Fish tree rings

Like the countable rings inside a log that show the age of a tree, most fish have three small bones within both inner ears that provide a concentric record of their lives.

Known as otoliths, these pebble-like objects are meticulously polished and then blasted with a laser that measures their chemical composition. Scientists then compare this information to a digital image of the otoliths—akin to tree rings—and piece together the life history of a fish.

OSU ecologist Jessica Miller studies otoliths and other marine biochemical markers to understand what traits and behaviors influence survival. By reading otolith rings, Miller can learn a fish’s hatch date, its growth rate in ocean and fresh water, and other indicators of activity and health.

“With nearly 99 percent of eggs released by fish failing to reach adulthood, otoliths can help us build knowledge about why some survive and others don’t,” said Miller. “We can take a successful Chinook salmon and see how important body size and the timing of when it swam to the ocean were to its endurance and survival.”

[caption caption="Close-up of an otolith reveals tree-like rings used to decode a fish’s life history. (Photo courtesy of Oregon Sea Grant.)"][/caption]

OSU researchers can even determine the hatchery and stock group a fish hails from, which wasn’t possible as recently as a decade ago. The more scientists understand what happens to fish after they leave hatcheries, why they’re surviving, and how they differ from naturally produced fish, the more they can contribute to the industry’s evolving idea of how some species should be managed, like Pacific cod, northern rockfish, and salmon.

State and federal governments in the Pacific Northwest have spent large sums on stream restoration and habitat modification in recent decades to improve the ability of freshwater systems to produce salmon. But whether these efforts make a difference to survival once they hit the ocean is still an open question.

“Managing complexity is really hard, so we tend to simplify systems. We’re trying to link the knowledge we can glean from biochemical markers in fish to the levers society can move to improve their success and survival,” Miller said. “We can’t influence the ocean much, but we can improve our rivers and estuaries for fish. This research can help get us closer to that goal.”