How Do We Count Fish? Understanding Fisheries Stock Assessments

Feature Photo: A small school of snook off the coast of Florida. Credit: Peter Leahy | Shutterstock

An important dynamic of fisheries management is the stock assessment, which provides the critical estimates necessary for setting sustainable fishing regulations. For advocates participating in the management process for our marine resources, this is foundational knowledge. While anglers and business owners don’t need to be able execute the intricate calculations input into these models, understanding the basics will increase your comprehension of available data and enhance your engagement in management meetings.

The following blog provides a breakdown of the essentials of fisheries stock assessments.

What Is a Stock Assessment?

A stock assessment evaluates the health of a fish stock by estimating its biomass, exploitation, recruitment, and population trends over time. It uses biological data, catch reports, and environmental factors to provide a scientific basis for determining whether a stock is overfished or experiencing overfishing. The definition of these two terms is important.

Overfished means the population biomass of a fish stock is below a level deemed sustainable, often due to long-term depletion, which can compromise its ability to replenish itself. In simplest terms, the fewer fish there are, the longer it will take for those fish to replenish their own stock through reproduction.

Overfishing means the the rate at which fish are being caught is too high, which risks depleting the population further or hindering recovery. While this may be confusing at first, it’s important to note that a fishery can be one of, both, or neither of these terms at any given time.

A small population that is not being fished hard could be overfished but not experiencing overfishing. On the contrary, a fishery can experience overfishing without yet being overfished, but continued overfishing may lead to an overfished status. Simple, right?

A stock assessment is crucial in setting catch limits and other management measures.

Core Elements of Stock Assessments:

1. Data Collection:
   • Fishery-Dependent Data: This includes catch data from commercial and recreational fisheries. These reports provide insight into the quantity of fish harvested, fishing effort, and spatial distribution of the fisheries.
   • Fishery-Independent Data: Scientific surveys and research provide unbiased data, including population trends, species distribution, and reproductive success rates.

2. Mathematical Models: Stock assessments use complex models to project future population trends. These models simulate fish population dynamics, including growth rates, natural mortality, and fishing mortality. They are built on assumptions about species biology, making accurate input data essential.

3. Important Terms & Key Indicators:
   • Spawning Stock Biomass (SSB): This refers to the total weight of all the fish in the stock that can reproduce. A decline in SSB can indicate overfishing, while a healthy SSB suggests a sustainable population.
   • Fishing Mortality Rate (F): The rate at which fish are removed from the population through fishing. If this rate exceeds sustainable levels, overfishing is occurring.
   • Recruitment: The number of young fish that survive to join the fishery each year. Good recruitment is essential for replenishing the stock.
   • MSY (Maximum Sustainable Yield): The largest catch that can be sustainably taken before overfishing begins.

How Stock Assessments Work:

The assessment process typically involves the following steps:

1. Data Compilation: Managers gather fishery-dependent and independent data, including catch reports, biological samples, and environmental factors.

2. Modeling: Scientists input these data into stock assessment models, which simulate the population’s response to current fishing pressures.

3. Peer Review: Assessments undergo rigorous peer review to ensure accuracy and scientific integrity.

4. Management Advice: Based on the results, managers may set or adjust catch limits, implement conservation measures, or revise fishing seasons and gear regulations.

Uncertainty and Challenges in Stock Assessments:

Stock assessments often face challenges due to data limitations. Variability in environmental conditions, changes in fish distribution, and difficulties in obtaining accurate catch data can all affect the precision of an assessment. Recognizing this, management bodies, such as the ASMFC, continually update their assessment methods, seeking to improve accuracy through more comprehensive data collection and advanced modeling techniques.

Why Stock Assessments Matter for Conservation:

For conservation-minded anglers and fisheries advocates, understanding stock assessments is essential. These assessments inform management decisions that balance the need for fishing opportunities with the long-term health of fish populations. They help protect spawning stock biomass, ensure sustainable harvests, and prevent overfishing, all while adapting to environmental changes and fishing pressures.

These stock assessments form the backbone of sustainable fisheries management. By understanding how these assessments work, advocates and anglers can better support science-based management practices and contribute to the long-term sustainability of marine resources.

For a more in-depth break down of Recreational Data & Stock Assessments, we encourage you to watch the following presentation from Katie Drew Ph.D., the Stock Assessment Team Lead from the ASMFC. Katie is one of the brilliant minds that helps our community better understand the state of our fisheries. The presentation was recorded at the National Saltwater Recreational Fisheries Summit in 2022: