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Research Methods for Salmon Abundance Estimation

  Mountain Goat

Abundance and survival are common metrics used for fish management decisions. For example, trends in salmon abundance are only meaningful if they are based on reliable data. An important goal of population monitoring efforts is to minimize the error associated with measurement (bias and uncertainty) so that real differences among populations and years can be identified and understood.

The most accurate study design for estimating fish abundance is a census count where every individual is directly counted. However, census counts are often impractical either because the entire population can not be observed or because the population is too large to count all individuals. In the case of salmonids, census counts are rare. A variety of techniques are used to partially sample the population and expand that sample to the entire population. Adult abundance assessments are typically accomplished with foot, boat, and aerial surveys and Mark-Recapture, Area-Under-the-Curve, or Count Expansion study designs. Juvenile abundance assessments are typically accomplished with a smolt trap operated near the mouth of the river and Mark-Recapture study designs.

When combined, adult and juvenile abundance from the same population provide measures of survival (or “productivity”) in the freshwater and marine environment. This monitoring strategy was termed “Fish In Fish Out” in the Washington Framework for Monitoring Salmon Populations and Associated Freshwater Habitat. In Washington State, combined monitoring of juveniles and adults is applied to geographically representative populations statewide and is used to assess factors limiting production of natural populations of salmon and steelhead.

WDFW Fish Science and Regional Fish Management Staff continue to improve upon both the survey methods (e.g., trap operation) and the analysis methods (e.g., abundance estimators) used for estimating the salmon and steelhead abundance at both juvenile and adult life stages.


  • Juvenile Trap Design
  • Adult Trap Design
  • Abundance Estimators
  • Testing Assumptions of Mark-Recapture Study Designs