Fish Stock Assessment & Sustainable Harvesting (HL) (DP IB Environmental Systems & Societies (ESS)): Revision Note

Fish stock assessment & sustainable harvesting

Importance of assessing fish stocks and monitoring harvest rates

• Fish stock assessment is the process of estimating the size and health of fish populations in a given area

  • Accurate assessment is essential to understanding the number of fish available for sustainable harvesting

  • It helps ensure that the population can reproduce and thrive over time

• Monitoring harvest rates involves tracking how many fish are caught by fisheries over a specific period

  • Sustainable fisheries aim to ensure that the rate of fish caught does not exceed the population’s ability to reproduce

  • This helps to prevent overfishing

• Why assessment and monitoring matter:

  • Without accurate assessments, there is a risk of overestimating fish stocks

    • This can lead to overfishing and population collapse

  • Monitoring ensures that fisheries

    • Follow regulations

    • Maintain harvest rates that do not threaten the long-term survival of fish populations

Methods of assessing fish stocks and monitoring harvest rates

• Trawl surveys:

  • Dragging nets through the water to sample fish populations

  • Provides data on species, size, and abundance for stock estimates

• Acoustic surveys:

  • Uses sonar to detect fish schools by bouncing sound waves off them

  • Ideal for pelagic species like herring and mackerel

  • Allows for quick, large-area population assessments

• Landing records (catch data):

  • Collects data from official port records

  • Tracks the amount of fish brought ashore by fisheries

  • Helps compare actual catches with estimated sustainable levels

  • Ensures harvest rates do not exceed stock regeneration

• Portside sampling:

  • Collects data on fish size, age, and species at ports

  • Provides insights into the health and reproductive potential of stocks

• On-board observations:

  • Trained observers on fishing vessels record catch details and monitor compliance

  • Observers provide independent verification of the data, ensuring accurate reporting of catches

  • Reduces underreporting and bycatch misreporting

• Research surveys:

  • Independent scientific expeditions assess fish stocks using trawl or acoustic methods

  • Provides unbiased, reliable data on population trends

• Tag and recapture surveys:

  • Fish are tagged, released, and later recaptured to estimate population size and movement

  • Often used for highly migratory species like bluefin tuna

  • Helps track fish growth rates and migration patterns

Risks of fishing at maximum sustainable yield (MSY)

• MSY is the largest number of fish that can be caught each year without reducing the fish population in the long term

• MSY is only an estimate because:

  • The MSY value is calculated using models

  • These models are based on assumptions about fish population growth, mortality rates, and environmental conditions

  • Because fish stocks and ecosystems are complex, MSY estimates are always uncertain and can be inaccurate

  • If fisheries aim to harvest at the exact MSY, they risk overestimating and depleting the population

• Risks of exceeding MSY:

  • Overfishing occurs when the harvest rate exceeds the population’s ability to reproduce

  • A reduction in fish numbers can lead to positive feedback

    • The reduced population becomes even more vulnerable to overfishing because there are fewer mature individuals to reproduce (lower reproductive potential)

    • This leads to a rapid decline in fish stocks