4BI1

Human Impact on the Environment

Ecology and the Environment · 8 question types

Practise

Why fish farming is needed

Wild fish populations have been severely overfished. Many fisheries that fed coastal communities for centuries have collapsed in the last 50 years; the Atlantic cod off Newfoundland is the most famous example. The UN estimates that around a third of all wild fish stocks are overfished and another half are at the limit of sustainability.

At the same time, demand for fish keeps rising as the world's population grows and as fish (especially oily fish) are seen as healthier than red meat. Fish farming (aquaculture) is supplying more and more of the world's seafood, now over half of it.

How fish farming works

Fish farms keep fish in enclosed cages or ponds, allowing them to be raised in controlled conditions:

  • Marine cages: salmon, sea bass, sea bream raised in coastal sea cages
  • Freshwater ponds: trout, carp, tilapia raised in inland ponds
  • Recirculating tank systems: enclosed tanks where water is filtered and reused

Fish farmers control:

  • Stocking density: how many fish per cage
  • Feed: usually a pellet mix containing fishmeal, soy, vegetable oils and vitamins
  • Water quality and oxygen levels
  • Disease: vaccinations and (sometimes) antibiotics
  • Temperature and lighting to maximise growth rate
Common exam question

Methods a fish farmer uses

Question: Explain the methods a fish farmer uses to maximise production, or describe how a farmer reduces disease in the fish (2–5 marks).

Set in 2 of the 23 papers, both on Paper 2, with one mark per method, and the specification's list is the menu: separate species, and separate sizes or ages, to prevent predation; feed protein-rich food in small, frequent amounts so waste does not build up; control disease with antibiotics, fungicides, pesticides or vaccination, by isolating or removing sick and dead fish, and by keeping the stocking density low; filter out faeces, waste and dead fish; aerate the water to keep oxygen levels up; and selective breeding or GM for faster growth. Unqualified answers earn nothing: "control their diet" needs the protein or the small portions, "clean the water" needs the oxygen or the waste removal, and "medicine" is ignored where antibiotics or vaccination are wanted.

Advantages of fish farming

  • Reduces pressure on wild fish stocks
  • Provides a reliable source of protein for a growing population
  • Fish convert feed into body mass more efficiently than cattle or pigs, so they need less feed per kg of meat
  • Creates jobs in coastal and rural areas

Disadvantages and concerns

  • Pollution: uneaten food, fish faeces and urine pollute the surrounding water with nutrients, sometimes causing local
  • Disease and parasites (especially sea lice in salmon farms) can spread to wild fish populations nearby
  • Escape of farmed fish that breed with wild populations, potentially reducing the wild population's genetic fitness
  • The feed contains wild fish: small fish are caught from the sea and ground up into fishmeal, so fish farming does not entirely remove pressure on wild stocks
  • Antibiotic use can drive resistance (see topic 14)
  • Habitat destruction: mangrove forests have been cleared in many tropical countries to make space for shrimp farms

Making fish farming more sustainable

Modern aquaculture is trying to reduce these problems:

  • Use of closed-containment systems that filter out waste
  • Plant-based fish feed to reduce reliance on wild fish
  • Vaccinations instead of antibiotics
  • Integrated multi-trophic aquaculture: farming fish, shellfish and seaweed together so the shellfish and seaweed take up the fish waste
Common exam question

How fish farms pollute the water and how to reduce it

Question: Explain how fish farms cause pollution, or how a change (a multi-trophic system, faster-growing GM fish, moving the farm offshore) reduces its effect on the ecosystem (2–5 marks).

Set in 3 of the 23 papers, all on Paper 2. The pollution answer is the eutrophication chain with a fish-farm source: uneaten food and faeces add organic waste and raise nitrate and phosphate levels; algae grow; bacteria decompose the waste and respire, so dissolved oxygen falls; pesticides, antibiotics and hormones add harm. A change that reduces pollution is marked as the chain in reverse: less waste, fewer decomposers and less bacterial respiration, so more oxygen stays in the water and less disease spreads; seaweed that takes up the nitrate prevents the algal bloom, and crabs or lobsters that eat the waste are extra products to sell. For an offshore site, any two of four points score: deep water, which dilutes the waste, and faster currents, which disperse it.

Sustainable wild fishing

For wild fish stocks, sustainable practices include:

  • Catch quotas that limit how much can be caught each year
  • Mesh size regulations so small juvenile fish escape and grow to breeding age
  • Marine protected areas where no fishing is allowed
  • Seasonal closures to protect breeding times
  • Better fishing gear that reduces bycatch (accidental catch of non-target species)

When properly managed, wild fish stocks can recover. The North Sea cod population has slowly recovered from overfishing thanks to strict EU quotas.