In 1883 Thomas Huxley convinced the public and fellow scientists that fish stocks could never be over-exploited because there are so many fish and they reproduce so quickly. Perhaps he didn’t estimate that the world’s human population would now be nearing 8 billion people and that around 50% of world fisheries are already fully exploited with 32% being over exploited. Given continued population growth it is necessary to produce an additional 40% of protein from the ocean by 2030 yet global fisheries are in a dire state. As most of the wild stocks are fully exploited the cultivation of fish produce seems to be the only viable option, this is called aquaculture. Aquaculture now provides 45% of global aquatic produce compared to just 12% in the 1970’s.
Aquaculture is projected to be the prime source of seafood by 2030, as demand grows from the global middle class and wild capture fisheries approach their maximum takes. Aquaculture is essentially the farming of aquatic livestock, much the same as the terrestrial systems. When practised responsibly, fish farming can help provide livelihoods and feed millions of people whilst relieving natural fish stocks from over stress. Unfortunately much like terrestrial farming, it is hard to find a happy medium between productivity and sustainability, with the industry posing many environmental threats.
There are two main kinds of aquaculture system. Firstly, extensive systems are used were naturally enclosed or semi-enclosed areas such as lagoons are exploited to capture stock. These systems have naturally high productivity and there is no need for added supplements. Traditional extensive fish farming is one of the most ancient aquaculture methods, and is still practised around the world. Natural aquatic vegetation and micro-organisms form a food-web that can be utilised with sensible levels of fish stock.
Secondly, the alternative and more commonly used system is the intensive system where anthropogenic stocking densities are involved and the input of nutrients is needed to enhance production. Stock are kept inside large enclosures suspended in the water column and must be managed intensively with feed and medication. It is this intensive farming of fish species which is of concern, although it could produce food for millions of people, it could also severely damage the environment.
Moving on, different species have specific needs when it comes to husbandry, so much research has been invested at species level. This species specificity means that there are no general rules when it comes to farming fish stocks. However, one of the concerns regarding the practice is the fact that the most desirable fishes are often carnivorous. Carnivorous fish account for just 10% of aquaculture biomass but contribute to 40% of all revenue. But why is this a problem for the industry?
The issue is that the whole point of aquaculture is to elevate the stress on wild populations. Unfortunately almost all carnivorous fish are currently fed from wild caught prey items such as sand eels. So although the target species wild population is allowed to recover, lower trophic levels are still being exploited. For example, it takes 20 tonnes of wild fish to feed 1 tonne of tuna. Imagine if humans started to farm lions for consumption, but to feed those lions we had to breed Antelopes. The process is extremely resource intensive and unsustainable, yet we let it happen to our ocean species. The exploitation of the worlds forage fish to feed farms is undermining marine food webs. Some 20% of the world catch is now forage fish to be used as aquaculture feed.
Furthermore, another problem with this industry is the way the farms are stocked. Most captive fisheries have a mono-culture system where there is only one species present and the species is often bio engineered. Cryo-preservation is used to keep a constant supply of genetically engineered larvae. Examples of genetic engineering are the alteration of sex ratios, sterilising individuals so that more energy goes on growth rather than reproduction and also head and tail sized are often artificially reduced to reduce the amount of waste.
These genetically enhanced fish can possibly escape and out-compete with the wild population, reducing the genetic diversity of the species. In North America hatchery reared fish were used to enhance wild salmon stocks, but instead the introduced ones just out competed the wild ones and it was a disaster. Escaped farm fish can compete for food and habitat, displace indigenous species and interfere with the life of wild species. These escaped fish are also able to breed with the wild stock which can dilute the natural gene pool and threaten the long-term survival and evolution of wild species.
Yet another issue with escapees is the potential spread of disease to wild fish. Mono-culture and high stocking levels increase stress in species and reduces their immunity. One major issue involving this is the occurrence of sea lice and other disease. These parasites can lead to mortality in fish with a parasite load of just 10 lice. As wild fish swim past the farms they pick up these lice, this is common in salmon farms. Chemicals cannot be used as treatments due to bio-accumulation issues, so instead cleaner wrasse are put into the cages to help maintain the lice and other diseases, but this is often not enough.
Moving on, potentially the most alarming issue when it comes to aquaculture is the build-up of excess food and faeces at the bottom of the nets coupled with the increase oxygen demand of the farmed fish. Farms are often placed in areas with little wave action as to avoid net destruction. However this and the fact that the nets reduce bottom-flow means that excess feed and faeces from under the cages can create anoxic conditions. This reduces the diversity of species under and around the net, also if conditions get really bad there may not be enough oxygen in the area, essentially making it a dead zone. Fallowing is used to elevate the pressure on the benthos, this is the process of regularly moving the cage to allow the benthos to bounce back. However this is not encouraged in areas of slow growing sessile organisms like mearl which can be heavily effected by anoxic conditions. Without extensive management systems put into place, fish farms can become extremely toxic to the waters they inhabit making it difficult for wild organism to survive.
In response to these threats the industry is under stress to fund scientists to find new sustainable ways to fuel this industry whilst having minimal impact on the surrounding environments. With the concern of feeding these systems, research has been done trying to replicate the fatty acid contents found in fish liver, and produce a similar amount in plant produce. If plants can give the target fish enough nutrition then there will be no need for wild caught fish. This fish food could also come from the trimmings produced in processing seafood for humans, recycling fish waste through the industry.
Furthermore, the process of farming poly-culture systems is becoming more and more popular, not just with the use of cleaning wrasse. Innovative systems have been developed to increase the productivity and reduce the environmental impacts of aquaculture by combining different types of production. Integrated multi-trophic aquaculture (IMTA) includes organisms from different trophic levels of an ecosystem (e.g. fish, shellfish, algae), so that the by-products of one become the inputs of another.
Following on from this idea, the rise of aqua-ponics could be used in conjunction with fish farming to not only fuel the industry itself but further aid developing nations with food production. Aqua-ponics refers to any system that combines aquaculture (usually in aquarium-like structures) with hydroponics (cultivating plants in water). The water from an aquaculture system is fed to a hydroponic system where the by-products are broken down by nitrification bacteria into nitrates and nitrites, which are utilised by the plants as nutrients, and the water is then recirculated back to the aquaculture system.
Integrated aquaculture along these lines, both large and small scale, has a variety of benefits for farmers in addition to the production of fish for consumption or sale. In Asia, for example, rice farmers use certain species of fish to fight rice pests such as the golden snail. With rice-fish farming, they boost their rice yields and harvest the fish. Farmers in Zambia are introducing small ponds into their home gardens for irrigation and aquaculture. Mud from the bottom of fish ponds is also an organic mineral-rich fertiliser.
The comparisons between terrestrial farming and aquaculture are clear. Firstly the practice is necessary in order to feed the growing populations of the world. Unfortunately, due to the monopolisation of the industries the scale of them threatens the natural environment and could lead to large scale destruction. However, clearly if done on smaller scales the idea of integrated systems could be used to revolutionise the ways in which developing nations feed their people.
The sustainability of the industry is questionable, due to the large scale that is needed. The truth is we need to ask ourselves if these industries are necessary. As affluent ‘developed’ nations, must we feast on intensively farmed animals? As we continue to consider our environmental foot print it seems unnecessary to sacrifice natural habitats for the sake of our plates. We could not only thrive as individuals on a plant-based diet, but we could also feed the world whilst halting the destruction of our planets natural lands. Whilst aquaculture may alleviate pressure on our natural stocks, and brings with it further problems, is it really the future of our diets?