Will fish farms sink the ecosystem?

When I was a child, smoked salmon was the height of luxury, only served on very special occasions.

The only access to salmon that I had was the dubious quality stuff that came in tins.

Now, no matter where you go, salmon is on almost every restaurant menu, in supermarket ready meals and even fish fingers.

Fish farming is now commonplace and more species are being farmed than ever before, including trout, catfish and even cod.

Prawns and other crustaceans are also harvested this way, bringing these protein sources to the masses, especially useful in developing countries where meat and conventionally-caught fish can be expensive.

However, fish farming is controversial and, according to its critics, causes massive disruption and damage to natural ecosystems.

Some scientists even suggest that widespread fish farming may account for the extinction of wild populations of trout and salmon in particular.

There are two types of fish farming, extensive and intensive, employing different methods of raising fish and each type is only suitable for certain species.

Extensive fish farming is generally for carp, catfish and perch and is not generally practised in the UK.

Intensive fish farming usually relies on fish being raised in cages, sometimes in cordoned off parts of natural waters such as lakes, rivers or inshore parts of the sea and sometimes in specially designed hatcheries that are separated from the natural environment.

As with any intensive farming system, intensive fish farming creates an unnatural population density that exposes the fish to greater vulnerability to disease and environmental problems.

If the fish are being raised in cages in natural waters, contamination of local waters quickly becomes apparent because of pathogens, uneaten food material and urine/faecal matter.

Nutrients from the fish farm can lead to a process known as eutrophication, or an excessive build-up of algae and microscopic plants, which can choke an aquatic ecosystem.

Salmon and trout in particular are very susceptible to polluted waters and contaminated water flushed out of a fish farm can bring disease and upset to the oxygen carrying capacity of the water.

Pristine water, which is required for successful breeding of trout and salmon, can be lost quickly in the vicinity of a fish farm.

The farmed fish are also unable to cope with polluted water, and losses can be quite high if the balance is not carefully maintained.

Diseases are usually controlled through drug applications, while purification systems treat the water to minimise pollution.

Failures of these systems are usually catastrophic.

While there is no doubt that fish farming has brought about a revolution in our diets, there needs to be a focus not just on price but the long-term health of aquatic ecosystems to enable natural fish populations to continue to thrive.