Can We Go On Farming Salmon?

10/02/2017 14:22 GMT | Updated 09/02/2018 10:12 GMT

It has been reported - in this week's Scottish Sunday Herald - that plans are afoot to develop the world's largest salmon farm. To be sited in the seas off Orkney, or perhaps Shetland, the farm is intended to hold six to eight hundred thousand tonnes of fish. The current limit is two thousand five hundred tonnes.

Environmentalists claim the site would generate pollution equivalent in bulk to all of Glasgow's sewage: six hundred thousand tonnes, more or less. They also predict increases in infectious diseases, sea lice infestations and polluting chemicals. Add to these genetic pollution and the environmental consequences of sourcing fish feed, and you have a catalogue of impending ecological disasters.

Salmo salar is the fish in question: the UK's most common commercially farmed fish. This is the same species as the wild Atlantic salmon whose territory is the North Atlantic Ocean and the rivers in which they spawn. Often they leap up great water falls to reach their natal rivers, hence their scientific name; salmo is the Latin for salmon and salar means leaper.

But the wild salmons' captive counterparts are reared in vast sea cages, a world that could hardly be in greater contrast to the open ocean. With no outlet for their natural behaviour the caged fish have nothing to do but swim ceaselessly in circles in water contaminated with waste food, faeces and chemicals.

Like all animals in intensive environments these fish are kept in such dense numbers that the only way to keep them free of disease is to routinely dose them with an array of chemicals, including antibiotics. The risk of passing disease-resistant bacteria to humans who ingest bacterial residues when they eat farmed salmon is very real. But resistant bacteria can also pass on to the wild fish that feed on the pellets that have fallen through the sea cages. From there they can be taken up by the sea birds and sea mammals that eat those fish, and on down through the food chain to sea lice.

Infestations of these parasitic insects can eat a fish alive and so the farmed salmon industry relies on an array compounds including pesticides, disinfectants and medications to control them. These too wreak their own ecological damage. The run-off from teflubenzuron, for instance - just one of several sea-lice pesticides - poisons marine worms, fish and shellfish.

Genetic pollution is another hazard. Every year tens of thousands of fish escape from farms when cages are damaged by storms or when management is negligent. Since the escapees have been genetically selected from eggs and sperm taken from a relatively few number of salmon they lack the diversity of their wild ancestors. As a result they are less able to adapt to environmental change and thus less fit to survive in the wild; they are also likely to have a diminished homing ability. Inevitably some wild and farmed salmon interbreed. The effect on the wild population is the corruption of genetic lines, lines that have evolved over thousands of years.

Just as breeding is key to rearing salmon that can grow fast and survive intensive conditions, so is diet. A diet that is high in fat and protein ensures fast growth, and fishmeal, which is mostly processed from wild caught fish, delivers. UK farmed salmon can reach their slaughter weight of 3 kilos in just two years.

About one quarter of the commercial global fish catch is processed into fishmeal for feeding, not just farmed fish, but also intensely farmed prawns, cattle, pigs and poultry. But already overfishing is depleting wild fish numbers. Yet we continue to haul out of the oceans as many species as we can possibly use; dumping those we can't; demolishing habitats; and depleting fish stocks beyond their ability to recover. According to a report for the United Nations Environment Program ("In Dead Water) if fishing carries on at its present rate all the species currently caught will have disappeared by 2048.

The consequences of over-fishing are not always obvious. For instance a shortage of anchovies has led to their reduction in fishmeal. A study at Scotland's Stirling University recently found that as a result of the anchovy shortage the Omega-3s in farmed salmon are half what they were 5 years ago. The advice from health experts is that, in order to get the same benefit, two portions of salmon a week should be eaten rather than one.

This is good news for the farmed salmon industry from which most supermarket supplies originate. But bad news for the farmed fish who, despite misconceptions to the contrary, are sentient beings, capable of feeling fear and pain (which are both necessary survival traits) and who can also suffer from psychological trauma. And, of course, an increase in salmon farming is also bad news for the caged salmon's wild counterparts.

All these production processes bring into question whether they are 'capable of being maintained at a steady level without exhausting natural resources and causing severe ecological damage.' This is the Collins English Dictionary's definition of sustainability - a word that is so over-used that it often sounds like meaningless jargon. But not in this case. The only way the world as we know it can remain sustainable is to cut back on rearing animals intensively.