9.4 - Fish Farming

Why fish are farmed

• So many fish are taken from the wild that, without care, wild populations could be driven down to very low numbers.
• Fish farming is one response: fish are reared for food in enclosures or tanks rather than caught at sea.
• A fish farm is run in a deliberately controlled way, aiming to produce as many fish as possible, because fish are a valuable source of protein in the human diet.

Controlling growth and survival

• Keeping the fish enclosed lets the farmer control the things that affect how well the fish grow and survive - impossible with wild fish:
  • The food they get
  • The predators and diseases they are exposed to
  • The quality of the water around them
• Selective breeding is also used - breeding from fish with useful characteristics, such as fast growth and less aggressive behaviour, so successive generations are better suited to farming.

Sea-cage farming and predation control

• Many farmed sea fish, such as salmon, are kept in large cages suspended in the sea.
• Penning the fish in a cage means they don't have to swim long distances, so they use less energy on movement.
• More of the energy from their food then goes into growth, so the fish grow faster and bigger.
• The cage also acts as a barrier against predators.

Two types of predation to control

1. Interspecific predation - Predation by a different species. The cage keeps out animals such as seals and seabirds that would otherwise eat the fish.
2. Intraspecific predation - Predation within the same species. When fish of very different sizes are kept together, larger fish eat smaller ones.

Preventing intraspecific predation

• Rear young fish separately - Keep them in special tanks, apart from larger fish, so as many as possible survive.
• Feed the fish regularly - So the bigger fish aren't driven to eat the smaller ones.

Keeping farmed fish healthy

• Crowding many fish together in cages lets disease and parasites spread easily, so farmed fish are more prone to infection than wild fish living at low density.
• A common parasite of caged fish is the sea louse, which attaches to the fish and damages it.

Controlling disease and parasites

• Adding pesticides to the water to kill parasites
• Treating the fish with antibiotics or vaccinating them
• Removing any dead or diseased fish
• Keeping the stocking density low so infection spreads less easily

Pesticides have a drawback - the chemicals can pollute the surrounding sea. An alternative is biological control: a small fish called a wrasse is kept with the salmon and eats the sea lice off their bodies, controlling the parasite without the pollution a chemical pesticide would cause.

Feeding and clean water

• The fish are given a controlled diet of protein-rich food pellets, supplied in the right amount and at the right frequency.
• Better-quality food gives more energy, so the fish grow quicker and bigger and the farm's yield rises.
• Feeding the right amount stops leftover food fouling the water.
• Uneaten food and fish faeces would otherwise build up, making the water dirty and using up its oxygen.
• The waste is filtered out of the water and the water is kept oxygenated.
• Keeping the water clean and well-oxygenated keeps the fish healthy and prevents pollution of the surrounding environment.