Since the 1990s, salmon hatcheries throughout Alaska have been using a variety of fish marking technologies to differentiate hatchery reared Pacific salmon from their wild counterparts to provide data essential for fisheries management.
One of the most popular and widely used methods for mass-marking hatchery fish is the otolith thermal mark. Otoliths, or “ear stones”, are paired structures made of calcium carbonate and protein located in the inner ear of bony fishes.
To create an otolith mark, embryos or alevins are exposed to a series of rapid shifts in incubation temperature — 3 to 4 degrees Celcius — that temporarily disrupts otolith growth to create a dark ring in the structure for each temperature change. By imposing a predetermined series of temperature changes for varying periods of time and specific developmental stages, a unique pattern of dark growth rings is produced.
Why mark salmon otoliths?
More than two billion otolith-marked Pacific salmon are released into the North Pacific Ocean each year by hatcheries belonging to member nations of the North Pacific Anadromous Fish Commission (NPAFC), which includes Canada, Korea, Japan, Russia, and the United States.
The NPAFC’s Working Group on Salmon Marking (WGSM) coordinates and documents the release of these salmon to ensure marked groups are unique within species. By doing so, each recovered mark can provide information about the fish’s country and hatchery of origin, brood year, release year, and release location.
Ultimately, these recovery data can provide insights about hatchery contributions to fisheries, stock-specific distributions, migration patterns, and stray rates to name a few, all of which can be used to inform management actions.
Coordination is essential
With more than 400 unique mark patterns produced each year by hatcheries throughout the Pacific Rim, it is important that all the countries involved coordinate their marking activities. This coordination minimizes duplication and avoids confusion so that mark recovery programs are informative and effective. As chairperson of the WGSM, it is my responsibility to ensure this happens.
Every spring, representatives from each country provide me with their proposed thermal marking plans for the upcoming year. These data are entered into a relational database and compared to identify potential duplicates. Although each country has a set of pre-assigned mark patterns that helps minimize such duplication, it does occur on occasion as plans can change as data needs evolve. These occurrences, however, are rare. For example, of the 430 unique mark patterns proposed for each of the last several years, the number of initial duplicates was less than five.
If any potential mark conflicts are identified, I work with each country to alter their mark plan so that duplications are eliminated before thermal marks are actually applied. Once all the plans are finalized, each hatchery produces their marks and enters voucher images of those marks along with the associated metadata (brood year, hatchery of origin, release location, release numbers, etc.) into an online reference database so that managers and researchers have easy access the patterns allowing them to identify any recovered marks accurately.
Over the last decade, mark duplication within species among NPAFC member nations has been virtually non-existent. These consistent outcomes are the direct result of a cooperative international coordination effort that reaches across economic, political, and cultural boundaries to encourage sound science and resource management.
