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Fish Stock Assessment 101: Part 2—A Closer Look at Stock Assessment Models

October 10, 2012

Catch, abundance, and biology data feed into mathematical models that represent the factors causing changes in harvested fish stocks.


In Part 1 of our Fish Stock Assessment 101 series, we presented the three primary types of data used in fish stock assessments—catch, abundance, and biology data. These three types of data feed into mathematical models that represent the factors causing changes in harvested fish stocks. The models produce estimates of the fishery management factors needed for managers to make informed decisions about how to best regulate a fishery. When possible, stock assessment models include information on ecosystem and environmental effects to improve the interpretation of historical information and the precision of forecasts. 

What is a stock assessment?

A stock assessment is the process of collecting, analyzing, and reporting demographic information to determine changes in the abundance of fishery stocks in response to fishing and, to the extent possible, predict future trends of stock abundance. 

Managers use stock assessments as a basis to evaluate and specify the present and probable future condition of a fishery.

What is a quota?

A quota is the maximum amount  of fish (number or weight) that can be caught within a specified time period. Quotas might apply to a total fishery, multiple fishing sectors, or individual fishermen under catch share programs (i.e., individual transferable quotas).

More About Models:

Individual stock assessment modeling packages offer different features. The models available for assessing fish stocks range from simple to complex based on the available data for a given stock. 

Scientists choose the model best suited for a stock’s life history and data availability and might try multiple models to find the best possible fit.

Fish stock assessment models can be divided into two very broad categories: 

Core Applications—Proven methods drive core applications. Used extensively in peer-reviewed stock assessments. 

Research Models—Research models represent new methodology tested (and often developed) by NOAA Fisheries stock assessment scientists, but not yet used to complete peer-reviewed stock assessments. 

Learn more about models and visit the Stock Assessment Toolbox.

Stock Assessments Provide Scientific Advice For Sound Fisheries Management

Stock assessments are one important piece of a dynamic cycle of management aimed at preserving our ocean resources. They provide scientific advice to decision-makers on the current health and future trends of a fish stock and its fishery. Assessments also offer the technical basis for setting annual fishery harvest levels (through quotas and catch limits) and other fishery management measures. 


For example, if a stock assessment model indicates that a stock has rebuilt to a healthy level, fishery managers recommend higher catch limits, longer fishing seasons, or fewer fishing area restrictions. Managers make recommendations with the intent of maintaining healthy fish populations and sustainable fisheries that provide for economically healthy coastal communities and a constant supply of seafood.

What Factors Go Into Fish Stock Assessment Models?

Fish stock assessment models represent the processes of birth, natural death, growth, and fishery catch that affect the fish stock over time. Scientists calibrate the model by using observed data from fishery catch, fish abundance surveys, and fish biology. Conceptually, this is similar to NOAA’s National Weather Service dynamic atmospheric models, which use multiple weather observations to calibrate complex atmospheric models that forecasters can use to make informed predictions. 

Even though fish stock assessments operate on much longer time scales than weather models—months and years rather than hours and days—they similarly combine and incorporate many different complex observations into a holistic picture of the situation. Learn more about models.

Like weather models, most of today’s stock assessment models work as computer simulations of fish populations. Hundreds of factors may be needed in complex situations involving multiple stock areas, several fishing fleets, and lengthy time series data. In the end, how closely a fish stock assessment model fits the actual data indicates the reliability of the historical estimates and future predictions for a fish stock. 

Many assessment models use graphical interfaces that help standardize assessments and make it easier for scientists to work together on projects and compare their work. Take a more in depth look at models—read Stock Assessments 101, Part 2b.

Understanding Population Assessments

Population assessments—also known as stock assessments—provide important information for marine resource management.

Last updated by Office of Communications on November 24, 2021