5.5 How do food systems affect fish stocks and marine habitats?

5.5.1 Pressures on wild fish stocks and threats to marine ecosystems

Wild fish stocks are under increasing pressure

Marine vertebrate populations declined 49% between 1970 and 2012
29% of marine fisheries are overfished
WWF (2015)

The last 50 years have seen dramatic reductions in wild fish stocks, due mainly to over-fishing and destructive fishing techniques by humans. Around 85% of fisheries are now fully exploited or overfished.

Fish are an important food source however, and nearly 3 billion people rely on fish as a major source of protein. Fisheries therefore need to be protected on grounds of ‘self interest’ – to safeguard global food security – as well as for intrinsic environmental reasons.

The MRAG report identifies the following key threats to wild fish stocks caused by fishery-linked activities:

  • Overcapacity – taking a larger number of fish out of the environment than can be replenished naturally, leading to decreasing populations.
  • Perverse subsidies – made to those in the fishing industry for such purposes as vessel construction or to offset fuel tax; this lowers the real cost of fishing, meaning that fishing activities can extend beyond the point at which they become unprofitable.
  • Poor governance – leading to little or no local sustainable management of fisheries (in contrast, good governance may involve implementing protected areas where fishing is restricted or forbidden, in order to allow fish stock recovery).
  • A lack of data – on, for example, the state of wild fish stocks, the state of the wider environment, or concerning the operation of fisheries. Such data provides the basis for understanding how sustainable management might be put into place and of measuring progress towards or away from key goals.
  • By-catch and discards – which can lead to damaging reductions in populations both of species that are deliberately caught (for example not returning live caught fish that are too small to sell) and of species caught incidentally and unintentionally (for example when nets and other fishing equipment are not able to distinguish between target and non-target fish).

(For more on the proposed solutions to these problems, see the MRAG report).

Trawling causes direct damage to ecosystems (especially coral reefs) irrespectively of the amount of fish caught.

Only 3.4% of the oceans are protected. Certification of sustainable fisheries does exist, although coverage is not high and illegal fishing continues.

Other environmental damage to marine ecosystems includes the increase in oxygen-depleted dead zones resulting from nutrient run-off from agriculture, loss of coral ecosystems and mangrove systems. Some of the mangrove loss is a consequence of aquaculture / seafood farming although the influences are diverse and changing (see Richard and Friess, 2015). See later in this chapter for more on aquaculture.

5.5.2 The rise of aquaculture

Aquaculture can reduce some of this pressure

FCRN (2016)

Critics of aquaculture production argue that the industry uses wild fish as feed ingredients for farmed fish, thereby depleting marine stocks, albeit at a lower trophic level. This criticism is based on the argument that catching fish in order to feed other fish does not make any sense, depletes wild fish stocks of smaller fish and disrupts marine food chains.

However, in recent practices fishmeal composition has changed, making use of marine catch byproducts such as fish guts as well as the byproducts from farmed fish processing. Additionally, fishmeal can be made from fish such as sand eels that are not usually eaten by humans, or from fish that have low demand from humans or are caught long distances from markets (such as anchovy) – although how ‘low demand’ is defined is still controversial. It is also the case that the major reduction fisheries (i.e. fish harvested as feed ingredients) such as the Peruvian anchovy are often well managed and stocks are not being depleted. Other components of fishmeal include grains and agricultural byproducts, and while the same arguments exist as for feeding grains to livestock, the energy conversion from grain to fish flesh is generally better for fish than livestock.

Using modern feed combinations, it is now the case that for every tonne of wild fish used for aquaculture, around 1.92 tonnes of farmed fish (averaged over all fish types) are produced. Obviously there are variations between types – salmon is still the highest user with a FIFO ratio of 1.68, meaning that for every tonne of whole wild fish used 0.595 tonnes of salmon are produced.

Consumption of farmed fish is increasing, but China dominates the industry

  • Aquaculture now contributes around 50% of fish consumption.
  • Aquaculture production increased from 36.8 million tonnes in 2002 to 66.6 million tonnes in 2012.
  • However China somewhat skews the figures significantly accounting for about 50% of the world’s aquatic production and consumption. China is also the world’s largest exporter of farmed fish.

FAO (2012)

In the last 20 years there has been significant increase in aquaculture production and consumption on a global scale, although China represents over 60% of global production, much of its consumption, and is the largest exporter of farmed fish. The sustainability of aquaculture, and by extension wild fish stocks, is therefore greatly influenced by China.

The next slide looks at this relationship in more detail and looks at what is needed to ensure sustainable wild fish stocks.

Aquaculture and its environmental impacts

Growth in aquaculture production has been linked to many environmental concerns including:

  • Mangrove destruction and habitat loss
  • Pollution of the aquatic environment (eutrophication, oxygen depletion, pesticide contamination)
  • Escapes and genetic interactions with wild fish populations; use of non native species
  • Transmission of zoonotic diseases to wild fish
  • Use of fish meal and fish oil as major feed inputs

5.5.3 The diversity of aquacultural systems

Aquaculture systems are highly diverse so it is hard to generalise about their environmental impacts


Aquaculture enterprises vary by:

  • Intensity of production: extensive systems (consuming nutrients naturally present in water), semi-intensive (fertiliser inputs to increase nutrient content, or some supplementary feeds); intensive (commercially prepared feeds based on wild fish, fish processing byproducts and/grains and soy)
  • Species type (from crustacea through to salmonids) and trophic level (filter feeders, herbivores, carnivores)
  • Water type and source – rainfed (natural pond) systems or irrigated; fresh, brackish or salt water
  • Containment type – ponds and sea through to tanks
  • Market orientation (subsistence/semi-subsistence, production for local market, production for national or international markets)
  • Degree of integration with other agricultural practices, seasonal or year round, and so forth.

The diversity of aquaculture systems

Aquaculture exists in a multitude of different forms. Each has a different balance of environmental and socio-economic benefits and drawbacks. A small sample of different systems is shown here (text adapted from and images courtesy of Dave Little (personal communication)):


Top-left: Cage culture: Cage culture is a popular, simple and relatively inexpensive form of aquaculture to establish, whereby fish are enclosed in a cage in either flowing or static water. A key design element is that water moves in and out of the cage either through the natural flow or through movement of the fish themselves. Such free exchange makes cage culture vulnerable to pollution from other water users and also liable to impact surrounding water quality through excess feed (leading to fertilisation effects and eutrophication).

Top-right: Gher system in Bangladesh: co-production of rice with prawns, shrimp and fish. Gher systems have helped diversify livelihoods as farmers produce prawn and shrimp (mainly for export) as well as rice, vegetables and fish species for local consumption. Co-production reduces the overall nutrients required (since the fertilisers applied to the rice increase natural feed available for stocked and wild aquatic animals that share the space) and reduces pesticide use compared to rice monocultures. However, the use of pesticides in the modified rice fields has to be carefully managed to avoid poisoning the aquatic animals.

Bottom-left: Extensive/semi-intensive aquaculture ponds: common for shrimp in countries like Bangladesh and the Philippines. The ponds retain nutrients but also provide environmental services by removing nutrients from surface water. Although mangroves have been lost in the past through conversion to shrimp farms, in many areas shrimp ponds have increasingly been established on formerly unproductive rice land. Inland, semi-intensive aquaculture relying on additional fertilisation and feed inputs tends to add nutrients to the farm or local environment which may have negative or positive impacts depending on context. Semi-intensive aquaculture ponds remain the dominant form of aquaculture in the Asia Pacific region, generating an increasing proportion of the fish consumed and supporting large numbers of poor livelihoods.

Bottom-right: Intensive white-legged shrimp production in China reared in intensive concrete lined ponds with on-farm effluent recycling and reuse and relatively high feed conversion efficiencies achieved through use of commercial feeds: The species has been subject to domestication and selective breeding in recent years. High stocking densities of shrimp and limited water exchange are made possible through use of commercial formulated water-stable feeds, the use of aeration and recycling of water on the farm. Such management reduces the risk to pathogen exposure and pollution of surrounding water resources. However feed and energy costs are high and comparative LCAs find overall environmental impacts to be higher in intensive than in semi intensive systems (Cao, L., Diana, J.S., Keoleian, G.A. and Lai, Q. (2011) Life Cycle Assessment of Chinese Shrimp Farming Systems Targeted for Export and Domestic Sales. Environ. Sci. Technol, 45, 6531–6538).

5.5.4 Addressing environmental concerns of aquaculture and overfishing

Progress is being made to address some environmental concerns:


  • Destruction of mangroves is slowing
  • Aquaculture production may be sited on unproductive agricultural land
  • Alternatives to fish meal are being sought (partly on cost grounds)

And in some contexts aquaculture can contribute positively to the environment:


  • Filter feeders (e.g. molluscs) can remove excessive nutrients from water ways
  • Aquatic systems, especially if well sited and designed, can attract birds and other wildlife

Increasing demand for aquatic products will require both sustainable aquaculture as well as marine protection and fishery regulation

 “Increasing aquaculture production can dampen the fishing pressure on wild stocks, but this effect is likely to be overwhelmed by increasing demand and technological progress, both increasing fishing pressure. The only solution to avoid collapse of the majority of stocks is institutional change to improve management effectiveness significantly above the current state.”

A recent study has modelled future pressure on wild fish stocks under different economic drivers of increased aquaculture production, increased demand for fish as a food, and technological advances in marine fishing. The study focused on four key fish: cod, salmon, tuna and seabass, both wild and farmed, under variable increases in demand. At expected rates of demand increase, it concludes that realistic increases in aquaculture alone will not protect existing wild fish populations, due to the magnitude of increase in demand.

If the increase in demand can be moderated, then the requirements on aquaculture and pressures on wild stocks will be reduced. More effective marine ecosystem protection and widespread fishery regulation are also needed to prevent stock collapse.