Last modified on Jan 21, 2021

Below is a list terms and expressions used while estimating age from Chinook salmon scales. Below this list are life history terms used to describe the variety of migrations, life cycle stages, and behavior patterns of Chinook salmon. A common set of age estimation and life history terms is useful for accurate interpretation of growth patterns found on scales.

The use of a terms in a consistent way is the first step towards common understanding and methods. These terms and their definitions were initially discussed at a Chinook salmon scale workshop held 2-3 April, 2015 in Juneau, AK. Additional terms and original sources used to develop this list are: Tobias et al. (1994), Ruggerone et al. (2002), Wilson et al. (1987), Kalish et al. 1995, Martinson et al. 2009, ADF&G 2010, and an Oregon Department of Fish and Wildlife draft scale age reading manual.

Age estimation terms

Term >Definition
Acetate Acetate or Vivak plastic® card with salmon scale impressions used to read fish age using a microfiche reader. Scale impressions are made on the acetate by applying heat and pressure to scales stored on a gummed card.
Age The length of time that an organism has lived or a thing has existed; measured in years, months, days, or other units.
Age accuracy The degree to which the age data close to the actual age
Age estimation Evaluation of fish age using biological data such as scale circuli growth pattern interpretation, length, or weight.
Age reading Technique for interpreting fish age from growth patterns on fish structures such as scales or otoliths.
Annulus (Annuli, plural) Growth zone representing one year of life. An annulus is commonly identified at the end of winter growth.
Anterior field The portion of the scale that is nearer to the head. Circuli and annuli form on this part of the scale. In live fish, this part is subcutaneous (under the skin).
Axis Plane of view or measurement from the posterior field of the scale to the scale's anterior margin.
Base Nexus of the posterior and anterior fields of the salmon scale and the location of the circuli "tails".
Bias The difference between an estimated value and the true value of the parameter.
Birth date Used to determine total age; important to consider when looking at the marginal growth (growth after the final winter annulus). January 1st is commonly accepted birthday for Chinook salmon in Alaska, though different run-types have different birthdays that are based on the timing of spawner returns. For example, Central Valley Fall-run Chinook salmon have an assumed birthdate of September 1 (O'Farrell et al. 2010), spring-run fish have an assumed birthdate of May 1 and winter-run fish have an assumed birthdate of March 1 (O'Farrell et al. 2012).
Check False annulus theoretically created by stressful conditions that cause decreased fish growth represented by relatively closely-spaced circuli within an annulus. Checks can be difficult to distinguish from annuli.
Circulus (Circuli, plural) Growth increment radiating out from the focus (Figure A2). Individual ring or ridge on a scale. The spacing between circuli reflects fish growth rate (Fisher and Pearcy 2005).
Circuli tails The termination or "ends" of circuli at the base of the scale. Generally, circuli tails turn away from the focus at the beginning of winter growth and towards the focus at the end of each winter. Circuli tail appearance is helpful for estimating of fish age.
European age notation Method of age notation for salmonid scales: the number of winters spent in freshwater after emergence from the redd before going to sea, for example 1 winter = age-1.X, followed by the number of winters spent at sea, three winters = age-X.3 or four winters = age-X.4.
Focus Point of origin of the scale and the area contained within the first circulus. The focus is frequently used as a reference point when referring to parts of a scale.
Freshwater circuli Circuli formed when the fish resides in fresh water. They tend to be more closely-spaced, thinner, and lighter in appearance than circuli formed after entry into an estuarine or marine environment (Figure A2). Generally, the ends of freshwater circuli ("tails") point towards the focus.
Freshwater plus Area on the scale between the last freshwater annulus and circuli formed in the ocean.
Freshwater zone The portion of the scale that was formed after fish growth in fresh water. Counting winter zones provides fish age in fresh water.
Gilbert-Rich age notation Method of age notation for salmon scales (Gilbert and Rich 1927): Nn , wherein N = the total age of the fish including the year in the gravel, and n = age at which the fish went to the ocean.
Gummed card Card with a "gummed" or sticky and numbered side. Salmon scales are placed on the sticky side. Sample information is recorded on the non-sticky side. High heat and pressure are used to create impressions of the gummed cards on an acetate for reading and long-term preservation of scale samples.
Hatchery fish Fish spawned and reared in a hatchery environment. Hatchery-origin fish often exhibit even and significant growth patterns prior to release (freshwater-rearing phase). In contrast, wild fish tend to have uneven growth during the freshwater-rearing phase and undergo a rapid compensatory growth upon entering the ocean.
Inverted scale A scale that was placed on gummed card upside down (e.g. with growth ridges (circuli) placed into the mounting medium) resulting in an unreadable impression on the acetate card.
Lateral line scale Scale collected near the groove on the skin along the side of the fish with a network of sensory canals (the lateral line). Scale shape is distorted around the pore and the scale margin has an indent.
Margin The border or edge of a scale. Outer circumference of scale where new growth accrues and scale material accrues.
Marine circuli Circuli formed while the fish is in the ocean. These circuli tend to be less closely-spaced and thicker than circuli formed in fresh water.
Non-preferred Description of a scale sampled from outside of the preferred sampling area on the fish (see preferred area, Mosher 1963).
Ocean entrance The border between the freshwater and marine zones. Delineated by the transition from the relatively slower freshwater zone growth to an increase in growth when a fish enters the ocean. Thus, after ocean entrance, an increase in circuli spacing and size is apparent and circuli tails turn away from the focus.
Posterior field Portion of the scale that is nearer to the fish's tail. In live fish, this portion is exposed to the fish's environment and is covered by pigment. Circuli are not present on this portion of the scale. This is the first portion of the scale to be resorbed and is resorbed to a higher degree than the anterior field.
Precision Repeatability of a given measure or age reading.
Preferred area Area on the fish located three scale rows above the lateral line between the posterior insertion of the dorsal fin and anterior insertion of the anal fin (Mosher 1963).
Preferred scale Scales sampled within the preferred sampling area for a species of fish.
Radius Distance from focus to a specific point on the scale.
Reference sample Scale taken from a fish of known origin, sample date, and/or age. Known-age samples are from validated by tagging (or other mark) studies by subtracting the year the fish was marked from the year the fish was recovered to determine age. This is used by scale readers to "calibrate" or become familiar with scale patterns and associate these with origin, sample date, and age.
Regenerated Area of a scale with missing circuli caused by reforming a lost scale. Sometimes freshwater age cannot be determined for regenerated scales due to subsequent reformation.
Resorbed Resorption (verb). Scales that are resorbed have portions that have lost their structure. The anterior field loses its circuli and the posterior field looses its characteristics. During resorption, the fish uses scale tissue (fibrous and calcified scale layers) as a source of mineral and somatic energy reserves during gonadal maturation, migration, and spawning that leads to a loss of scale tissue (Bigler 1989). Resorption of the scale margin generally starts on the posterior field and progresses near the base of the anterior field along the lateral margins of the scale until the whole scale margin is affected (Mosher 1968).
Reticulation Apparent dissipation or dispersion of circuli tissue located between and around circuli tails at the base of the scale. Reticulation can appear globular or ladder-like. Presence and appearance can be used as part of discerning species.
Saltwater plus (Plus growth) Circuli formed after the ocean winter (annulus) on the margin of a scale. This zone is often resorbed in mature salmon, where the amount of resorption increases the closer the salmon is to spawning. Depending on life history, stock, or sample day, scale appearance information from reference scale samples, known-age reference samples are preferred, can be used to justify adding another not complete annulus to a scale reading.
Validation The method of checking or proving the validity or accuracy of an something. Age validation is usually performed by using known-age specimens collected from a tagging or marking studies. Some validation methods include: coded wire tag (CWT), Parentage Based Tagging (PBT), or PIT tagging fish at a known life stage, typically smolts, then subtracting release year from the recovery year to validates the number of years as the age, typically ocean age.
Zone Region of similar structure or definition. For example, the "annual growth zone" consists of one year of growth, usually one summer zone and one winter zone. Most Chinook salmon have one freshwater zone with zero to one annulus, and one marine zones with two to four annuli.

Life history terms

Term Definition
Fall-run Salmon that return to fresh water during the fall to spawn.
Jack Sexually mature male salmon of an age group younger than the youngest females in the population (Quinn 2005). For example, in the Columbia Basin, male Chinook salmon that return to spawn after their first year in the ocean (one-ocean fish); total age 3. Alaska (Yukon): Male Chinook salmon returning after spending two years in the ocean (two-ocean fish); total age 4.
Jill Female that returns to spawn one year earlier than the bulk of the female salmon in the run. Jills are very rare (Quinn 2005).
Mini-jack A male Chinook salmon that smolts and matures in the same year. Return to fresh water occurs before forming an ocean annulus. Mini-jacks are defined for the Columbia River as an upstream migrating Chinook salmon with a fork length between 15-30 cm. Scale patterns, mean number of circuli in fresh water and salt water, and circuli spacing are described for scales from mini-jack Chinook salmon in Idaho and Bonneville Dam by Johnson et al. (2012).
Ocean-type (sub-yearling) Life history strategy of emigrating from fresh water to the ocean before their first winter; no freshwater winter annulus is apparent on the scale.
Reservoir-type Life history strategy of rearing in a reservoir before emigrating to the ocean (Connor et al. 2005). Fish size at ocean entrance is generally larger for this life history than others; most scales show the freshwater annulus and ocean entrance at the same position.
Resident Fish that are not anadromous, although their cohorts may be. Examples include kokanee (sockeye salmon); rainbow trout (steelhead); Chinook salmon, and coho salmon. Resident fish are identified by their small size compared to same age anadromous fish, lack of saltwater annuli, and large fresh water zone on the scales.
Spring-run Salmon that return to fresh water during the spring to spawn.
Stream-type (yearling) Life history strategy of rearing a year or more in fresh water before emigrating to the ocean. Scales show at least one freshwater annulus.
Summer-run Salmon that return to fresh water during the summer to spawn.


ADF&G (Alaska Department of Fish and Game). 2010. Alaska Department of Fish and Game Writer's Guide Alaska Department of Fish and Game, Juneau,

Bigler, B. 1989. Mechanism and occurrence of focal scale resorption among chum salmon (Oncorhynchus keta) of the North Pacific Ocean. Canadian Journal of Fisheries and Aquatic Sciences 46(7): 1147-1153.

Connor, W. P., J. G. Sneva, K. F. Tiffan, R. K. Steinhorst, and D. Ross. 2005. Two alternative juvenile life history types for fall Chinook salmon in the Snake River basin. Transactions of the American Fisheries Society 134(2): 291-304.

Fisher, J. P., and W. G. Pearcy. 2005. Seasonal changes in growth of coho salmon (Oncorhynchus kisutch) off Oregon and Washington and concurrent changes in the spacing of scale circuli. Fishery Bulletin 103: 34-51.

Gilbert, C. H., and W. H. Rich. 1927. Second experiment in tagging salmon in the Alaska Peninsula fisheries reservation, summer of 1923. Fishery Bulletin (42):27-25.

Johnson, J., T. Johnson, and T. Copeland. 2012. Defining life histories of precocious male parr, minijack, and jack Chinook salmon using scale patterns. Transactions of the American Fisheries Society 141(6):1545-1556.

Kalish, J. M., R. J. Beamish, E. B. Brothers, J. M. Casselman, R. I. C. C. Francis, H. Mosegaard, J. Panfili, E. D. Prince, R. E. Thresher, C. A. Wilson, and P. J. Wright. 1995. Glossary for otolith studies. Pages 723-729 in D. H. Secor, J. M. Dean, S. E. Campana, and A. B. Miller, editors. Recent Developments in Fish Otolith Research. University of South Carolina Press, Columbia, South Carolina.

Martinson, E. C., J. H. Helle, D. L. Scarnecchia, and H. H. Stokes. 2009. Alaska sockeye salmon scale patterns as indicators of climatic and oceanic shifts in the North Pacific Ocean, 1922–2000. North Pacific Anadromous Fish Commission Bulletin 5:177-182.

Mosher, K. H. 1963. Racial analysis of red salmon by means of scales. International North Pacific Fisheries Commission Bulletin 11:31-56.

Mosher, K. H. 1968. Photographic atlas of sockeye salmon scales. Fishery Bulletin 67(2): 243-280.

O'Farrell, M. R., M. L. Palmer-Zwahlen, and J. Simon. 2010. Is the September 1 river return date approximation appropriate for Klamath River fall Chinook. National Marine Fisheries Service Southwest Fisheries Science Center., NOAA-TM-NMFS-SWFSC-468.

Quinn, T. P. 2005. The behavior and ecology of Pacific salmon and trout. University of Washington Press.

Ruggerone, G. T., J. L. Nielsen, E. Farley, S. Ignell, P. Hagen, B. Agler, D. Rogers, and J. Bumgarner. 2002. Long-term trends in annual Bristol Bay sockeye salmon scale growth at sea in relation to sockeye abundance and environmental trends, 1955–2000. North Pacific Anadromous Fish Commission Technical Report 4:56-58.

Tobias, T. M., D. L. Waltemyer, and K. E. Tarbox. 1994. Scale aging manual for Upper Cook Inlet sockeye salmon. Alaska Department of Fish and Game, Commercial Fisheries Management and Development Division, Regional Information Report 2A94-36, Anchorage. (PDF)

Wilson, C. A., R. J. Beamish, E. B. Brothers, K. D. Carlander, J. M. Casselman, J. M. Dean, J. A. Jearld, E. D. Prince, and a. A. Wild. 1987. Glossary. Pages 527-530 in R. C. Summerfelt, and G. E. Hall, editors. Age and Growth of Fish. Iowa State University Press, Ames, Iowa.