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Fish Stock Assessment 101 Series: Part 1—Data Required for Assessing U.S. Fish Stocks

May 23, 2012

A biological fish stock is a group of fish of the same species that live in the same geographic area and mix enough to breed with each other when mature.


A biological fish stock is a group of fish of the same species that live in the same geographic area and mix enough to breed with each other when mature.  A management stock may refer to a biological stock, or a multi-species complex that is managed as a single unit.

Why Do We Conduct Fish Stock Assessments?

Yellowfin tuna jumping out of the water near Venezuela. 

NOAA Fisheries’ scientific stock assessments are key to fisheries management. They examine the effects of fishing and other factors to describe the past and current status of a fish stock, answer questions about the size of a fish stock, and make predictions about how a fish stock will respond to current and future management measures (Marine Fisheries Stock Assessment Improvement Plan). Fish stock assessments support sustainable fisheries by providing fisheries managers with the information necessary to make sound decisions.

Why Are Fish Stock Assessments Important?

Fisheries in the United States contribute significantly to the American economy and generate over 1.5 million jobs economy-wide. Healthy fisheries also provide recreational fishing opportunities to millions of Americans. To continue enjoying these benefits, we must carefully manage fish stocks to ensure sustainable use for current and future generations.

Stock assessments provide important science information necessary for the conservation and management of fish stocks. The Magnuson-Stevens Reauthorization Act calls for the best scientific information available to manage U.S. commercial and recreational fisheries. More than 500 fish stocks in the United States are managed under fishery management plans produced by eight regional fishery management councils. Additionally, coastal states and international organizations rely on NOAA Fisheries’ stock assessments for the management of non-federal and joint jurisdiction fish stocks.

Data for Complete Stock Assessments—Catch, Abundance, and Biology

Stock assessments are based on models of fish populations that require three primary categories of information: catch, abundance, and biology.  To ensure the highest quality stock assessments, the data used must be accurate and timely. 

Catch DataThe amount of fish removed from a stock by fishing. 
A national network of fishery monitoring programs continuously collects catch data and makes this information available to stock assessment scientists and managers. Sources of catch data include:

  • Dockside monitoring: Often conducted in partnership with state agencies and Fishery Commissions, dockside monitoring records commercial catch receipts to give an accurate measure of commercial landings and provides biological samples of the length, sex, and age of fish.
  • Logbooks: Records from commercial fishermen of their location, gear, and catch.
  • Observers: Biologists observe fishing operations on a certain proportion of fishing vessels and collect data on the amount of catch and discards. 
  • Recreational sampling: Telephone interview surveys and dockside sampling estimate the level of catch by the recreational fishery (Read more about the Marine Recreational Information Program).

Abundance DataA measure, or relative index, of the number or weight of fish in the stock

Data ideally come from a statistically-designed, fishery-independent survey (systematic sampling carried out by research or contracted commercial fishing vessels separately from commercial fishing operations) that samples fish at hundreds of locations throughout the stock’s range. Most surveys are conducted annually and collect data on all ecosystem components. NOAA Fishery Survey Vessels and chartered fishing vessels use standardized sampling methods to collect data the same way each year, providing a relative index of abundance over time.  In some situations, catch rates by fishermen can be calibrated to provide additional abundance measures as well. 

Biology DataProvides information on fish growth rates and natural mortality. 

Biological data includes information on fish size, age, reproductive rates, and movement. Annual growth rings in fish ear bones (otoliths, pictured on right) are read by biologists in our laboratories. The samples may be collected during fishery-independent surveys or be obtained from observers and other fishery sampling programs. Academic programs and cooperative research with the fishing industry are other important sources of biological data.

Improving Data Collection—Good Stock Assessments Require High Quality Data Inputs

How is NOAA Fisheries working to improve data collection programs?

  • Electronic catch data collection for rapid access.

  • Advanced monitoring equipment attached to traditional sampling gear to collect concurrent environmental information during surveys.

  • Visual surveys in complex habitats using imaging systems on robotic and autonomous underwater vehicles (AUVs - click here for more details on AUVs from one of our Teachers at Sea).

  • Non-extractive (does not harm or remove samples) abundance sampling using hydroacoustic technology.

  • Better define stock boundaries, habitat use, and fish movements by using electronic fish tags, genetic analysis, and research on the chemical structure of fish bones.

Stock AssessmentsDesigned to Answer Difficult Questions:

  • What is the current status of a fish stock relative to established targets? (e.g. Is a stock experiencing overfishing? Is the stock overfished?)

  • How much can fishermen catch while maintaining a healthy and sustainable fish stock?

  • If a stock is depleted, what steps must be taken to rebuild it to healthy abundance levels?

Answers to these important questions help managers make the best decisions to ensure a healthy balance between sustainable fish stocks, ecosystem health, and productive coastal communities.


Understanding Population Assessments

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

Last updated by Office of Communications on August 25, 2021