Aquaculture: A Sustainable Alternative?

Aquaculture (or fish farming) has grown at an average annual rate of 8.4%, according to a new report ‘Blue Frontiers: Managing the Cost of Aquaculture’ by the WorldFish Center and Conservation International, making it one of the world’s fastest-growing means of food production. Fish farms support a huge worldwide production industry, from the US to South-East Asia.

With stocks of the world’s wild fisheries depleting at a frightening rate after decades of unsustainable fishing methods and water pollution, focussing on controlled, farmed fish production seems like a smart ecological move, allowing for greater control of output and giving devastated wild populations time to replenish.

In reality, while aquaculture holds a great deal of promise for conserving the world’s oceans and satisfying demand for plentiful inexpensive seafood, lessons still have to be learnt if fish farming is to become a truly sustainable alternative.

Chris Lo talks to Mark Stevenson, population biologist at Queen Mary, University of London, about methodology, management and the mountain that aquaculture has to climb.

Chris Lo: How could you categorise the different types of aquaculture?

Mark Stevenson: There are two major types – extensive and intensive. Extensive-style hatcheries will have a high water-to-fry ratio, they won’t dump lots of chemicals in them, they’ll tend to take longer to mature and they’ll be more expensive to raise. The intensive style is maximum everything – especially in America, where they have growth hormones – even manipulating the temperature of the water the fish are in to get them to grow faster.

Depending on whether it’s extensive or intensive – and there’s a gradient between these two methods – you get different qualities of return. But the more intensive you become, the more you get mortality, especially in eggs. Intensive farms tend to produce grown-up fish faster, but at the same time the health of the fish is inferior, and they often have yields that get wiped out by diseases.

How do techniques differ across the varying marine environments?

With a salmon fishery, for example, there’s a two-part model. You grow them up to a certain point, and like wild salmon they commute out to sea, so you have to get a tank somewhere in either a lake that has sea water coming into it or a big pen out in the ocean. In ocean salmon pens you’ll have a double-pen; one that keeps the salmon from getting out and another sturdier one that prevents predators getting in.

If you’re doing freshwater fish farming, it’s often a lot easier. If you fish out a female carp, you just dump it into a new pond and pretty soon you’ll have lots of carp in that pond. While there are a lot of commercial carp farms producing lots of return, the main application with freshwater farming is at a small-scale local level.

The other major technique comes from the shellfish industry. Shellfish farming is probably one of the best developed techniques. It’s actually really good for the environment, relative to some of the other practices. For example, you collect large adult female mussels, because they’re the super-breeders, known as seeding females. These are collected from the wild, then seeded on the bed, so you have lots of little squares on the seabed that have nice habitat in shallow water, especially in coastal areas where there is a lot of tidal difference, like in the UK. Pretty soon, these females will produce ten, 15, 100 times as many mussels, and when the tide goes out you just pick them up. The native range of the species creates a more diverse, structured habitat. It’s pretty good, provided it’s the species’ native range.

Do you think aquaculture is having a positive impact on depleted wild populations?

I think this has to be a ‘yes, but’ answer. The state of the technology in our wild fisheries is the equivalent of going out to the Amazon rainforest and clear-felling everything; it’s not sustainable. The technology that we’re using – trawling, long-lining – is scouring the sea. If you look at aquaculture as a way to take the heat off capture fisheries, which are globally over-exploited, it’s a good thing. You can use relatively small, managed areas and get a lot of productivity without as much environmental impact overall.

A lot of intensive aquaculture has very intense local impacts; the water where fish are being farmed gets absolutely decimated. Prawn farming is a good example; in prawn aquaculture, you take pristine mangrove beds and you plough them up and turn them into channels where you can raise shrimp and prawns. If you’re doing this very intensively, you put loads of fertiliser in. They produce lots of waste and the area turns into the industrial wasteland. You get five years of good productivity, and then for the next 25-30 years the land is completely loaded with nitrogen and poisons and heavy metals. So that isn’t sustainable. Depending on how the aquaculture is used, it could be a useful tool for taking the heat off capture fisheries. But you also have to ask what the other impacts are.

Is there evidence to suggest any differences in flavour or health between wild fish and farmed fish?

I don’t think it’s very conclusive in terms of taste. Where there is really good evidence for differences is in things like Omega-3 and Omega-6, the fatty acids that are in fish that are related to brain development in infants. If you look at American-farmed tilapia, these fish are not fed their native diet. Their wild diet includes bugs and plants, a mixed diet with protein from small insects. When they’re farmed en masse in America, they’re just fed corn. That’s fine, a lot of this can be quite sustainable, but they’re not getting the same nutrients that they would in the wild.

There’s a significant difference between a wild tilapia and a farmed one in terms of the content of fatty acids. There are measurable differences between wild and farmed in certain species. Also, intensively farmed fish in general can build up heavier concentrates of heavy metals and pesticides. It doesn’t pose a risk to human health, but there are significant differences in the heavy metal build-in salmon.

In terms of the health of the fish, there are really big differences. The more intensively you farm the fish, you’ll get more return, but more fish will be abnormal. You get really big growth abnormalities because you’re pushing nature as hard as you can. In eggs, you get deformation. You also get swim bladder control problems, disease outbreaks and parasites. If one parasite gets in, the fish are in such close contact that it spreads everywhere.

What leaps would you like to see made in the future to make fish farming a more sustainable practice?

Some of the really exciting stuff in terms of sustainability is in tuna farming. Tuna are open ocean fish and need to swim continuously. There are some guys in Japan that have giant, oceanic tanks to raise tuna from eggs to fully grown adults. There’s always been massive mortality, but these guys are getting really good at simulating the environment they need. It’s very expensive, but we need to think about farming them if people want to keep eating them.

The next generation of hatchery will be combining really sound ecological theory with up-to-date hatchery technology. There are people working on this. It’s an area called aquaponics. This is where you grow plants in water and raise fish with them, creating a mini eco-system that is fully sustainable: everything feeds into everything else. You’re building a farm based on the model of nature. It requires maintenance and expertise, because you need to control the balance, but you get really good returns out of it, in theory. It hasn’t been done on a large scale yet, but that’s the optimistic outlook on the future.