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Fisheries science: Stock assessment

Stock assessment is Step 2 in the Data to Decision process.

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About stock assessment

It is the scientific process of analyzing available data to evaluate the abundance, productivity, and options for harvest levels of fish stocks in the past, present, and future.

Stock assessments help to:

Stock assessments provide the information needed for evidence-based decision making. Stock assessments also help evaluate the expected biological impacts and benefits of proposed fisheries management measures.

Stock assessment methods

Stock assessment methods

Fishery-dependent and fishery-independent data are often used in stock assessments as part of mathematical models which can describe the changes observed in the stock and fishery in the past.

Stock assessment models are used to estimate how a stock has changed over time and can be used to predict what could happen in the future, such as how a stock might respond to different levels of catch or different environmental conditions.

Computer simulations

Computer simulations test how a stock might be affected by:

Simulations can help determine what level of harvest can help maintain the stock above a certain limit.

Fishery managers, industry, and Indigenous communities, work together to create harvest decision rules. These rules help decide when the catch:


Uncertainties can be grouped into two categories:

  1. measurement uncertainty, where perfect estimates of what is being measured, like stock size and harvest, can’t be guaranteed
  2. process uncertainty, where how a stock behaves or interacts with the environment is not fully known

More uncertainty means less precise estimates of stock status, harvest rates, or future trends. During stock assessments, biologists identify the main uncertainties in their advice. They also explain how they have taken those uncertainties into account. This advice helps indicate how uncertainty may affect possible risks associated with management decisions, or where more research is needed.

Data availability

Stock assessments often require large amounts of data, collected over multiple years. The amount of data collected for a stock, as well as the schedule for data collection and stock assessment, can vary depending on various factors such as resources for research or area accessibility. There are different techniques to deal with different amounts of data availability.

One way to deal with limited data is to provide a different kind of science advice, one that uses alternative methods like:

This approach can help provide similar advice on the possible impacts of different harvest levels.

Another way to deal with limited data is to use a “data-poor” stock assessment method that doesn’t need as much information going in. It can give some of the same kinds of results that the best “data-rich” methods can provide, and it does this by making more assumptions in areas where we are uncertain. For example, we can compare a data-poor stock with a stock of a closely-related species that has more information, to make predictions about how the data-poor stock may respond under similar harvest rates or environmental conditions.

Technical Expertise in Stock Assessment

The Technical Expertise in Stock Assessment (TESA) program at Fisheries and Oceans Canada promotes stock assessment excellence and expertise of our science staff through:

TESA greatly improves our ability to develop tools related to stock assessment, for example:

Since the program began in 2009, hundreds of our science staff across Canada have benefited from TESA training.

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Taking an ecosystem approach to fisheries management

Canada currently uses a single-species approach for the majority of the stocks we assess and manage. This approach examines one species at a time and uses stock assessment to consider the impact of fishing on the abundance of a fish stock. The results inform management decisions about the fishery.

An ecosystem approach to fisheries management takes a single-species approach and incorporates environmental variables into the stock assessment. This approach results in more informed decision making because it examines the impact of not only fishing but also environmental factors like:

We use this approach for some of the stocks we manage, and are working to include it in most stock assessments, which is a challenge because we often do not have enough data on environmental variables and understanding of how they influence the stock. We are also working to make sure that the scientific advice provided to fisheries management always considers environmental variables.

Moving towards an ecosystem approach to fisheries management will:

Considering the impact of environmental variables on fish stocks by implementing an ecosystem approach to fisheries management is a critical step on the path towards adopting ecosystem-based fisheries management, an even more comprehensive approach.

Ecosystem-based fisheries management is an approach that requires fisheries management decisions to consider the impact of the fishery on a wider set of factors including:

Learn more about the principles of ecosystem-based fisheries management.

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