Otolith Photos

Pacific salmon are anadromous fish, meaning they return to fresh water to spawn (or lay their eggs). Once the eggs hatch, the fish migrate to the ocean where they remain for one to seven years, depending on the species. They then return to their fresh water birth stream or hatchery release site to lay their eggs. As part of their lifecycle, after spawning, they die.

Introduction to Thermal Marking

In prior years, it was believed that all DIPAC produced fish must come with a label identifying them as hatchery fish. Since 1991 this has been a reality, and now all Gastineau Hatchery salmon are thermal marked. Much of the credit for our ability to do so goes to Dr. William Smoker of the University of Alaska.

What is Thermal Marking

Thermal marking is an efficient means of marking 100% of the fish at the hatchery. Therefore, we can take any salmon in the ocean, remove its otoliths or ear bones and be able to tell if it is a DIPAC hatchery fish or not. The otoliths, found in pairs and located beneath the brain, are composed of calcium carbonate and protein. The otoliths lay down the calcium carbonate and protein daily in a pattern of rings similar to that of trees, external factors such as temperature influence these patterns. The hatchery salmon are marked prior to hatch or soon thereafter in the incubators. By manipulating the water temperature in the incubators, hatchery technicians can place a series of rings on the otoliths that will identify them by hatchery and brood year. This process forms a type of "bar code" on the otolith that remains with the fish for its lifetime. These patterns of bands can be customized for each hatchery and brood year by varying the number of bands and the width and spatial placement of these bands.

Thermal Marking as a Tool

Thermal marking is used as a means of identifying hatchery fish once they return from the ocean. Previously tagging the fish with coded-wire tags was our only method of identification. Unfortunately, only a small percentage of the fish were tagged. With thermal marking, 100% of the hatchery fish are marked. We then use this information to check that our fish are returning to the hatchery and to our special harvest areas, and to determine what percent of our fish are caught in the common property fishery. In most cases, this information can be available within 24 hours of the samples being collected. In addition, thermal marking is now being used to determine where the salmon go during their ocean migration. Therefore, through thermal marking we have the means of tracking our fish in order to learn more about their migration patterns and as means of illustrating our contribution to the commercial catches.

Results of Chum Salmon Sampling from the 1997 Season

Below is a condensed chart showing our sampling efforts for 1997. The total catch is the number of chum salmon recorded by Alaska Department of Fish & Game for the 1997 season. The DIPAC percent marked column is the overall percentage of chum salmon found in a sub-sample of the catch with the Gastineau Hatchery thermal mark. DIPAC Contribution to the total catch is the estimated number of fish out of the total catch with the Gastineau Hatchery thermal mark.

Thermal Marking Efforts for 1997

In 1997, 104.9 million chum fry, 8.8 million pink fry, 825,000 coho salmon, and 656,615 chinook salmon will receive a thermal mark with a pattern of 5 rings. The chum salmon to be released at Boat Harbor and Limestone will receive an extra set of bands called accessory marks that will identify them from those released at the hatchery.

For Information or Comments

For more information or comments on otolith marking and recovery e-mail me at dipacoto@pobox.alaska.net or check out the ADF&G otolith page at http://tagotoweb.adfg.state.ak.us.