Farming the water: Aquaculture farms fish, molluscs (mussels, oysters), crustaceans (shrimp, crabs) and aquatic plants (seaweed). Since 2022 it has supplied more of the world's aquatic animals than wild capture.
The points to remember
- More demand: a growing, richer population wants more protein, and fish is seen as healthy.
- Wild stocks are depleted: capture fisheries cannot catch much more.
- Little spare farmland: water can produce food where crops cannot grow.
- Profitable: it needs fewer boats and fishers than wild fishing, and its technology keeps improving.
- Economic development: jobs and exports for coastal and rural areas, especially in Asia.
- Food security: a steady supply all year, close to where people live.
Example (SL): China's carp ponds. Chinese farmers have raised carp in ponds for over a thousand years. Today China grows over half of the world's farmed aquatic food, much of it carp that eat plants and plankton, not fishmeal.
A reason for growth needs a link: Say what changed and why it raises output: 'wild fisheries are depleted, so demand shifts to farmed fish' scores; 'fish is popular' alone does not.
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Data questions often show a chart of seafood harvests by group: wild capture, aquaculture, and a projection for 2050. Read it in three steps: which bar, which colour, which year.
The points to remember
- An increase in tonnes = projected value - today's value. Do not give a percentage unless asked.
- A percentage increase = (new - old) ÷ old x 100, for example in the number of farm sites.
- Molluscs are easy to farm: small, barely move, packed close, and many filter-feed, so they need no feed.
- Crustaceans such as shrimp grow fast, take high stocking densities and sell at a high price.
- Freshwater fish (carp, tilapia) eat plants and cheap feed and grow well in ponds.
- Marine fish are hard to farm: many are big, migrate, need lots of feed and space; wild capture is cheaper, and many buyers prefer wild fish.
Example (SL), from the chart: molluscs harvested 6 + 18 = 24 million tonnes in 2019 and are projected at 34 in 2050, an increase of 10 million tonnes. A reason: more people, with more money, want high-protein food.
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Most farms keep many animals in a small space and feed them. That brings six main impacts, each with a clear cause: from clearing mangroves, to waste that causes eutrophication, to escaped fish that are genetically modified.
The points to remember
- Habitat loss: mangroves and wetlands are cleared for ponds; nets and cages trap wild animals.
- Feed and faeces: waste sinks and feeds algae, causing eutrophication, low oxygen and smothered sea beds.
- Anti-fouling agents: copper paint that stops weed growing on nets is toxic to shellfish and algae.
- Antibiotics and other medicines: they reach the water, harm wild life and breed resistant bacteria.
- Disease and parasites spread from crowded pens to wild fish.
- Escapees compete with wild fish and interbreed, diluting wild genes; some are non-native or GM.
Name the pollutant and the ecosystem: 'Pollution' alone is not enough: say 'uneaten feed sinks and causes eutrophication under the cages'. Other impacts: farmed fish eat fishmeal, so wild fish are overfished; inland ponds can use up fresh water and make soil salty.
Example (SL): salmon farming in Norway. Norway grows more farmed salmon than any other country, in floating net pens in its sheltered fjords.
The points to remember
- Sea lice from farms infect wild young salmon heading out to sea.
- Escapees interbreed with wild salmon, weakening their genes.
- Feed, faeces and copper from nets pollute the fjord bed.
- Managed by: vaccines in place of antibiotics, and cleaner fish that eat the lice.
- A traffic light system lets farms grow only where wild salmon are least harmed.
Using an example in an answer: Name the place and species, give two or three impacts with their effect, then one management technique and how it reduces an impact.
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Each impact has a technique that reduces it. The best known is integrated multi-trophic aquaculture (IMTA).
The points to remember
- IMTA: mussels filter out small particles; sea cucumbers or lobsters eat the large ones.
- Seaweed takes up dissolved nutrients and CO2 and gives out oxygen: lower BOD, no eutrophication.
- Recirculating systems: no escapes, waste collected.
- Vaccines and cleaner fish in place of antibiotics and lice drugs.
- Lower stocking density, and farms sited away from wild salmon rivers, in well-flushed water.
- Farm herbivores (carp, tilapia) that need no fishmeal; protect mangroves; certify farms.
Example (SL): the Bay of Fundy, Canada. Trial farms grew salmon with mussels and kelp beside them. The mussels and kelp grew well on the salmon's waste, and gave the farmers two extra crops to sell.
An integrated farm is not pollution-free: For its negative impacts, give disease, escapees, habitat lost to cages, or medicines. Do not give eutrophication or low oxygen: those are what the system reduces.
Questions ask you to evaluate aquaculture as a food source for the future, or to compare it with wild capture fisheries. Give both sides, then a judgement.
Strengths
- Less pressure on wild fish stocks.
- Food for a growing population: food security.
- Jobs and income, especially for coastal and developing countries.
- Efficient: fish turn feed into meat far better than cattle.
- Healthy protein; technology keeps improving it.
- IMTA gives several crops and recycles nutrients.
Limitations
- Habitat loss, such as mangroves.
- Pollution from feed, faeces, antibiotics, copper.
- Disease and escapees (some non-native or GM).
- Carnivores need fishmeal from wild fish.
- High costs; big farms push out small ones; hard to certify.
- Not possible for many large marine fish, such as tuna.
The points to remember
- Efficiency: farms produce more food per hectare and burn less fuel than boats; wild capture needs no feed.
- Impacts: capture causes overfishing, bycatch and trawl damage; farms cause pollution, disease and escapees.
- Scale matters: small, mixed farms (pond polyculture) are efficient and low impact; large monocultures of carnivores have the most impacts.
Example (SL): two kinds of farm. A carp pond in China, fed on plants and pond plankton, adds food with little harm. A salmon pen in Norway needs fishmeal and spreads lice. Both are 'aquaculture', so a good judgement says which kind.
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How this comes up: Paper 2 Section A reads a harvest chart (identify, calculate, a reason, two reasons [2], evaluate [4]); Paper 1 asks about one farm system [2]; Section B asks you to evaluate aquaculture [7] or compare it with wild fisheries [9].
Scotland's salmon farms produce over 150 000 tonnes of fish a year in the sea lochs of the west coast.
Evaluate the potential value of aquaculture for providing food for future generations.
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