Seawater fish farming

Until recently, ocean based aquaculture has focused on a limited range of fish species. Atlantic salmon remains the predominant species, with smaller stocks of ocean trout, sea bass, cod, and halibut.

The farmed salmon industry began in Scotland and Norway during the seventies and the technology was gradually distributed to other locations, including the southern hemisphere. Today, salmon are intensively farmed in the cool coastal regions of Iceland, North America, Ireland, Chile, Australia, and New Zealand.

When affected by deficient planning or management, seawater farms have the capacity to severely threaten coastal environments and disrupt their natural ecosystems. Choosing an appropriate site is probably the most critical consideration for effective waste management and pollution control. From the perspective of raising large fish with minimal losses, well-protected bodies of water were originally preferred. In many cases their lack of tidal movement caused steady accumulations of uneaten food and fish droppings. A release of nitrogen and other alterations in water chemistry resulted in algal blooms which suffocated and poisoned the fish and other living creatures by depriving them of oxygen. When affected regions were surveyed, their aquatic habitats were often found to be significantly altered. As a result of this, stocks of wild fish were depleted, with several important species failing to recover.

Burdening marine habitats

Like other primary sectors, the aquaculture industry is subject to numerous restrictions imposed to limit the risk of environmental damage and depletion of wild stock species. While several of these restrictions are clearly inadequate or poorly enforced, progress is limited by the small amounts of research undertaken each year. Even when there is conclusive evidence of problems, it may still require years before all of the concerns are addressed in the manner which enables a practical ban or restriction to be imposed. When uncertainties or differences of opinion occur, the process may drag on indefinitely. With the exception of some organic representatives, the conventional aquaculture industry has failed to address many of the concerns raised by scientists monitoring marine habitats and wild fish populations.

Ocean based aquaculture has the potential to influence the health and diversity of wild fish populations in several ways: As described above, the biological waste and pollutants can alter the habitats of wild fish species. There is the ongoing risk of disease and parasites transferring from farmed fish to wild fish stocks. This has already occurred with infectious anaemia and sea lice. Despite best efforts to contain them, farmed salmon do escape in significant numbers. It is assumed that some of these escapees will eventually breed with wild fish of the same or related species. Because farmed salmon are selectively bred for production purposes, their genes can potentially interfere with the process of natural selection occurring among wild stocks.

The real cost of processed pellet food

The Atlantic salmon is largely a carnivorous species, normally dependent on a diet of crustaceans and insects. During their accelerated growth cycles in captivity, they consume several times their adult weight in fish meal and oils which are usually processed as a pellet feed. The raw materials for this are removed in vast quantities from the North Atlantic and Southern Pacific oceans. An excess of five million tons of small pelagic fish are currently required to feed captive salmon stocks every year. The species involved include sardines, herring, anchovies, and mackerel. While still considered abundant, the slow decline for many of these populations is undoubtedly impacting on their natural predators in the ocean. These include many pelagic fish species, migratory birds including penguin, albatross, and the oceanic mammals like seal and dolphin.

It has been suggested that every kilogram of farmed salmon requires two and a half kilograms of fish to be taken from the ocean and processed into farmed stock feed. The apparent inefficiency of salmon farming suggests that ocean resources are being utilised in an unsustainable manner. While several attempts have been made to identify and utilise alternative feed sources, salmon farming remains largely dependent on fish meal.

The future may lie in developing farming technologies for other ocean species, particularly those which provide superior food conversion efficiency.