Aim for FiveEnvironmental Science

Unit 5 · Topic 5.16

5.16 Aquaculture

Aquaculture is raising fish, shellfish and seaweed in controlled areas of water. It produces a lot of food in a small space with little fuel and can ease pressure on wild fish, but crowded farms can pollute water, spread disease and parasites to wild fish, and release escaped fish that compete with wild ones.

Key terms

  • aquaculture
  • fish farming
  • water pollution
  • disease spread
  • escaped fish

What aquaculture is

Aquaculture is the farming of aquatic organisms, such as fish, shrimp, oysters, mussels and seaweed, in ponds, tanks, net pens in the ocean or enclosed areas of lakes and rivers. It's the fastest-growing kind of food production, and farmed seafood now makes up about half of the fish and other aquatic animals people eat worldwide.

Common farmed species include carp and tilapia (mostly in freshwater ponds), salmon (in ocean net pens), shrimp (in coastal ponds) and shellfish like oysters and mussels (on ropes or racks in coastal water).

Benefits

  • High yield in a small area: farms can produce large amounts of protein per hectare of water.
  • Low fuel use compared with fishing fleets that travel long distances.
  • Efficient feed conversion: fish are cold-blooded and supported by water, so they turn feed into body mass more efficiently than cattle or pigs.
  • Less pressure on wild stocks, if the farm doesn't rely heavily on wild-caught fish for feed.
  • Filter feeders like oysters and mussels need no added feed and can even clean water by filtering out algae and particles.

Environmental problems

  • Water pollution: fish waste, uneaten feed and chemicals from crowded pens add nutrients and organic matter to surrounding water, which can cause algal blooms and low oxygen.
  • Disease and parasites: crowded fish get sick easily, and diseases and parasites like sea lice can spread from ocean net pens to nearby wild fish, such as young wild salmon.
  • Escaped fish: farmed fish that escape can compete with wild fish for food and habitat, and if they interbreed, they can reduce the genetic fitness of wild populations.
  • Antibiotics and pesticides: used to treat crowded fish, they can enter the water and contribute to resistance.
  • Wild fish for feed: farmed carnivorous fish like salmon and shrimp are fed fishmeal and fish oil made partly from wild-caught small fish, which can add to overfishing.
  • Habitat loss: in many tropical countries, mangrove forests have been cleared to build shrimp ponds, removing nursery habitat and storm protection.

More sustainable aquaculture

Farms can lower their impact by raising plant-eating or filter-feeding species, using closed tank systems that recycle and treat water, keeping stocking densities lower, placing pens where currents disperse waste, and replacing fishmeal with plant-based or algae-based feeds. Some farms grow shellfish and seaweed near fish pens to absorb the extra nutrients.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1Calculator allowed

    Wild fish in the feed (hypothetical numbers)

    A salmon farm produces 1,000 metric tons of salmon a year. It uses 1.3 kg of feed per kg of salmon, and 25% of the feed is fishmeal. Making 1 kg of fishmeal takes about 4.5 kg of wild fish. How many metric tons of wild fish does the farm use per year?

    Show the solution
    1. Step 1: Feed = 1,000 t × 1.3 = 1,300 t of feed.
    2. Step 2: Fishmeal = 25% of 1,300 t = 0.25 × 1,300 = 325 t.
    3. Step 3: Wild fish = 325 t × 4.5 = 1,462.5 t.
    4. Step 4: Interpretation: in this example, the farm uses more wild fish (about 1,460 t) than the salmon it produces (1,000 t), so it doesn't reduce pressure on wild fish overall.

    Answer: About 1,460 metric tons of wild fish per year.

  2. Example 2

    Evaluating a salmon farm

    A company plans ocean net pens for salmon in a bay where wild salmon migrate. Describe two environmental risks to the wild salmon and one way to reduce them.

    Show the solution
    1. Step 1: Risk 1: parasites like sea lice and diseases spread from crowded pens to wild salmon swimming past, especially young fish.
    2. Step 2: Risk 2: farmed salmon that escape compete with wild salmon for food and spawning sites, and interbreeding can weaken wild populations' adaptations.
    3. Step 3: Reduction: raise the salmon in closed, land-based tanks (or closed containment pens) that separate farmed and wild fish and let waste be treated.

    Answer: Risks: spread of sea lice and disease, and escaped fish competing or interbreeding with wild salmon. Using closed containment systems reduces both.

Common mistakes

  • Assuming aquaculture always reduces pressure on wild fish. Farms that feed carnivorous fish with fishmeal can increase it.
  • Forgetting habitat destruction. Shrimp farming has cleared large areas of mangroves.
  • Listing benefits or costs without explaining them. Say how waste or escaped fish cause harm.

On the exam

  • Expect to give one benefit and one environmental drawback of aquaculture with an explanation.
  • Connect aquaculture to overfishing (5.8) and eutrophication (8.5) when explaining its impacts.

Connected topics

Videos

Check yourself

4 questions on 5.16 Aquaculture. Pick an answer to see if you got it, and why.

Question 1 of 4

Atlantic salmon are raised in crowded net pens in coastal waters. Which is a likely environmental drawback of this kind of aquaculture?

Question 2 of 4

Farmed salmon are often fed pellets made partly from fishmeal and fish oil, which come from wild-caught small fish such as anchovies. What is a likely environmental concern with this practice?

Question 3 of 4

In parts of Southeast Asia and Latin America, large areas of coastal mangrove forest have been cleared to dig ponds for shrimp farming. Which is the most likely environmental cost?

Question 4 of 4

Which is an advantage of aquaculture compared with catching wild fish at sea?

0 of 4 answered