Understanding Population Assessments

A population assessment—also known as a stock assessment—measures the health and abundance of a given species population. It provides high-quality science information needed by fishery managers for decision-making. Our scientists conduct stock assessments for fish and for marine mammals.

Fish stock assessments

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. “Stock” may also refer to a multispecies complex that is managed as a single unit.

For fish stock assessments, we collect, analyze, and report demographic information to determine changes in the abundance of the stocks in response to fishing. To the extent possible, we also use this information to predict future trends of stock abundance.

Assessments provide the scientific basis for determining whether a stock is overfished or experiencing overfishing, and for calculating a sustainable harvest rate and forecasting catches that correspond to that rate.

Learn more about fish stock assessments

Marine mammal stock assessments

The Marine Mammal Protection Act [link to MMPA tab on Laws topic page] defines a stock as “…a group of marine mammals of the same species or smaller taxa in a common spatial arrangement, that interbreed when mature.” The act requires NOAA Fisheries to prepare stock assessment reports for each marine mammal stock that occurs in U.S. waters. (The U.S. Fish and Wildlife Service prepares stock assessment reports for marine mammals under their jurisdiction--manatees, polar bears, sea otters, and walruses.)

Managers use stock assessment reports to:

• Identify and evaluate the status of marine mammal populations and the effects of human activities upon them.
• Authorize the "taking" of marine mammals incidental to human activities.
• Design and conduct appropriate conservation measures.
• Evaluate the progress of each fishery in reducing its incidental mortality and serious injury rates.

Learn more about marine mammal stock assessments

Data for fish stock assessments

Two types of data are collected for fish stock assessments: fishery-dependent and fishery-independent.

• Fishery-dependent data are collected directly from the commercial and recreational fisheries and may include fishing effort, total amount of fish removed from the ocean (landings and discards), species, and biological information.
• Fishery-independent data are collected from at-sea surveys, where scientists from our regional fisheries science centers and from partner organizations gather data on fish stock abundance, biology, and their ecosystem.

Although NOAA Fisheries manages approximately 500 fish stocks, we only have data and resources to assess about 200 stocks each year. So, we work with our regional partners to determine which stocks are assessed each year.

Data for marine mammal stock assessments

Data on marine mammals are collected in a variety of ways, including aerial and ship-based surveys, acoustic monitoring, photo ID studies, biopsy sampling for genetic studies, and tagging. We analyze and interpret the data through our marine mammal research programs at the regional fisheries science centers.

Each stock assessment report includes:

• A description of the stock's geographic range.
• A minimum population estimate.
• Current population trends.
• Current and maximum net productivity rates.
• "Potential Biological Removal" levels.
• Status of the stock.
• Estimates of annual human-caused mortality and serious injury.
• Descriptions of other factors that may be causing a decline or impeding the recovery of “strategic” stocks.

Fish stock assessment reports provide estimates of fish stock abundance and fishing mortality rate relative to the targets and limits set for the fishery. These assessments are critical components of fishery management decisions.

For marine mammals, we use stock assessment reports to identify and evaluate the status of marine mammal populations and the effects of human activities upon them, authorize the "taking" of marine mammals incidental to human activities, and develop conservation measures. We also use the reports to evaluate the progress of each fishery in reducing its incidental mortality and serious injury rates.

NOAA Fisheries is about to start using a new fish stock assessment prioritization process, which will provide a consistent approach for establishing assessment priorities in each region. By considering numerous stock and fishery attributes, we will be able to identify stocks in need of assessment, and set target assessment frequencies and levels for each stock.

For marine mammal stock assessments, the Marine Mammal Protection Act provides only general guidance on assessment methods and on the content of the reports. So, since 1994 NOAA Fisheries and the U.S. Fish and Wildlife Service have held a series of workshops to develop guidelines that can be applied nationwide. The most recent revision of the Guidelines for Assessing Marine Mammal Stocks was completed in 2016, and the latest updates will be reflected in the 2017 draft stock assessment reports.