitat and 5-10% in the basin with poor habitat. Reductions in habitat quality up to 60% in 99 years resulted in reduced coho salmon populations i n all basins and significantly increased extinction risk in the basin with poor habitat.