Marine fish farming: solution or delusion?

Mariculture is farming of marine organisms in cages, coastal enclosures, or containers with sea water. Presently almost a half of all fish consumed by humans are farmed fish exceeding in value and/or in tonnage many capture fisheries worldwide. Mariculture is becoming the dominant factor in fish supply with production that has increased by 10 times since the late 1970s.

By all estimates, the world population is approaching 8 billion. The resources of wild fish being finite, mariculture is the sole solution. In 2007, the total fisheries yield was over 140 million mt, with some 90 million produced by capture fisheries, and over 50 million by (marine and inland) aquaculture. If capture fisheries production (92 million mt in 2006) and the non-food uses of fish (33.3 million mt in 2006) remain constant, to maintain the current per capita consumption of 16.7kg, aquaculture needs to surpass 80.5 million mt by 2030, or sooner.

According to Danish-led recent study, gradual transfer of production of animal protein from land to the ocean is inevitable. Others believe that mariculture could provide alternative employment in fishing communities at a time of massive decline in fishing fleets.

Asia alone is producing 80% of the world’s aquaculture yield. A recent book ‘Success Stories in Asian Aquaculture’ (Springer Science, www.springer.com, De Silva, S.S. and F.B. Davy (Eds)), explains much of this development. Although the bulk of the Asian fish farming is inland, its shrimp mariculture has been growing at a fast rate, despite diseases epidemics and growing protests against uprooting of coastal mangroves and, thus, denuding beaches and exposing them to erosion and the nearby areas to tsunamis.

Not everyone is happy with mariculture development. Some think that not enough is known about the impact of farming new species, such as cod or halibut. Residual feed from cages causes algal blooms, which every so often are toxic. With 1.4 million tonnes of farmed salmon there are problems of dissemination of parasites and diseases to wild fish, of fish escapes, with possible genetic contamination of wild populations, and of huge discharge of nutrients, drugs and other pollutants.

Mariculture sometimes hurts capture fisheries. Fishermen are blaming the sea lice epidemic and the resulting dwindling of wild salmon stocks on salmon farms situated on wild salmon’s migrational path, and demand moving cage-farms out of that path, and to employ closed containment non-polluting systems. Cage farms anchored over extensive segments of fishing grounds deoxygenate and pollute the sea bed. Much of the conflicts could be avoided, if mariculture developers would follow such procedures as those appearing in the ‘Guide for the Sustainable Development of Mediterranean Aquaculture 2: Aquaculture site selection and site management’ (IUCN 2009, Gland & IUCN, 303p, http://iucn.org/mediterranean). This useful handbook is also relevant worldwide. Unfortunately, some mariculture enterprises take advantage of enforcement weakness to make as much profit as fast as possible, while ignoring environmental and social aspects.

Constraint of fishmeal supply

Carnivorous farmed fish must be fed fishmeal, mostly produced of small forage fish, which account for some 37% of the world’s sea fish yield that 25-30 million mt of them are reduced into fishmeal and oil. In 2006, aquaculture consumed approximately 3.06 million mt or 56 per cent of world fishmeal production and 0.78 million mt or 87 per cent of total fish oil production with over 50 per cent of fish oil going to salmon farming.

Between 1992 and 2006, in spite of increasing share of vegetable fodder, the expanding aquaculture’s consumption of fishmeal and oil tripled, and rather sooner than later it must face supply constraint. Besides in fishmeal or fishmeal-based diets, an approximate 5 to 6 million mt of low-value/trash fish are used as direct feed, notably in tuna cage culture. But the resources of fish that can be economically utilised for reduction are finite, and under favourable market conditions some of those resources may be reallocated for human consumption.

The solution consists in improved feed-production and utilisation. According to Israeli studies, vegetable protein feeds are fully sufficient for tilapia culture (apart from fingerlings), and any fishmeal mixed in is due to conservatism. Reduction of fishmeal content in seabream farming is also indicated, but only if more expensive than soybean meal and other vegetable products. FAO predicted that feed management efficiency is going to be improved. Now, one-third of fishmeal is made from various wastes, such as fishing bycatch and discards, fish-processing offal and industrial non-food fishes. Mixing poultry by-products and vegetable oils in feeds is experimented with.

Options

In sea cages, filtering shellfish culture is more environmentally benign than finfish farming and it can reduce the residual food and nutrients-products from the former. On the other hand, shellfish tend to bio-accumulate toxins from toxic algae.

Growing marine plants, including algae, as a basis for fodder for farmed sea fish and for bio-fuel production could be done on the basis of nutrients from waste, effluents and other coastal pollution, and offer significant environmental benefit.

Offshore cage culture could be the least damaging to the environment, if certain rules are followed. The option of free-floating cage farms migrating in oceanic currents is considered in USA.

Carrying sea fish farming ashore opens several interesting options. One is to regard the fish as only one component of an integrated system in which also other organisms are grown. Such aquaculture could be based on algae or other plants, and designed to exert less pressure on the environment and even enhance it. In addition, the option to grow sea fishes in land-based installations or in ponds represents a solution for local situations where sea-borne farming in cages is excluded.

Anyway, as long as it produces fish at affordable prices, mariculture will keep expanding along with the expanding world population. Constraining and obstructing it under the banners of “sustainability” and “precautionary principle”, is unsustainable. Reducing its negative impact – is.