Conveniently, fish harbor a perfect tool to map their habitat mosaic: ear stones, or otoliths, that accumulate mineral layers across their lifespan. In a sliced ear stone, these layers looks just like tree rings—and relay similar information. The stones form as calcium carbonate from the river water accumulates over time. These layers alone wouldn't indicate a location, but natural waterways also contain small amounts of strontium, which also gets laid down within the calcium carbonate layers. Depending on where in the fish are, the ratio of strontium isotopes varies, creating a unique chemical signature that researchers can use to determine where salmon have been living over their entire lifetime. And, the size of ear stone rings can reveal how much a fish grew in a particular environment. The scientists plucked these stones from sockeye and Chinook returning from sea at the mouth of the Nushagak. They analyzed the otoliths of a total of 1,377 salmon, collected during 2011, 2014, and 2015.