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Aquatic Food Production: Productivity, Fishing and Aquaculture

Seas are productive where light and nutrients meet: shallow continental shelves, upwelling zones, estuaries and coral reefs. The open ocean is poor in nutrients. About 90% of energy is lost at each feeding level, so food from low trophic levels (seaweed, shellfish, small fish) is more efficient. Fishing methods differ in how selective they are and how much bycatch and seabed damage they cause. If catch is larger than the stock's growth, the stock collapses (overfishing). The maximum sustainable yield (MSY) is the largest catch that can be taken year after year, usually at about half the unfished stock. Aquaculture (fish farming) now supplies over half of the fish we eat; it can reduce pressure on wild stocks but can cause pollution, disease, escapes and habitat loss.

🎬 Step-by-step story

  1. The sea is not equally rich everywhere. Where there is light and nutrients together, like shallow shelves and upwelling zones, plankton grow fast. The open ocean is nearly a desert.
  2. About 90% of energy is lost at each feeding level. 1000 units of plankton make only 1 unit of tuna. Eating low on the food chain feeds more people.
  3. Here are four fishing methods. Some also catch animals we did not want. This is called bycatch. Count the red blocks under each one.
  4. Overfishing: catching 40% of the stock every year. Young fish cannot replace them fast enough, so the count keeps falling.
  5. Aquaculture: farming fish in cages or ponds. Feed goes in. Waste and chemicals come out into the water. Some farmed fish escape.
  6. Your turn. Set the yearly catch and run 20 years. Which catch gives the most fish in total and still keeps the stock healthy?

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

πŸ€” Common doubts, cleared

Why is the open ocean poor if it gets so much sunlight?

Light alone is not enough. Dead plankton sink and carry nutrients down, so the sunlit layer runs short of nitrate and phosphate.

Why not just eat more tuna and less sardine?

Each step up the food chain loses about 90% of the energy. A tonne of tuna needs about 1000 tonnes of plankton; sardines need far less.

Is any fishing method completely harmless?

Selective methods such as pole and line catch little bycatch, but every method removes fish. Harm depends on the method and how much is caught.

If the stock does not hit zero, why is 40% a problem?

The stock settles at a low level, where it grows slowly, so the yearly catch becomes small. It is also more likely to collapse in a bad year.

Does fish farming solve overfishing?

Only partly. Waste, disease, escapes and fishmeal feed can harm wild fish and water quality.

Why does a medium catch give more fish in total than a big catch?

A medium catch keeps the stock near half its maximum, where it grows fastest, so it can be harvested every year.

Marine productivity: why some seas are rich

Productivity is how fast plants and algae make new living matter. In the sea most of it comes from tiny floating algae called phytoplankton. They need two things: light (only in the top layer, roughly 0–200 m) and nutrients such as nitrate and phosphate.

Energy passes up food chains with about 10% efficiency. So 1000 kJ of plankton supports about 1 kJ of a top predator such as tuna. Farming or eating species low in the food chain (seaweed, mussels, carp, sardines) produces far more food from the same sunlight.

Fishing methods and overfishing

Fishing methods differ in what they catch and the damage they do:

Modern technology (sonar, GPS, bigger boats, freezer ships) makes catching easier. Overfishing happens when catch is larger than the stock's natural growth. Signs: smaller catch per hour of fishing, smaller and younger fish, and loss of breeding adults. Removing top predators can also change the whole food web.

Maximum sustainable yield (MSY) is the largest catch that can be taken every year without shrinking the stock. Population growth is fastest at about half the carrying capacity, so MSY is taken when the stock is near half its unfished size.

Management tools: catch quotas, minimum mesh and landing sizes, closed seasons (such as India's monsoon fishing ban), no-take marine protected areas, limits on boats and days at sea, bycatch-reduction devices (turtle excluder devices), and eco-labels for sustainable fish.

Aquaculture: farming the water

Aquaculture is farming fish, shellfish or seaweed in controlled conditions: ponds, tanks, sea cages or ropes. It now produces more than half of the fish people eat worldwide. China, India, Indonesia and Vietnam are leading producers.

Advantages

Problems

Better systems: farming herbivores and filter feeders (carp, tilapia, mussels, seaweed); integrated multi-trophic aquaculture, where seaweed and shellfish take up waste from fish cages; recirculating tanks on land; and rice–fish farming.

Try it: find the best catch

In the free-play step, try a yearly catch of 10%, 25%, 40% and 60%. Write down the total catch and the stock left after 20 years. You should find that a middle value (around 25%) gives the biggest total while keeping the stock. That is the idea of maximum sustainable yield.

Key formulas and definitions

Worked examples

1. Plankton in an area store 50 000 kJ of energy. Using 10% transfer, how much reaches a third-level consumer (plankton β†’ small fish β†’ bigger fish β†’ tuna)?

Small fish: 5000 kJ; bigger fish: 500 kJ; tuna: 50 kJ.

2. A fish farm feeds 1300 kg of feed and the fish gain 1000 kg. Find the feed conversion ratio.

FCR = 1300 Γ· 1000 = 1.3.

3. Explain why the waters off Peru are so productive while the central Pacific is not.

Off Peru, winds push surface water offshore and cold deep water full of nitrate and phosphate rises into the sunlit layer, so phytoplankton grow fast and feed huge anchovy shoals. The central Pacific has light but few nutrients, because dead matter sinks below the sunlit layer.

4. A stock's carrying capacity is 200 000 tonnes. At about what stock size is MSY taken?

At about half the carrying capacity: 100 000 tonnes.

Common mistakes

Practice quiz

1. Phytoplankton grow best where there is:
2. Roughly how much energy passes to the next trophic level?
3. Which fishing method damages the seabed most?
4. MSY is usually taken when the stock is at about:
5. Which is a problem caused by fish farming?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is maximum sustainable yield?

The largest catch that can be taken from a fish stock every year without making the stock smaller over time. It is usually taken when the stock is around half its unfished size.

Why are continental shelves so productive?

They are shallow, so nutrients from rivers and the seabed stay in the sunlit water, where phytoplankton can use them.

What are the main disadvantages of aquaculture?

Water pollution and eutrophication, disease and parasites spreading to wild fish, escapes, habitat loss such as mangrove clearance, and fishmeal from wild fish for carnivorous species.

Where this is taught

England (GCSE, A level)Year 133.5 Biological resources

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