The rings of a tree tell us how old the tree is and gives us clues about its environment. That’s because the layers of wood become darker toward the end of summer as the tree’s growth slows down. Like all vertebrates, salmon have rings too — and they’re found inside their ears. Otoliths, the scientific term for the salmon’s ear bone, form when salmon encounter temperature changes in the water. The rings that form are called thermal mark codes.
In the summer, when salmon are feeding, a wide, opaque layer of calcium forms. In the winter when fish eat less, a more narrow, translucent layer forms. These layers allow scientists to tell how many seasons a salmon has been alive. In Alaska, salmon hatcheries can also create a marking on the otolith before the salmon hatches from its egg. When hatchery technicians study adult salmon, these thermal mark codes tell biologists which hatchery raised the fish.
Thermal Mark Codes and Salmon Eggs
Before salmon hatch, Alaska aquaculture workers create otolith markings by heating up and cooling down the water temperature in incubation bins, essentially putting some stress on the eggs. A 48-hour warm-water cycle produces a large dark mark while a cycle that alternates between warm and cold water makes a lighter mark. The Alaska Department of Fish and Game gives each hatchery in the state a unique sequence each year.
As the salmon mature, the mark remains on the ear bone, which means hatchery technicians and biologists can tell the hatchery and the year the fish was born.
How the Rings of a Salmon’s Ear Bones Are Used
Pacific salmon hatcheries can clip the fins of juvenile salmon or tag them with coded wires, but the rings of a salmon’s ear bone are the only method that’s built right into the fish. The thermal mark code travels with the fish its entire life and can’t be altered. These artificial markings are the only difference between a hatchery salmon and one that was hatched in the wild. Thermal mark codes are used in the following ways:
- Alaska salmon hatcheries use ololiths to evaluate the success of a hatchery program. Data collected from otoliths can also help create models that forecast adult returns and improve the accuracy of future projections.
- Otoliths can show if a Pacific salmon is from a hatchery, or if it hatched in the wild.
- State fishery managers use otoliths to distinguish fish produced by non-profit hatcheries like Cook Inlet Aquaculture Association from state hatcheries that produce sport fish for inland lakes and rivers.
- Otoliths can show the age of both hatchery and wild-born salmon.
- The Alaska Department of Fish and Game keeps otolith markings in a state database, so otoliths can show where hatchery broodstock comes from.
- While Alaska salmon hatcheries don’t use this method, otoliths can be used to determine the species of a fish based on the shape of the ear bone.