Fish Farming, Introduced Species and Eutrophication

Young fish are kept at high density in cages and fed with pellets of food, often made from other fish. The farmer can control feeding, protect the fish from predators and treat disease, and then harvest them.

Fish farms provide a reliable supply of protein for a growing human population and reduce the pressure of overfishing on wild stocks, which gives wild populations a chance to recover. They also provide jobs in coastal and rural areas.

Uneaten food and fish faeces fall to the seabed and add nutrients to the water, which can cause eutrophication and harm the organisms living on the seabed. Chemicals used to treat disease can also harm other marine organisms.

Parasites such as sea lice and diseases spread easily from the crowded farm fish to wild fish passing by. Farmed fish that escape can compete with wild fish and breed with them, which can reduce the genetic variation of the wild population.

Some are introduced deliberately, as crops, garden plants, pets or to control pests. Others arrive by accident, in cargo, in the ballast water of ships, or on boots and vehicles.

A new species often has no natural predators or diseases in its new home, so it can multiply quickly. It may outcompete the native species for food and space, eat them, or bring a disease that kills them, so biodiversity falls.

Many introduced species are useful: potatoes, tomatoes and sheep are not native to Britain, and they feed millions of people. A species that is introduced as a biological control agent can reduce the numbers of a pest, so fewer pesticides are needed.

A species that does no harm in its own habitat can do great harm in a new one, so introductions should be assessed before they happen.

Cane toads were brought to Australia in 1935 to control beetles that were damaging sugar cane. They did not control the beetle, but their skin is poisonous, so native animals that ate them died, and the toads have spread across northern Australia.

These large crayfish were brought to Britain from North America. They carry a disease called crayfish plague, which kills the native white-clawed crayfish, and they also compete with them for food and shelter.

This tall plant was brought to Britain as a garden plant. It grows along river banks, shades out native plants and then dies back each winter, which leaves the banks bare and open to erosion.

Farmers spread fertilisers containing nitrates and phosphates to increase crop yields. When it rains, some of the fertiliser is washed from the soil into streams and ponds. Untreated sewage and detergents add more.

The extra minerals act as a fertiliser for algae in the water, which grow and reproduce much faster than normal. The effect is a thick green layer of algae on the surface, called an algal bloom.

Plants beneath the algae lose their light and die. Bacteria multiply on the dead material and use up the oxygen, so fish and other animals that need oxygen die too and the biodiversity of the water falls.

Farmers can use only the amount of fertiliser the crop needs, avoid spreading it before heavy rain, and leave a strip of vegetation along the edge of fields to take up run-off. Sewage works can remove phosphates before water goes into rivers.

Farms can be placed where the water moves quickly to carry the waste away, use better feed so less is wasted, keep fish at lower density to reduce disease, and use strong nets and regular checks to stop escapes.

Strict rules on moving plants and animals, checks on ships and cargo, and fast action to remove a new invader before it spreads all help to keep biodiversity high.