Age Validation

Last modified on Feb 08, 2023

Coded wire tags (CWT)

Coded wire tags (CWTs) are used in Alaska to track cohorts of fish (Magnus et al. 2006). CWTs are small pieces (0.25 x 0.5 or 1.0 mm) of stainless-steel wire that are etched with a decimal or binary code (Jefferts et al. 1963). Each CWT code identifies a release group (e.g. 2017 spring Chinook salmon release at a specific river or hatchery). CWTs are injected into the snouts of juvenile fish, usually when the fish are parr (fish that are rearing in fresh water) or smolts (fish that are transitioning from fresh water to ocean residency). Each fish that receives a CWT also has it's adipose fin clipped to signify the presence of a CWT when the fish is re-captured. When a fish with a missing adipose fin is found, the fish's head is sent to the ADF&G Mark, Tag, and Age Lab in Juneau. There, the CWT is excised from the head, decoded, and the tag code is entered into a database along with the fish's catch data. Then, CWT release and recovery data are summarized in online reports (

Scale age can be validated from CWT data. Subtracting the CWT release year from the recovery year results in a known number of years between tagging and recovery. As many Chinook salmon are CWT-marked as smolt during their outmigration to the ocean, in most cases, CWT data provides validated marine ages. Few studies mark juvenile parr at a known fish length to validate freshwater age. Age validation is effective for Chinook salmon, which predominantly spend 1 year in fresh water before out-migrating as smolt, except for some notable stocks with a 0-freshwater age component in SEAK (Situk and Keta rivers). Age validation for coho salmon can be difficult due to their varying freshwater life history and the difficulty of capturing juvenile coho salmon in fresh water.

Age validation of salmonids increases confidence in analyses such as:

  • Scale age training
  • Assessment of scale pattern interpretation
  • Age at maturity
  • Size at age
  • Cohort strengths
  • Migration information
  • Life history models
  • Recruitment

Age and stock information from CWT recoveries are a crucial component of various population assessments. CWT data provides information about total adult production (harvest plus escapement), exploitation rates, smolt production, marine survival or return rates, and spawner-recruit relationships (McPherson et al. 2000; Pahlke et al. 1995). CWT-based cohort analyses are a main component of the Chinook salmon abundance model used for pre-season harvest forecasts and management of Chinook salmon harvest in Pacific Salmon Treaty (PST) areas of Southeast Alaska and British Columbia (Peterman et al. 2016).

Otolith thermal marks

An otolith, or ear stone, is a dense structure composed primarily of calcium carbonate and protein that is located within the inner ear of teleost (e.g. bony) fishes that is essential for maintenance of balance and hearing, similar to the bones in the inner ear of mammals (Barton 2007). Otoliths form when the fish is an egg and grow as the fish grows. Thus, otoliths are often used to estimate age and retain these marks throughout the fish's life. Changing the water temperature during incubation at set cycles of cold and warm temperature intervals results in patterns of high-contrast rings in the otolith (Volk et al 1990). By planning a sequence of temperature changes, a hatchery can produce a specific otolith pattern on the otoliths of a group of fish. When different thermal marks are applied at each hatchery and year, thermal-marked otoliths can be used to identify fish origin and age. Otolith thermal-marks can be used to validate scale age by subtracting the release year from the recovery year. Otolith thermal mark age validation is practical at hatcheries, but not for natural-origin fish.

Age validation for hatchery Hood Canal summer chum salmon, Lower Columbia River fall-run chum salmon, and Cedar River sockeye salmon otolith and scale age estimates was performed by Anderson and others (2023) using thermal marks. Otolith and scale total age estimates were similarly accurate (scales: 96.9% accurate; otoliths: 95.3% accurate), although the rarer (<2.6%) younger ages were overestimated and older ages were underestimated.

Other age estimation techniques

  • Vertebrae
  • Fin rays (Chilton and Bilton 1986; Ferreira et al. 1999)
  • Inter-opercular bones
  • Humerus bones
  • Teeth


Anderson, A., A. Claiborne, and W. Smith, 2023. Validation of age estimates for Chum and Sockeye salmon derived from otolith and scale analysis. Fisheries Research(259): 106556.

Barton, M. 2007. Bond's Biology of Fishes, Third Edition. Thomson.

Chilton, D., and H. Bilton. 1986. New method for ageing Chinook salmon (Onchorhynchus tshawytscha) using dorsal fin rays, and evidence of its validity. Canadian Journal of Fisheries and Aquatic Sciences 43(8): 1588-1594.

Ferreira, L. C., R. J. Beamish, and J. H. Youson. 1999. Macroscopic structure of the fin-rays and their annuli in pectoral and pelvic fins of Chinook Salmon, Oncorhynchus tshawytscha . Journal of Morphology 239(3): 297-320.

Jefferts, K. B. 1963. A coded wire tag identification system for macroorganisms. Nature 198(4879): 460-462.

Magnus, D. L., D. Brandenburger, K. F. Crabtree, K. A. Pahlke, and S. A. McPherson. 2006. Juvenile salmon capture and coded wire tagging manual. Alaska Department of Fish and Game, Special Publication No. 06-31, Anchorage. (PDF).

McPherson, S. A., D. A. Bernard, and J. H. Clark. 2000. Optimal production of Chinook salmon from the Taku River. Alaska Department of Fish and Game, Fishery Manuscript No. 00-02, Anchorage. (PDF).

Peterman, R. M., R. Beamesderfer, and B. Bue. 2016. Review of Methods for Forecasting Chinook Salmon Abundance in the Pacific Salmon Treaty Areas.

Pahlke, K. A. 1995. Coded wire tagging studies of Chinook salmon of the Unuk and Chickamin Rivers, Alaska. 1983-1993. Alaska Department of Fish and Game. Alaska Fishery Research Bulletin 2(2):93-113. (PDF).

Volk, E. C., S. L.Schroder, and K. L. Fresh. 1990. Inducement of unique otolith banding patterns as a practical means to mass-mark juvenile Pacific Salmon. American Fisheries Symposium 7:203–215.