Influence of Streamflow on Productivity of Stream Type Chinook Salmon Populations in the Salmon River Drainage, Idaho

We compared whole life cycle population productivity to population density and streamflow experienced by rearing juveniles for ten Chinook salmon populations in the Salmon River drainage of central Idaho, USA. Three of the populations were in drainages with heavily developed water resources (developed drainages) and seven were in drainages with very little water use (undeveloped drainages). For two of the populations, one in a developed and one in an undeveloped drainage, we also compared productivity measured at the juvenile outmigrant and smolt life stages to population density and rearing streamflow. Productivity was positively related to flow experienced by rearing juveniles across the entire range of flows. The strength of the relationships increased with age, with the weakest and strongest relationships, respectively, for the outmigrant and adult return life stages. Both population size and productivity were substantially higher in undeveloped than in developed drainages, but the relationships of population productivity and rearing flow were similar. Productivity was negatively related to population density in both developed and undeveloped drainages and, as with flow, the strength of the relationships increased with age. Adding population density to the regression models usually did not improve relationships for flow, possibly due to high leverage of the low population density data points, especially for undeveloped drainages. Removing all data points in the lowest 25 percentile population density increased the strength of the productivity versus rearing flow relationships in both developed and undeveloped drainages, but did not appreciably change the slopes of the relationships. The positive relationships across the entire range of flows suggest that instream flows should be protected and enhanced whenever possible.