Fisheries for the future: life cycles, harvest and aquaculture
Evaluate marine life histories, fisheries management and aquaculture using ecological, social, economic and food-supply evidence.
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 8.1, 8.2, 8.3 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What is one role of a dispersive larval stage for a sessile marine adult?
What you will learn
- Compare simple and complex life cycles, larval dispersal and reproductive strategies
- Explain stock decline and evaluate methods for more sustainable fisheries
- Compare intensive and extensive aquaculture and assess long-term viability
- Consider ecological, economic and social trade-offs in resource decisions
Life cycles and reproduction
Metamorphosis changes body form between life stages. Many crustaceans have complex life cycles with dispersive larvae; marine mammals have simpler development without a larval stage. Sessile adults may rely on mobile larvae for dispersal; non-sessile adults move directly. External fertilisation can release many gametes but fertilisation success is uncertain; internal fertilisation protects gametes but requires greater investment. Compare tuna, sharks and whales using the syllabus examples and explain offspring-care trade-offs.
Sustainable fisheries
Fishing pressure can reduce a stock below a level where it recovers. Assess abundance, age/size structure, recruitment, catch and effort rather than relying on landings alone. Management can include quotas, size limits, seasonal closures, protected areas, gear rules, bycatch reduction and enforcement. Sustainable yield is context-dependent; consider uncertainty, habitat impacts and the livelihoods and food security of communities.
Aquaculture
Intensive systems control high stocking density and inputs; extensive systems use more natural conditions and lower inputs. Salmon, mussels and shrimp illustrate different methods. Long-term success requires stock, clean water, feed and feed efficiency, labour, disease management, suitable location, market demand, market access and return on investment. Evaluate habitat damage, feedstock pressure, pollution, escapes, invasive species, disease and resource competition alongside reduced pressure on wild stocks, jobs and food supply.
Sources for this lesson
Pause and explain
Which evidence best separates catch volume from stock condition?
Worked example
A proposal replaces part of a wild fish catch with salmon aquaculture. Give one possible benefit, two risks and evidence needed to decide whether the project is sustainable.
Show the worked solution
- Aquaculture could provide food and reduce some harvest demand on wild stocks.
- Risks include feed-fish pressure, disease, pollution, escapes or habitat damage.
- Compare lifecycle impacts, wild-stock trends, feed conversion, effluent, escapes, jobs and market viability over time.
Answer Salmon farming may add food and reduce pressure on some wild catches, but can depend on fish-based feed and introduce disease, waste or escapes. Compare stock status and lifecycle feed impacts with repeated water-quality, escape, disease and economic data before calling it sustainable.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A fishery catches 12 000 tonnes this year compared with 9 000 last year, but fishing effort doubled. Explain why this does not prove the stock improved.
Give me a hint
Compare catch per unit effort and consider independent stock evidence.
Compare my reasoning
- Calculate or compare catch relative to effort.
- Explain that more effort can raise catch despite declining abundance.
- Request stock, size structure and recruitment evidence.
Catch rose by one third while effort doubled, so catch per unit effort fell from 9 000/E to 6 000/E if effort changed from E to 2E. Greater catch can result from more effort rather than a healthier stock. Independent abundance, size and recruitment data are needed.
Look for these in your work
- Compares catch with effort
- Avoids equating landings with abundance
- Names stock indicators
A shrimp farm creates jobs but discharges nutrient-rich water and sometimes escapes stock. Propose two indicators for evaluating the trade-off.
Give me a hint
Use at least one ecological and one social/economic measure.
Compare my reasoning
- Measure downstream nutrients or dissolved oxygen against a comparison site.
- Track escape frequency or changes in local species.
- Record jobs, income or local food supply over the same period.
Monitor nutrients and dissolved oxygen downstream against a suitable comparison site, and record escapes and native-species changes. Also track jobs, income and local food supply. A sustainability judgement should compare ecological outcomes with social and economic benefits over time.
Look for these in your work
- Includes ecological indicators
- Includes social/economic indicators
- Uses comparison and time evidence
A mobile marine mammal and a sessile adult crustacean have different life-cycle needs. Explain why dispersal and offspring care may differ.
Give me a hint
Compare larval stages and the trade-off between many exposed gametes and fewer protected offspring.
Compare my reasoning
- Describe the crustacean’s possible dispersive larva and metamorphosis.
- Describe the mammal’s simpler development and internal fertilisation.
- Compare dispersal opportunity with survival and parental investment.
A crustacean may release many gametes and produce larvae that disperse before settling and metamorphosing, spreading offspring but with uncertain survival. A marine mammal uses internal fertilisation and greater parental care, usually producing fewer offspring with higher investment. Species details vary, so use the named organism’s syllabus example.
Look for these in your work
- Compares complex and simple development
- Links dispersal to life stage
- Explains reproduction-care trade-offs
Common mistakes
- Equating higher catch with larger fish populations
- Treating all aquaculture as environmentally beneficial
- Ignoring larval dispersal in a life cycle
- Evaluating sustainability only by profit or only by ecology
What can you explain now?
Which evidence best separates catch volume from stock condition?
Compare with the explanation
Catch and effort alongside independent stock and recruitment measures
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What evidence would you need before deciding that an aquaculture project reduces pressure on wild fisheries?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.