Aquaculture is the farming of marine and fresh water fish, shellfish and aquatic plants which can be done in a marine or freshwater environment. It is an important sector as it reduces the pressure on the wildfish population due to overfishing (Naylor et al. 2000). Currently aquaculture supplies one-third of the world seafood supplies and is expected to increase due to the high demand of fish as a main protein source (Naylor et al. 2001). In Southeast Asia, the commonly used species for aquaculture are tilapia, shrimps (Penaeus monodon), crabs (Scylla serrata) and catfish to name a few (SEAFDEC 2009). Even though aquaculture is expected to provide a net increase of the global fish production, it has several negative impacts towards the environment. In this paper, major environmental impacts will be discussed such as discharge of effluent and chemicals from farms, introduction of exotic species, spread of disease, habitat loss and dependence of fish-based fish feed. Together with these, management of the impacts which are currently used will be discussed such as the integrated coastal zone management, management of farm effluent, disease control and the use of native species and low trophic level fish.
Environmental Impacts
Water pollution around the aquaculture farms is a common problem mainly due to the accumulation of fish feces and uneaten fish food. The untreated waste is released into open waters causing nutrient pollution. High concentration of nitrogen waste consists of ammonia or nitrite is toxic to marine animals (Hargreaves 1998). The increase in nutrients can also change the biochemistry of the benthic communities, sediment chemistry and cause eutrophication at a local scale (Naylor et al. 2003; Goldburg et al. 2004).
Chemical products and antibiotics are also commonly found in the waters surrounding the shrimp farms in Southeast Asia to control the water quality and the spread of diseases. However antibiotic from the feces or uneaten fish feed and excess chemical products will accumulate and affect non target species (Naylor & Burke 2005). The resistance of pathogens in the wild fish population will increase. If not, it will decrease microbial activity and denitrification leading to formation of anaerobic conditions and a decrease in water quality (Gräslund & Bengtsson 2001). It is also possible that the excess use of antibiotic can transfer resistant DNA/RNA to human pathogens (Tookwinas et al. 1999). Besides that, the use of excess chemical can affect the health of the person handling it.
Farming of non-native species can lead to the introduction of exotic species if any fish were to escape into the wild population. According to Naylor et al. (2001), the release of exotic species into the wild population are mostly accidental. This can affect the wild population by creating intense competition for food and habitat and the interbreeding of these different species. If the exotic species survive well in the new environment and outcompete the native species, there will be an explosion of these species. This might also increase the chance of the spread of parasites, pathogens and diseases which the native species never had before (Naylor et al. 2001).
The risk of the spread of disease is especially high when the farms have dense population leading to disease epidemics. One of the reasons could be caused by bacteria, virus or parasite from the large volume of waste discharged from the farms (Minchin 2007). The disease can then be transmitted to the wild fish population. This can occur when the wild population comes in contact with the farmed fish or infected escapee goes into the wild or wild population moving through the area of the infected farmed fish area (Naylor et al. 2005).
Another negative impact caused by aquaculture is the destruction of natural habitats such as mangrove forest and coastal wetlands which are turned into milkfish and shrimp ponds. These would indirectly affect the wildfish population because the mangroves act as a nursery habitat for juvenile fish as it provides food and shelter before they venture out into the open seas (Robertson & Duke 1987). In Southeast Asia, species which are dependent on the mangroves makeup one-third of the wildfish caught by fisherman. According to Martosubroto & Naamin (1977), they observed in Southeast Asia that there is a positive relationship between wildfish and shrimps landings with the area of mangrove present. Besides that, the loss of mangroves also affects humans because it protects our coastal area, control floods, traps sediment and treats water before it is released into the sea (Naylor et al. 2000).
The production of fishfeed for farmed fish is also giving negative impacts to the environment. This is especially true for farmed carnivorous fish where processed fishmeal and fish oil are fed to them. A large amount of wildfish is used to feed a small amount of farmed fish. So instead of aquaculture increases the net production of world fish supplies, it decreases the net production, increasing the pressure of fisheries resources (Tacon 1997). Another problem is that manufacturers of the fishfeed have insufficient data of certain species especially on omnivorous species, leading to production of over formulated feed. If sufficient data is gathered, omnivorous catfish can have fish feed which also included plant based protein because they have the ability to absorb them better(Naylor et al. 2000).
Managing Aquaculture Farms
Integrated farming system or polyculture is a good way to promote the sustainability of aquaculture. It is the farming of different species in close proximity and each of the species will have a responsibility, keeping the ‘ecosystem’ going (Jones & Iwama 1991). In China the most used combination is the silver carp which feeds on phytoplankton, herbivorous macrophyte feeder grass carp, omnivorousdetritus bottom feeder common carp and the big head carp which is a zooplankton filter feeder (Tacon & De Silva 1997). According to Neori et al. (2004) and Jones & Iwama (1991), mussels and seaweed thrive in waste water from the farms so it can be used to reduce the nutrient concentration of the waste water before expelling them. With polyculture, food and water resources will be used efficiently resulting in a reduced cost and increased productivity.
Farm effluent can be reduced by modifying the feed, incorporating higher percentage of plant-based proteins. In France, according to Papatryphon et al. (2004), the use plant-based feed on Rainbow trouts (Oncorhynchus mykiss) shows a significantly lower nutrient discharge compared to fishmeal-feed. According to Baruah et al. (2004), “addition of microbial phytase in plant-based aquafeed can improve the bioavailability of phosphorus in fish resulting in a reduced in phosphorus waste from farms.” Then, to reduce the accumulation of waste around the farms especially in marine aquaculture, these areas should be well flushed to avoid water quality problems. If this is not done, dead zones will form and may affect the bentic community (Beveridge 1996). By incorporating more plant-based protein into the fishmeal, less pressure is placed on the wildstock fisheries.
As for disease control, by reducing the waste which carries pathogenic bacteria, fewer diseases would occur. Dense population of farmed fish should be avoided together with the overfeeding of antibiotics. The risk of diseases can be reduced by having a “biosecurity programme with monitoring of determined high risk entry point with pre-border and post border management” (Hewitt et al. 2004). According to Tookwinas et al. (1999) diseases in shrimp farms can be prevented by providing good nutrition and overall stress reduction which we should be focusing on rather than disease treatment.
Lastly, it should be encouraged to farm native species so that there would be a lower risk in spreading new diseases to the wildfish together with other complications which come along with the introduction of exotic species. Any accidental escapes would not be much of a problem. It is also encouraged to farm low trophic fish where there is less demand for fishmeal based feed and more for plant-based feed (Naylor & Burke 2005).
As for the use of mangrove forest as shrimp farms, Gräslund & Bengtsson (2001) has shown that mangrove soil are acidic and therefore not suitable for shrimp farming. However compared to other environmental impacts caused by aquaculture, habitat destruction consists of a small percentage only.
The conclusion of this essay is that even though aquaculture is meant to improve the production of fish, if managed improperly, it will have the opposite effect. The main issues of it are water pollution caused by the waste and excessive chemicals and medicines, impacts of introduced species and large usage of fishmeal-based food. But as what have been stated in this essay, there are methods which are currently used to properly manage these farms to reduce its environmentally negative impacts and achieve its primary objective.
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