To increase wild salmon populations in Alaska, hatcheries raise batches of salmon from eggs and release them into the wild. The little fish are held just long enough to remember the scent of the stream where they will later return to spawn.
How do scientists tell the difference between a salmon born in a wild fish hatchery and one born in a stream? The answer, barcodes.
For almost thirty years, fish were tagged by removing an unnecessary fin and placing a metal id tag in the fish's nose. However, this process was time consuming, and only about a tenth of the fish that came from hatcheries could be marked.
Researchers then discovered that a kidney bean shaped bone in the salmon's inner ear, called an "otolith," is sensitive to water temperatures during embryonic development. When the temperature of the water running over the salmon eggs is raised by a few degrees, the developing otolith adds a darker layer of calcium to its surface. This is done repeatedly to the embryos. Later on when the adult fish return to spawn and are harvested, the otolith can be removed and cut in half to reveal a pattern of dark rings.
By reading this "barcode," the hatchery can tell which batch the fish came from and how long it had been in the wild.
Scientists need to be able to tell where a salmon was born so they can determine the health of wild salmon populations that return upstream to mate. Since all hatchery born salmon can be tagged this way, smaller samples can be taken to determine the success and health of each salmon run.