The big picture

Humans continue to explore how natural resources can meet the needs of a growing population. Aquatic ecosystems provide us with a diverse range of potential resources.

Aquatic organisms at various different trophic levels have historically been exploited. Consumption of primary produces such as seaweeds continues to be highly popular in many parts of the world.

Figure 1. Seaweed salad.

Research is on-going on how we could further harness phytoplankton, the microscopic primary producers as a:

• Direct food resource e.g. the commercial production and use of Chlorella and Spirulina species as a protein and vitamin rich food.
• Biofuel of the future (This website highlights some of the reasons why there is investment in the development of phytoplankton as a biofuel).

Phytoplankton and algae are also important in:

• Supporting trophic levels within a food web including species harvested by humans.
• Absorbing carbon dioxide and therefore playing a significant role in the regulation of our global climate.
• Producing oxygen through the process of photosynthesis.

Figure 2. Generalized aquatic food chain.

The primary producers are eaten by the primary consumers that include zooplankton, microscopic animals. They in turn are eaten by secondary consumers such as shellfish (e.g mussels and clams) and fish (e.g. herring and sardines). The next trophic level is the tertiary consumers such as predatory fish (e.g. tuna), mammals (e.g. seals and dolphins) and seabirds followed potentially by quaternary consumers such as larger fish (e.g. sharks) and mammals (e.g. polar bears).

Humans tend to harvest aquatic animals such as shellfish, fish and mammals from different trophic levels. Some species are more intensively harvested than others and this can potentially have a knock on effect on other species within the ecosystem. E.g. if a large numbers of secondary consumers are removed, it may reduce food available for tertiary consumers, reducing the growth rate and population number of these tertiary consumers. In addition excessive removal of an aquatic species can reduce the rate of population growth and replenishment. In extreme situations, harvesting of a species could lead to its extinction.

Figure 3. Hawksbill turtle eaten as a delicacy is classified by the IUCN Red List as critically endangered.

In order to augment the amount of aquatic food available from the wild (also referred to as capture fisheries), there has been a growth in the farming of aquatic organisms. This has led to increase in production of a wide variety of species from seaweeds to predatory fish. In addition to providing a food source, aquaculture provides a livelihood for many people around the world helping to alleviate poverty. More than half of global aquaculture occurs in China. The FAO estimated that in 2013, around 57 million tonnes of food (13.5 million tonnes of algae and 43.5 million tonnes of aquatic animals including fish) was produced through aquaculture in China.

Figure 4. Crab farm in China.

Aquatic ecosystems

An aquatic ecosystem is a body of water which can be either marine or freshwater.

Marine ecosystems

Marine ecosystems include oceans, estuaries, saltmarshes, mangroves and coral reefs. They cover more than 70% of the earth’s surface. Salinity of the water varies, seawater typically contains around 35g/l of salt. The water will usually be less saline in intertidal ecosystems such as estuaries, saltmarshes and mangroves due to the inflow of freshwater.

Oceanic Zones

The ocean can be divided into zones (you do not have to remember the exact names but it may help your communication marks if you do. The principle of zonation is the key idea to learn):

• Epipelagic zone (depth of 0 to 200m) into which light penetrates allowing primary produces to grow. It is the most productive zone, with little photosynthesis occurring outside this area.
• Mesopelagic zone (depth of 200m to 1,000m), where there is insufficient light penetration to allow for plant growth. This zone contains a diverse range of organisms.
• Bathypelagic zone (depth of 1,000m to 4,000m) is also known as the dark zone due to the absence of light apart from that produced by any bioluminescent organisms present.
• Abyssalpelagic zone (depth of 4,000m to 6,000m) is dark and the water temperature is just above freezing. Few organisms can withstand the high pressure in this zone.
• Hadalpelgic zone (depth of more than 6,000m), usually includes trenches and canyons. The water again is very cold and life here needs to be adapted to extremely high pressure.

Figure 1. Oceanic zones.

Marine trophic levels

Trophic levels with marine ecosystems include:

• Primary producers form the base of the food web and include phytoplankton and seaweeds. Phytoplankton are microscopic plants such as diatoms and dinoflagellates that float freely in the water and are distributed by the currents. The seaweeds are found near to the shore attached to rock or other substrate to prevent them being swept away by the currents and tides. The shallow depth of the water near the shore allows sufficient sunlight to reach the seaweed.

Figure 2. Diatoms have cell walls made of silica.

• Primary consumers include the zooplankton, small floating animals in the sea that graze on the phytoplankton. They consist of a diverse range of animals including ciliates, copepods and animal larvae.
• Secondary consumers are small predators such as some fish (e.g. sardines, menhaden and herring) and the young stages of larger varieties of fish and jellyfish.
• Tertiary consumers include top predators such as large fish (eg sharks, tuna and mackerel), marine mammals (e.g dolphins, seals and walruses) and birds (e.g penguins and albatross).

Figure 3. Example of trophic pyramid for marine ecosystem.

Levels of productivity

Primary producers support a diverse range of food webs including many fisheries which humans are dependent on. Primary production is dependent on the amount of sunlight, suitable temperature and available nutrient levels. The nutrients required include nitrates, phosphates and silicates. Over time particulates including nutrients precipitate out of the water column and during calm warm weather conditions thermal stratification could occur that prevents movement of the nutrients back into the epipelagic zone.

Thermal stratification occurs when:

• Sunlight heats the upper layer of water and surface movements create a layer with a fairly consistent temperature.
• Water movement below the surface mixed layer is reduced due to the calm weather and the sun continues to warm the water. Sunlight penetration declines with depth resulting in a temperature variation from the top to the bottom of the thermocline.

This thermal stratification prevents mixing occurring.

Figure 4. Thermal stratification.

Strong winds and coastal currents can cause mixing of the water and break up the thermocline and redistribute nutrients back into the water column. This process is referred to as upwelling and contributes to an increase in primary production. El Nino events reduce surface current and wind driven upwellings, which has a negative effect on productivity in the area.

In high latitudes during the winter vertical mixing also occurs when the surface waters gets cold and sinks. In the tropics, thermal stratification is fairly stable and production levels tend to be lower but constant.

Coastal waters and shallow seas tend to be productive, because:

• In shallow water any nutrients that precipitate out are re-suspended by wind and currents.
• River input brings in more nutrients.
• Sunlight may penetrate down to the sea floor resulting in relatively high levels of light that drives photosynthesis.

Aquatic ecosystems - freshwater

Freshwater ecosystems include lakes, rivers, streams and wetlands. Freshwater has a low salinity of usually less than 0.5 g/l of salt.

Lake zones

Lake ecosystems include the following zones (you do not have to remember the exact names but it may help your communication marks if you do. The principle of zonation is the key idea to learn):

• Littoral zone is the shallow area of the lake that goes up to the shore area. This is where large freshwater plants called macrophytes occur.
• Limnetic zone covers the open water in the lake where there is enough light for phytoplankton to photosynthesize.
• Euphotic zone includes both the littoral and limnetic zone where there is sufficient light for photosynthesis to occur.
• Profundal zone is deep water where there is no light penetration.
• Benthic zone is the lake bottom, where organisms live within the sediments or on the surface of the lake sediments.

Figure 1. Lake zones

Freshwater tropic levels

Trophic levels in freshwater ecosystem:

• Primary producers: phytoplankton and macrophytes. Phytoplankton includes freshwater varieties of diatoms, dinoflagellates, and cyanobacteria.
• Primary consumers: zooplankton (e.g. waterfleas, copepods and rotifers) and water snails.
• Secondary consumers: fish (e.g. perch, smelt, minnows), birds (e.g. ducks) and frogs.
• Tertiary consumers: large fish (e.g. trout, charr and piranhas), large birds (e.g. kingfisher) and mammals (e.g. otters and humans).

Figure 2. Typical food chain from freshwater systems.

Food products from aquatic ecosystems

A diverse range of plants and animals obtained from oceans and inland waters contribute to our food resources. In many cultures around the world algae, fish, shellfish and other aquatic organisms are an important component of the diet.

Algae

Large amounts of algae are harvested for human consumption. In 2013, aquaculture contributed to the production of over 26 million tonnes of algae. Farming of algae is popular especially in South East Asia.

Examples of edible algae include:

• Green algae e.g. sea lettuce (Ulva sp.).
• Brown algae Japanese kelp (Laminaria japonica) and Bladdewrack (Fucus vesiculosus).
• Red algae: Laver, known as Nori in Japan (Porphyra spp.) and Irish moss (Chrondus crispuscrispus).

Figure 1. Nori algae is commonly used to make sushi rolls.

Chemicals extracted from algae have many different uses including as stabilizers, emulsifiers and thickening agents. They are commonly used by the food, pharmaceutical and cosmetic industries. Algae is also used as animal feed, fertilizer and to produce fuel.

Fish not used for human consumption is used to produce:

• Pet food.
• Feed for livestock.
• Aquaculture feed.
• Fish oil.
• Gelatin
• Fertilizers

Global production of fish has more than doubled since 1960 and continues to rise. Consider the significance of the following two annual growth rates?

• Annual growth rate of global fish supply is 3.2%.
• Annual growth rates of global population of 1.6%.

Figure 3. World per capita fish consumption (kg) between 1961 and 2011.

Factors contributing to the increase in demand include:

• Growth in human population.
• Promotion of health benefits of consuming fish. Many health authorities around the world suggest eating fish as part of a healthy diet because:
  • Fish is a high source of protein and nutrients.
  • Fish contains essential fatty acids e.g. omega 3 fatty acids, which are required for brain functions and development of the nervous system (especially important for young children and pregnant women).
  • Fish is low in saturated fats, carbohydrates and cholesterol which reduce the risk of heart disease.
• Growth of a more affluent global population which can afford to import fish. Instead of availability of fish being confined to local and seasonal produce, the importation of fish has allowed for a diverse range of fish being available throughout the year in many places around the world.

The rising demand for fish has been met through expansion of fish production and more efficient distribution routes.

Shellfish and other groups of animals

Figure 4. Variety of shellfish for sale in a market.

Many other groups of aquatic animals are used as a food resource and include:

• Molluscs which are a group of invertebrates that usually have a calcium carbonate shell enclosing a soft non-segmented body. Types of edible mollusc include:
  • Gastropods e.g. sea snails and whelks.
  • Bivalves e.g. clams, oysters, scallops, mussels and cockles.
  • Cephalopods (meaning ‘head footed’) e.g. octopus, squid and cuttlefish.
• Crustaceans that have a calcified exoskeleton and include, shrimps, prawns, crabs, lobsters and krill.
• Echinoderms that are invertebrates with radial symmetry such as sea stars. Echinoderms consumed by humans include sea cucumbers and sea urchins.
• Reptiles which include turtles, crocodiles and alligators.
• Amphibians such as frogs.
• Aquatic mammals which include whales, dolphins and seals.

Hunting of seals

Hunting and use of some species such as whales and seals is controversial. The hunting of Humpback Whales is discussed in subtopic 3.3.

Seal hunting has been traditionally carried out for over 4,000 years by the Inuit communities found in Canada, Greenland, Alaska and Russia. Seals were used as a source of meat, clothing, materials for tools and shelter to help the Inuit people survive. Seal hunting is considered an integral part of Inuit culture and traditions. Today the Inuit communities harvest only about 3% of the total number of seals hunted globally each year.

The threat to seal numbers and populations comes from commercial exploitation, especially in Canada where they kill more seals than anywhere else. Large scale commercial hunting of seal is considered controversial today because:

• The large numbers of seals that are killed every year have led to concerns over some species becoming threatened.
• Methods used to kill seals are contentious and believed by some people to be inhumane. Seals have been clubbed or shot and then skinned for their pellets. There has been enormous public outcry across the world over images of seals being killed.
• Only the pellets are used, most of the meat is wasted.
• Seals have been incorrectly blamed for the collapse of the cod fishery in Newfoundland. Scientists agree this was due to overfishing by humans and not the seals feeding on the cod.
• Melting of ice as a result of global warming is reducing the habitat of seals and threatening populations.

Figure 5. Protest in Paris against seal hunting in Canada.

Canada has attempted to regulate seal hunting by:

• Use of quotas i.e. set limits on how many seals, can be hunted.
• Adoption of open and close season for hunting.
• Limit to number of catches per day.
• Limit to number of boats allowed to hunt.
• Banning hunting of new born harp seals and young hooded seals.

There is much debate about how effectively these regulations are enforced and continued protest against hunting. The European Union have imposed a ban on all seal products from Canada, with exception of products from the Canadian Inuit communities.

Research demonstrated that in 2008 the income generated from seal watching of about US$2 million was almost four times the amount of revenue made from sea hunting. Hence, the economic value of seals through tourism is significantly higher than through the trading of seal pellets.

Figure 6. Seal watching can generate significant more income than the products from seal hunting for local communities.

Energy efficiency of aquatic food systems

Aquatic food systems are often considered to be less efficient than terrestrial food systems:

• Primary producers in aquatic systems receive less light than terrestrial plants because some of the incoming light is absorbed or reflected by the water.
• Compared to terrestrial foods, humans generally tend to eat organisms from higher up in the aquatic food chain. Not all the energy is transferred from one trophic level to the next, hence the longer the food chain and the more transfers, the greater the energy loss.

However, in aquatic systems less of the biomass may be lost as indigestible skeletal material (e.g. jellyfish have no skeleton) resulting in more efficient energy transfer.

Capture fisheries

Capture fisheries globally account for about 90 million tonnes of fish annually, the majority of which comes from marine ecosystems.

Figure 1. FAO estimated in 2012 capture fisheries accounted for 91.3 million tonnes of production comprising of 79.7 million tonnes from marine ecosystems and 11.6 million tonnes from inland freshwater ecosystems.

Most major fisheries are coastal focusing on demersal (bottom dwelling) species e.g. cod, haddock, hake, sole; or pelagic (open water) species e.g. sardines, anchovy, tuna, mackerel. Fish populations tend to be greatest where there is high primary production supporting the food chain e.g. in shallow waters and where there are upwellings which increase the level of nutrients available for growth.

Figure 2. World capture fisheries production from freshwater and marine ecosystems.

Growth in marine capture fisheries

Figure 2 illustrates that capture fisheries in marine waters increased until around 1990, thereafter growth in capture fisheries has been predominately limited to freshwater ecosystems. So, what caused the growth in fish production and then why did the marine based capture fisheries plateau out?

The growth in fisheries has been driven by demand due to rising human population and the popularity of fish.

In order to meet demand the intensity of fishing effort increased. Many countries moved from small scale local fishing to large scale commercial fishing involving:

• Growth in number and size of fishing fleets.
• Improvements in shipping vessels which allowed fishing to occur further from the shore and in deeper waters.
• Larger ships allowing for longer periods at sea resulting in greater harvest of fish.

Technological developments have also increased the efficiency of harvesting fish. These include:

• Development of sonar, radar and satellite technology to detect and track schools of fish.
• The ability to process, preserve and freeze aquatic produce on the ship whilst still out at sea.
• Changes to fishing gear, allowing for larger catches.

Use of nets

In addition to catching the intended species of fish, other organisms are often also accidentally caught in the fishing nets. These non-targeted organisms are referred to as bycatch. Animals can also become entangled with nets discarded or lost at sea, commonly referred to as ghost nets. Types of nets used include:

Trawler nets used to catch demersal fish by dragging a funnel shaped net along the seabed. As the net moves along it also damages the seabed and can kill organisms that live there such as corals and sponges.

Figure 3. Use of trawler nets.

Purse-seine nets used to catch schools of pelagic species. The fish are surrounded by the net, which is then closed like a draw string purse to trap the fish. Other species commonly reside within schools of fish and hence when purse-seine nets are used they become bycatch. This is particularly problematic when the bycatch are species that are under threat of extinction.

Figure 4. Use of purse seine fishing nets.

Drift nets hung vertically in the water are used to catch pelagic schools of fish such as sardines, swordfish and tuna. They have also been referred to as "curtains of death" and bycatch include a wide range of species from sea turtles to sharks and dolphins. In 1992 the United Nations banned the use of nets more than 2.5km long within non-territorial waters, prior to which drift nets as long as 64 km had been used. In 2002 the European Union banned any use of drift nets.

Figure 5. Use of drift nets.

Why did marine capture fisheries not continue to expand?

After many decades of growth in marine capture fisheries, it now continues to fluctuate at around 80 million tonnes of production each year. Whilst in some parts of the world marine capture fisheries have increased, in other areas they have declined. This reduction in some regions has been a consequence of overfishing and habitat degradation. The FAO estimated that in 2011 over a quarter of all marine fish stocks assessed were overfished, meaning that catch sizes were biologically unsustainable.

Fish yield and maximum sustainable yield

Fish stocks are only renewable if the rate of their removal does not exceed their growth rate (i.e. their increase in natural capital). However, with low fishing effort, fish yields are relatively low and the population or area can be considered as being under-fished. The maximum sustainable yield (MSY) is the optimum harvest that can be obtained annually without affecting the standing stock and its ability to replenish itself. With a very high fishing effort, species are harvested at a greater rate than they can reproduce and grow, hence exceed sustainable levels. The overall population will decline and fishing becomes uneconomical. Intensive fishing efforts to meet high demand of fish can lead to overfishing. As stocks dwindle, juvenile fish are often caught which further reduces the ability of the fish stock to replenish.

Figure 6. Catch-effort curve.

The use of MSY can still lead to over-exploitation of the fish. This is because the calculated MSY can be overestimated as it may be based on inaccurate or incomplete datasets and due to gaps in our understanding of fish ecology.

Why is overfishing common?

Although we all understand that overfishing can result in long term decline in fisheries and potential extinction of some species, given the choice we often continue to fish beyond sustainable levels. What causes this behaviour? Two contributing factors are issues of property rights and the related theory of zero sum game:

• Property rights: no one owns the fish. Fish swim through large areas and do not respect national boundaries. People often do not want to spend money on conserving the fish, if other competing countries will harvest them.
• Zero sum game: this is the idea that in many situations someone gains at the expense of others. In order to conserve fish stocks you need to convince people to sacrifice short term gain to benefit in the future. For this to be a favourable option, the long term gain needs to be sufficiently lucrative and low risk. If you take action to conserve fish stocks, can you ensure your competitors will do the same and not harvest the fish? If not, then the group that choose conservation lose out completely.

As a result of these difficulties, over-fishing can be considered as a perfectly sensible choice by rational people but that will lead to collapse of fish stocks and potential species extinction. Hence to deal effectively with the problem, action at an international as well as national level is necessary.

Theory of Knowledge

Over fishing is a common problem. How can a society whose decisions are knowledge based cause over exploitation and destruction of the resource it dependents on.

Managing fish stocks

To try and prevent overfishing quotas are often used and where overfishing occurs the focus is often to reduce fishing effort and hence the amount of fish harvested. In addition approaches which enable the fish stock to grow are increasingly employed.

Use of quotas

Fish biologists estimate the maximum sustainable yield based on current stock levels and rates of replenishment. This information can be used by politicians to agree on catch limits called the Total Allowable Catches (TAC). The TACs are shared out and used to set individuals limits referred to as quotas.

A key criticism of the use of a quota system is the problems of bycatch. TAC are based on individual species but often non-target species are also accidentally caught. In many cases, the bycatch is discarded in the seas as waste due to penalties of landing non-allocated species or fish that are below the allowable size limits imposed.

Reduction in fishing effort

This is achieved through:

• Reducing the number of boats fishing.
• Restricting boat size.
• Restricting type of fishing gear used, including limits on size of nets and mesh size. Large mesh sizes are used to reduce catch of juvenile fish.
• Setting limits on the minimum size of fish that are allowed to be caught. This prevents fishing of small and young fish.
• Restricting fishing times e.g. having a closed season when fishing is not allowed.

Use of exclusion zones including Marine Protective Areas (MPAs)

Fishing is banned in exclusion zones and in MPAs. The latter are formal conservation areas which may represent areas of high biodiversity or provide habitats to threatened species. Exclusion zones can often act as breeding and nursery grounds for fish and can overtime contribute to recovery of fish stocks.

Often these different measures to manage fish stocks are enforced through legislation at international and national level. Many aspects are difficult to enforce, especially out at sea. Why do you think these actions are often unpopular with fishermen?

Management of cod fisheries: two contrasting cases

Figure 1. Cod.

Newfoundland

Cod fisheries in Newfoundland, Canada is covered more extensively within the sub-topic Tragedy of the Commons. In summary, Newfoundland, had the largest cod stocks in the world. However in the 1950s, with the adoption of modern technology, the level of fishing effort increased considerably. This involved use of:

• Large shipping fleets with more efficient engines that allowed boats to stay out longer and cover more fishing grounds.
• Factory fishing boats with the capability of processing and freezing fish on board.
• Huge trawl nets that that covered a larger area but also damaged the seabed.
• More efficient detection methods to find fish.

Catches peaked in 1968 with an estimated 800,000 tonnes of cod. Despite warnings from scientists that the level of fishing should be reduced, the government decided not to cut quotas in fear of losing jobs and overfishing continued. By 1992 the number of mature fish able to reproduce had fallen considerably and the government decided to take radical action. They closed the entire fishery resulting in the loss of over 42,000 jobs. Fish stocks were expected to recover within five to ten years but by 2003 there was still little sign of improvements. Only in 2011 were there indications that fish stocks were beginning to recover although numbers were still low.

Iceland

Figure 2. Fishing boats in the Port of Reykjavik in Iceland.

Cod is also an important fishery in Iceland. Following a decline in cod fish stocks, the Iceland government took action to enable them to continue fishing but at a rate that did not lead to collapse of stocks as had occurred in Newfoundland. This included:

• Protecting territorial waters from fishermen from other countries (action also undertaken by Newfoundland to protect fish stocks).
• Restrictions on fishing gear and fleet sizes.
• Strict quotas that can be traded between fishermen.
• Banning the disposal of any bycatch including undersize cod.
• Use of exclusion zones (where fishing is banned) e.g.:
  • Permanent closure of nursery areas.
  • Seasonal closure of some areas during spawning times.
  • Temporary closure of fishing areas if fish caught are too small in order to conserve juvenile fish.
• High level of enforcement by inspectors that police and monitor the seas.

Today fisheries continue to contribute significantly to the economy of Iceland. Cod is exported to markets across the world including to the USA, Russia, Spain, France and the United Kingdom.

International-mindedness

Fish often travel large distances crossing many national boundaries, hence effective management of many fisheries requires international collaboration.

Aquaculture

“Aquaculture is the farming of aquatic organisms: fish, molluscs, crustaceans, aquatic plants, crocodiles, alligators, turtles, and amphibians. Farming implies some form of intervention in the rearing process to enhance production, such as regular stocking, feeding, protection from predators, etc…”. (FAO definition 2015)

Figure 1. Crocodile farming.

Whilst catches from wild capture fisheries are expected to level off, aquaculture continues to grow. The FAO predicts the amount of fish consumed by humans from aquaculture will rise from about 50% in 2012 to 62% by 2030. About 60% of the production is from freshwater systems compared to 35% from the sea and 5% from brackish waters.

Figure 2. Contribution of aquatic ecosystems to total aquaculture.

In 2012 aquaculture contributed 66.6 million tonnes of fish and shellfish and 23.8 million tonnes of aquatic algae produced globally. Aquaculture not only provides a vital protein source in many regions but provides a livelihood for millions of people. The FAO estimate that in 2012, over 18.8 million people were working within the aquaculture sector.

Open vs semi-closed aquaculture systems

Aquaculture systems are usually either open systems or semi-closed systems.

Open based systems are the most popular and involve farming the organisms within a natural aquatic ecosystem such as the sea or a lake. This includes fish cages, clam beds and oyster rafts that are submerged in the water. Often juvenile fish are transferred from hatcheries (where fish eggs are incubated and hatched) into the fish cages to develop and grow. The fish farmer has little control over the environmental factors such as temperature which may affect growth rates of a species. Other potential issues include predation and poachers.

Figure 3. Open based system of marine aquaculture in Norway.

Semi-closed systems involve the abstraction and use of water from the sea or lakes within tanks or ponds situated on land. This allows for greater control over environmental conditions such as temperature and water velocity. If required the water can be filtered to remove any predators or pathogens. Semi-closed systems tend to be more expensive than open-closed systems.

Figure 4. Semi-close system of industrial aquaculture.

Tilapia is a popular fish for aquaculture that can be bred in either open or semi-closed systems. The following video ‘Raising Tilapia Fish - Why Farm Tilapia’, gives some of the reasons why Tilapia is chosen for aquaculture by FishFarmingBusiness's channel.

Figure 5. Farming of Tilapia on the Mekong delta, Vietnam.

Environmental impacts of aquaculture

Loss of habitats can occur to make way for aquaculture. For semi-closed aquaculture systems land is frequently cleared. Mangrove forests have been cleared in many areas of Asia and South America to make space for ponds used for shrimp farming. This not only results in loss of habitat but can also reduce the natural storm protection provided by the mangrove ecosystems.

Aquaculture can also result in loss of aquatic habitats. Favorable sites for fish cages are often in sheltered areas where there is less likelihood of storm damage. These same areas also provide good habitats for other organism that may be lost as a consequence of fish farming.

Figure 6. Mangroves are important nursery grounds for fish and other organisms.

In an open system, the environmental impacts are influenced by the volume of water and the retention time of the water. The latter is reduced by tidal movement and sea currents that contribute to water circulation. In a small enclosed area, with little water movement the impacts are greater than in a larger open area with high water circulation that help remove and breakdown any potential pollutants.

Further environmental impacts of fish farming are covered below and range from increase in organic sediments to other organisms becoming entangled within the fishing cages.

Increase in organic sediments

The intense rearing of fish generates waste which comprises of uneaten fish food, fish faeces and medicines. Some of the waste precipitates to below the fish cages where it may accumulate if water movements are not strong enough to disperse them. This sediment is predominately organic and can smoother organisms that live on the sea or lake bottom reducing overall biodiversity. Over time the organic material is broken down initially by aerobic bacteria to form carbon dioxide, water, nitrates, phosphates and sulphates. However, if there is little water circulation and oxygen is used up, anoxic conditions will prevail. Without oxygen, anaerobic breakdown occurs resulting in the formation of methane, ammonia and hydrogen sulphide which are potentially toxic gases, killing organisms in the vicinity. In such, cases the fish within the cages are also under threat. To avoid this situation the siting of fish cages needs to consider the movement and sufficient aeration of the water.

Increase in available nutrients

Soluble nutrients released from the uneaten fish food and fish faeces increase the overall dissolved levels of nutrients in the water. These nutrients can increase primary productivity resulting in an increase in phytoplankton and algae. This could lead to algal blooms that could physically damage fish gills and stop them from functioning. In addition some algal blooms are toxic and could cause fish kills. Where there is high water circulation the nutrients and phytoplankton may be dispersed at a rate that reduces the risk of algal blooms.

Figure 7. Algal bloom.

Use of medicines and hormones

Medicines such as antibiotics and hormones used to treat the farmed fish can contaminate the water and affect other aquatic life.

Use of antifouling agents

Antifouling agents are also used to prevent growth of algae and other organisms on the cage. Historically the antifouling paint, tributyltin (TBT) was used but is now banned. TBT is toxic to larvae of molluscs and also causes female molluscs such as dog whelks to develop male sexual organs. In some regions the use of TBT led to a collapse of mollusc populations.mollusc populations.

Spread of disease

Within intensely stocked cages, disease can easily spread from one fish to another and even potentially to other fish outside the cages.

Escaped fish

If fish escape from the cages they may threatened wild stocks by competing for habitat and food or transmitting diseases. Escaped fish may also change the ecological dynamics in the area e.g. by feeding of other fish or breeding with them. These problems may be exacerbated if the farmed fish are genetically modified organisms with particular characteristics that provide them with an advantage for survival over wild fish.

Attracted predators

Predators attracted by the farmed fish can become entangled and caught within the nets of the fish cages. In some cases, underwater acoustics are used to deter the predators away from the fish cages.

Figure 8. Environmental impacts of open-ocean aquaculture.

Managing environmental impacts

Action to reduce some of the environmental impacts of fish farming includes:

• Reducing the waste from uneaten feed by careful selection of appropriate feed and not overfeeding (e.g. timing feeding sessions with care).
• More effective application of any medicines to reduce losses to the environment.
• Regular removal of any dead fish from the cages.
• Moving the cages at regular intervals to prevent build-up of organic sediments and give the area time to recover.
• Locate fish farms where there is sufficient movement and exchange of water to:
  • Reduce nutrient levels in the water.
  • Reduce phytoplankton levels in the water and disperse any blooms.
  • Reduce build-up of waste by dispersing it.
• Aerate the water to prevent anoxic conditions.

Figure 9. Use of mechanical paddles to aerate water at a shrimp farm in Thailand.

• Removing the deposited waste from below the fish cages.
• Using predator resistant netting material for cages. The following video 'Fish Farming - Creating Shark Resistant Nets' by DSM provides an example:

Shrimp aquaculture in Thailand

In Thailand shrimp aquaculture grew dramatically during the 1980s. In 1985 annual production was estimated to be about 10,000 tonnes and by 2009 had increased to around 539,000 tonnes. Shrimp farming contributes to food security and the economy, with over a million people employed in the industry. Thailand is a key exporter of shrimps to countries such as the USA and Japan.

During the 1980s many land based farmers switched from production of rice to shrimps. Coastal fields and adjacent mangrove forests were changed to accommodate shrimp ponds. This flourishing business earned many farmers significantly more income than from rice farming.

Figure 1. Collection of shrimp at a shrimp farm.

Environmental impacts included the loss of mangrove ecosystems which served to provide:

• Breeding areas for many species including fish.
• Habitats for many species.
• Protection from coastal erosion, flooding and storm damage.

Approximately two thirds of the mangrove forests in Thailand have been destroyed as a result of shrimp farming activity.

High density farming of shrimps can result in rapid transmission of diseases such as early mortality syndrome (EMS) or Yellowhead disease. The shrimp ponds can accumulate waste products from uneaten food and faeces. The subsequent biodegradation of this organic waste can lead to anoxic waters. Artificial aeration can be used to reduce the risk of anoxic conditions occurring or to restore oxygen levels. The high level of nutrients in the water can result in toxic algal blooms or the growth of other harmful bacteria and viruses. Without adequate management, the life span of a shrimp pond is only two to four years. There are many deserted shrimp farms across Thailand.

During the 1990s there was significant international pressure for Thailand to reduce environmental degradation and produce shrimps more sustainably. This in turn led to new national legislation and more stringent controls over management of wastewater and the use of mangrove areas.

The following video ‘Thailand shrimp industry’ by Soraphat Panakorm shows how shrimps are farmed on a commercial scale in Thailand together with some of the issues and how they are managed: