New Readers

Last modified on Jan 21, 2021

Throughout Alaska and the Pacific Northwest, Chinook scale age estimation training varies by location. At most sites, inexperienced personnel mentor with experienced personnel. Usually trainees begin by studying materials to gather background on the stocks that they will be aging. Some sites emphasize learning life histories, such as basic biology and hatchery practices (releases, sizes, marks, etc.) of target fish stocks from local biologist(s), district reports, hatchery plans, or other sources. Some locations have manuals describing the process, and a trainee likely would study these documents. Trainees would study the reference collection at sites where they exist. Reference collections consist of scales from known-age fish (CWT- or PIT-tag validated) and scales previously aged by several individuals or groups. Some locations use PowerPoint presentations in addition to actual samples to practice estimating age. Random samples of reference structures can be inserted into regular samples to ensure that readers remain consistent with their estimations (Buckmeier 2002).

Once a trainee becomes familiar with age patterns by species and area, various exercises are implemented, such as:

  1. Review literature.
    1. Review life histories, reports, hatchery plans, etc.
    2. Review existing manual, this wiki.
  2. Study reference collection.
    1. Study known age scales, if they exist.
    2. Study previously aged scales, important to use consensus-aged scales where more than one reader agreed upon age (Campana 2001).
    3. Some locations - trainees review previous 3-5 years of data depending on fishing area, gear, and species.
  3. Introduce trainee to easy to read scales and have them estimate ages.
    1. Trainee and experienced reader estimate the age of a set of easy to age scales together.
  4. Trainee ages independently.
    1. Trainee independently estimates ages of a set of scales previously read by an experienced reader.
    2. Trainee usually has access to scale age estimates for further study when doing the first set.
    3. Next, trainee does a "blind" read (i.e.; no knowledge of age data) on another set of scales.
    4. Scale sets range in size from 50 (Prince William Sound) fish per species to 500 (Canadian Department of Fisheries and Oceans, DFO) fish per species.
  5. Review.
    1. Review age discrepancies/patterns.
    2. Review difficult ages/projects with experienced reader.
    3. After review, trainee and trainer discuss differences in age estimations.
  6. Agreement.
    1. Some locations have trainee read same scale set repeatedly until trainee reaches agreement targets. Other locations have trainee read different sets of scales until agreement targets are reached.
    2. For most locations, agreement levels (average percent error) must reach >90%. Department of Fisheries and Oceans Canada (DFO) uses staged agreement levels: 60-70% for first set then >80% agreement to begin to first reading (MacLellan and Gillespie, 2015). According to Buckmeier (2002) bias is negligible for readers scoring ≥90% on tests for accuracy.
  7. Types of tests.
    1. Idaho Fish and Game produces separate tests for adult versus juvenile scales (Wright et al., 2014).
      1. Adult scales come from known saltwater age fish.
      2. Juvenile tests are produced by experienced readers.
  8. In some locations, new readers shadow experienced readers on current year's data (6 months – 1 year), but this is not always possible.


Buckmeier, D.L., 2002. Assessment of reader accuracy and recommendations to reduce subjectivity in age estimation. Fisheries 27 (11), 10-14.

MacLellan, S., and Gillespie, D., 2015. Chinook Salmon (Oncorhynchus tshawytscha) scale age determination procedures. Canadian Technical Report of Fisheries and Aquatic Sciences 3123, 69.

Wright, K.K., Hernandez, K., Hohman, C., Reinhardt, L., Schrader, W., and Copeland, T., 2014. Laboratory methods for assigning ages to anadromous salmonid scale sample. In: Report to U.S. Department of Energy, Bonneville Power Administration, Boise, ID.