Simply put, the smolt-to-adult return ratio (SAR) is the percentage of smolts that survive and return to spawn:

ADULTS / SMOLTS = SAR

For example, if 100 steelhead smolts pass Lower Granite dam on their downstream migration and 2 adult steelhead from that group return and survive to pass Lower Granite on their way to spawn, the SAR would be 2 percent (adults/smolts).   

SAR is an important metric because it is the only metric that captures (most of) the cumulative impacts of the hydro system on salmon and steelhead, telling us how sustainable the returns of adults are over time. This is critical because even if high quality habitats produce a lot of smolts, the population will only be sustained if those smolts can make it out to the ocean and survive to return and spawn as adults.   

For Snake River stocks SAR is often calculated by dividing the number of returning adult salmon and steelhead that pass the uppermost lower Snake River dam, Lower Granite, on their way to spawn, by the total number of smolts (juvenile salmon and steelhead) that previously passed Lower Granite dam earlier as they were migrating to the ocean.

The scientific consensus is that SARs must be at least 2 percent for Snake River adult salmon and steelhead to replace themselves and simply avoid extinction. To rebuild stocks, that percentage will need to be 4 to 6 percent.