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Dry Marking and Thermal Marking: A Biological Tracking Code

Biologists can tell which salmon hatchery raised a particular fish based on mark codes on its ear bones. Here's how it's done.

by | December 9, 2022

An otolith with mark codes. Biologists use dry marks and thermal marks to track salmon.
An otolith is a salmon’s ear bone. Here, the bone has a thermal marking code. CIAA

As pink salmon eggs are slowly and comfortably incubating, we will throw a major curveball in their direction. We will need to dry mark these eggs.  What is dry marking? First, we need to know a little about otoliths.

Fish have three pairs of inner ear bones, called otoliths. These calcium carbonate structures help fish hear and keep their balance underwater. Otoliths also change color when the fish is under environmental stress, such as when water temperatures change. It’s like the growth rings around the core of a tree trunk.

How we use otoliths

Otoliths are a handy way for fishery managers to track the birthplace and age of a fish. The Alaska Department of Fish and Game assigns each hatchery a unique otolith mark. The mark has to be in place before a hatchery releases the fish to begin its migration to the sea. These marks are sort of like a barcode that helps trace back where a particular fish hatched.  

There are a few different ways to mark an egg, including thermal marking and dry marking. 

Dry marking in and out of the water

A dry mark is exactly what it sounds like. Prior to eggs hatching, we drain the water from the incubator completely, leaving the eggs moist. By using sheets of pool foam cut to the size of the incubator, we can lay these over the eggs to trap moisture and help regulate temperature. Surprisingly, these eggs can survive using the humidity in the air for quite some time.

Our eggs only need to go dry for 12 hours to leave an identifiable mark. During that time, we carefully monitor the air temperature. After the 12 hours is up, we turn the water back on for 12 hours. The mark appears after we expose the egg to cold water after a period of dryness. We repeat this process until the hatchery mark forms as a series of light and dark rings on the otolith.

As an example, Tutka Bay Lagoon Hatchery pinks have an assigned dry mark of 5,3,H. This requires five (the “5”) marking periods followed by one longer period where there is no marking (the “comma” indicates this), and then three (the “3”) more marking periods, prior to the eggs hatching (the “H”).  Locations assigned post-hatch markings, such as 5,3H3, cannot fully dry mark because the incubator cannot be drained after hatch. Facilities wanting to dry mark a release group assigned a post-hatch mark can request an alternate pre-hatch mark.

Thermal marking is all about heat

Thermal marking uses an opposite process to dry marketing. Water is heated by a boiler to create otolith marks with the same timeframes as dry-marking, alternating between warmer and cooler water. Thermal marking is expensive because diesel fuel is used to heat the water. Dry marking reduces hundreds of hours of boiler run time.

Dry and thermal marking for the best results

We use a unique combination of both dry marking and thermal marking simultaneously to give us the best results possible. Many other Alaska hatcheries are not able to do this because they don’t have the ability to control the air temperature in their incubation rooms.

Each of our hatcheries has come up with a unique way to maintain a three to five-degree Celsius temperature differential between the air in the incubation room during the dry period, giving us a dry and thermal mark at the same time, but without the added cost of heating and pumping the water.  We started this at the Trail Lakes Hatchery in 2020 and as of this year, all of our hatcheries are using both methods successfully.

Marking otoliths requires diligence and attention to detail. When we leave a good mark, hatchery and fishery managers can use that information in future years to help project runs and manage salmon harvests. 

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