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First teaching 2024

First exams 2026

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

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

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

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.