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Fish Stock Assessment 101 Series: Part 3—Ecosystem Factors and Assessments

Why study food webs?

Studying a particular fish’s prey and predators helps scientists understand how that fish is connected to other marine resources in the ocean. Fish, marine mammals, sea turtles, seabirds, and plankton all live in a balance known as the food web. Researching food web dynamics helps NOAA Fisheries scientists better understand the important factors to be considered in stock assessments. These stock assessments are used as tools for setting catch limits. Read about an interesting food web conundrum when one endangered species eat another.


Scientists from the NOAA Alaska Fisheries Science Center conduct a groundfish survey in the Gulf of Alaska. Credit: Pamela Goddard, Thalassa (used with permission)

What is ecosystem data?

Factors other than fishing can have an important role in determining the health and abundance of fish stocks. Ecosystem factors such as interactions among species in the marine food web, changes in marine coastal habitat, and constantly changing ocean environmental factors may be important. Examples of ecosystem data that may inform stock assessments include:

June 17, 2013

Fishery Stock Assessments—It's All About the Science
Part 1—Data Required to Assess Fish Stocks
Part 2a—Stock Assessment Models
Part 2b—More on Models
Part 4—Future of Stock Assessments

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 data feed into mathematical models that represent the factors causing changes in harvested fish stocks.

In Part 2, we took a more detailed look at how stock assessment models work. 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.

Ecosystem Factors and Stock Assessments

Factors other than fishing can have an influential role in determining the health and abundance of fish stocks. These factors are not only important to fish populations; by including them in the analysis of fishing effects, we can better interpret stock assessment results. Ecosystem factors such as species interactions, habitat, and large-scale climate patterns may be important.

A Holistic Approach

Traditionally, fish stock assessments have relied on direct measurement of fish stocks and catch to determine a stock’s abundance and potential catch levels. This approach is effective for looking at present and historical conditions, but limited when trying to understand why changes occurred because it only accounts for the effects of fishing. This approach is also limited when trying to make accurate forecasts of sustainable catch levels because it does not account for changing ecosystem factors that could impact fish abundance.

The integrated analysis models described in Part 2 of this series allow environmental and ecosystem factors to be included in a stock assessment model, which can help scientists better understand historical stock changes and improve forecasts. For example, research indicates that the annual catchability of several stocks of flatfish in the Bering Sea is affected by bottom water temperatures. For these species, colder water influences the timing of spawning migrations and also slows activity, making the fish less likely to be caught in survey trawls. Modeling the relationship between annual bottom temperatures and survey catchability for these species improves the fit of survey biomass estimates and reduces overall uncertainty in the model results. Ecosystem food web studies can also provide more accurate values for important fish assessment parameters, such as natural mortality. For example, several species of forage fishes like the Atlantic herring assessments have begun to include estimates of predation removals. Modeling the dynamics of Atlantic herring inclusive of predation has provided a more accurate accounting of what has been removed from the stock each year, and hence a more realistic understanding of how that stock behaves, consistent with scientific and fisher observations on the water.

More research is needed to determine which factors are most important to fish populations so these factors can be included in stock assessment models appropriately. It is clear that very often more than just catch can affect fish stocks. Fish stock assessment results often feed back into holistic ecosystem studies by providing long time series of information on historical fish abundance and productivity. NOAA Fisheries is committed to supporting science and research to move us toward effective ecosystem-based management. Developing tools and approaches for incorporating ecosystem factors will allow us to deal with the impacts of climate and other environmental change on our marine trust species.