Farmed fish: the good, the bad, and the algae

Fishing is still our main source of seafood and that’s surprising when you think about it. It is after all ‘hunter-gathering’ - making a living from wild animals – and for the most part, we gave up on this rather strenuous lifestyle over 10,000 years ago, when our ancestors began farming crops and livestock. And yet in the 21^st Century we still hunt fish. The growth of developing countries means there are more people with more money to spend on expensive foods like meat and fish. But with stocks overfished and the industry stagnating, fishing cannot supply the growing demand. Aquaculture, the farming of seafood (fish/shellfish/seaweeds) is filling the gap and is now the fastest growing food industry. For the industry to grow sustainably, fish farmers might need to use algae (= seaweeds and microscopic plant cells).

The good

Fish farming bypasses some of the environmental problems linked with modern fisheries. For example, most commercial fishermen use large nets; although they are generally interested in selling a few species of fish, ‘non-target’ animals get caught and hauled up too, often dying before they can be thrown back. Other fishing practices such as dynamite fishing and seabed trawling damage habitat, including coral reef. Fish farming doesn’t usually cause serious habitat damage (although shrimp farming certainly can.)

Fish farming avoids the by-catch problem.

Farming is intrinsically a more efficient way of sourcing food. This is because the farmer can manage their farm to maximize the health and productivity of his or her crops/livestock/fish stock.  For example, because a fish farmer is likely to have a good idea of how many fish they own, they can regulate the amount and type of food provided and this is especially true in the case of an intensive farm.

So long as it is sustainable, fish farming represents a more efficient way to supply our demand for animal protein than livestock farming.  This is because fish are cold blooded (or ‘poikiliotherms’ as zoologists like to say). This means more of the food they eat will be used to make muscle (meat) rather than being used to maintain a high and unvarying body temperature as is the case with our warm-blooded livestock. So is fish farming sustainable?

The answer depends on what type of fish farm you consider. Broadly, there are two main types: intensive and extensive. Extensive fish farming uses natural water bodies where a relatively small number of fish are contained and there is less need to feed them since the smaller number of fish can feed on natural food sources. The other extreme is an intensive fish farm – in order to maximise the amount of fish produced, fish are raised at high density in tanks or cages rather natural water bodies. Most of their food comes from man-made ‘aquafeed’.

Obviously, intensive farming offers the best hope for meeting rising food demands. Unfortunately, intensive farming, especially of carnivorous fish such as salmon, has been maligned for its unsustainability. At the same time, it is increasingly recognised that salmon has important health benefits due to the omega oils found in their tissues, so the cultivation of salmon will continue. Why is intensive farming of fish like salmon seen as the bad part of fish farming?

The bad (e.g. environmental problems caused by intensive salmon farm)

Disease spreads fast in large populations and intensive salmon farms are no exception: parasites such as ‘sea lice’ are rife. Worryingly, farmed salmon may be spreading their diseases on to wild relatives, many populations of which are in decline. Disease transmission is encouraged when farmed salmon escape (e.g. if a storm breaches the containment facility). This also raises the possibility of interbreeding between farmed and wild salmon – another bugbear for anyone concerned about sustainable salmon farming. This is because farmed salmon have been bred for growth, but may be genetically inferior when it comes to surviving in the wild (not least completing epic migrations). So, wild populations that have mixed with farmed fish could suffer.

Farmed salmon diets currently contain fish meal and fish oil derived from small fish. On the other hand, wild salmon survive on a varied carnivorous diet ranging from krill and zooplankton (lower in the food chain) to small fish. The result is that farmed salmon are feeding higher in the food chain than their wild counterparts. Because of ‘biomagnification’ (the accumulation of non-degradable toxins higher up food chains), farmed salmon are more likely to contain toxins such as dioxin than wild salmon – this is a shame given that salmon should be such a healthy food.

Quite apart from the toxin issues, feeding salmon with fish meal and fish oil derived from fisheries puts more pressure on fish stocks. It takes ~2kg of fish to make 1kg of salmon. Herbivorous fish such as carp and tilapia don’t need to be fed lots of fish meal so that’s why I singled out intensively farmed carnivores. Actually, most aquaculture has a relatively small ecological footprint...the most commonly farmed organisms are low in the food chain – herbivorous fish such as carp, filter-feeding molluscs such as oysters, and seaweeds which photosynthesise:

salmon, trout and catfish are carnivores. 

Finally there is the problem of waste – uneaten feed and fish faeces contain nutrient minerals such as nitrogen and phosphorous, too much of which can cause eutrophication. It’s well known that lakes and ponds suffer from eutrophication if a nearby farmer is over-zealous with fertilizer – mats of microscopic algae form at the surface of the pond, and deprive plants below of light. Waste from a fish farm can cause the same ‘algal bloom’ effect in a small part of the sea. Algal blooms get really bad when the algae that happen to be blooming are toxic - shellfish poisoning in humans is caused by the shellfish taking up algal toxins.

While microalgae cause problems when they bloom, macroalgae (macroalgae = seaweed) may be the solution to this problem and even the notorious microalgae may be able to improve the fish farming industry! Let's see what the algae have to offer...

The algae

Some seaweeds are already heavily cultivated and marketable. For example, sushi wrap is made of a blackish seaweed called nori. Seaweeds require nitrogen and phosphorous for growth, and fast-growing seaweeds are especially good at extracting nutrients from seawater quickly. The idea is to farm fast-growing and marketable seaweeds close to a fish farm. The seaweed would prevent eutrophication and toxic blooms, and even better, turn the fish farm waste into a sellable food product! In fact, some recent aquaculture projects have put the idea into practice.

In future, algae could also help to provide a source of sustainable feed to replace fish meal and fish oil. Although vegetable oils are currently being used to partially replace fish meal, vegetable oils lack the omega-3 and -6 fatty acids that are believed important for human health. As a result salmon fed on vegetable oils will have reduced omega oils in their tissues. Interestingly, some algae contain omega oils in abundance, especially certain microalgae. Even though salmon are carnivores, they will eat a diet derived from algae, provided nutrients are found in the correct ratios and indigestible components are removed. Although it is possible to feed salmon on an algal derived diet, the microalgae with the highest omega oil concentrations are not widely marketed, so the idea is not economically feasible...yet - a report published by the Scottish Aquaculture Research Forum suggests that algae-based fish feed will become viable as algae are increasingly cultivated for the bio-fuel sector.


The blue revolution

To feed 9 billion people in 2050, we will have to become effective and sustainable farmers of fish rather than relying too heavily on fishing and livestock. Hence, this century will see the arrival of the ‘blue revolution’. But the blue revolution should be ecologically friendly: algae, by extracting excess nutrients from fish waste and providing sustainable fish feed, could help to ‘green’ the blue revolution.