Is Aquaculture Good or Bad for the Environment?

Zack Nash (BCT)

Molly Korowotny (NRC)

Rachel Grabar (Pre-Vet)

A fish farmer smiles as he looks out over his 11,000 hectare (about 42 square miles) farm near the Guadalquivir River in Spain, where a feast is taking place. Except he wasn’t watching the fish feasting on pelleted feed, it was the local population of birds devouring the stock of this natural aquaculture farm. Dan Barber, an American chef, asks: “[Why are you smiling?] [Aren’t] they feasting on your fish?”  The farmer emphatically replies: “Yes!”  The farmer goes on to describe that the farm loses 20% of their fish and fish eggs to birds (Abend, 2009). However, this is an interconnected ecosystem. The booming bird population eats the shrimp and the shrimp eat the phytoplankton. In this system, the healthier the predatory birds are, the healthier the rest of the system becomes. The farmer created an ecosystem that relies on each part to be healthy, and he extracts the excess. This means the farmer does not need any extra feed for the fish, because all of their food is produced naturally within the farm by algae and the other complimentary organisms (Barber, 2010). This technique for growing fish has not only produced large amounts of fish (1,200 tons a year to be exact), but has proven to be beneficial to the Guadalquivir River and the surrounding environment (Abend, 2009). This Spanish fish farm illustrates that aquaculture can be productive and beneficial to the environment if done correctly. 

Aquaculture “…refers to the breeding, rearing, and harvesting of plants and animals in all types of water environments including ponds, rivers, lakes and the ocean” (NOAA).  This includes the production of freshwater and marine species of finfish and shellfish. In the United States, aquaculture is a $1.2 billion industry that is expected to expand as the population grows and the demand for fish increases (NOAA).  Aquaculture is a diverse system that exists in vastly different forms.  One form is pond aquaculture, which can occur in freshwater or brackish water.  Brackish water occurs where salt and freshwater meet, such as in an estuary.  Pond aquaculture can vary, with some ponds being refilled only by rainwater, and others being connected to streams, canals or wells.  Most pond aquaculture operations utilize manure or fertilizer to increase the carrying capacity of the ponds, allowing them to add more fish to a smaller sized pond than previously possible.  Fish are usually fed pelleted feed to ensure fast growth (FAO, 1987).  Brackish water ponds are very similar to other pond aquaculture operations, except they occur in tidal zones on creeks or streams, and there are systems of gates that control the movement of water.  The Food and Agriculture Organization notes that “Brackish water fish farming is a fast growing science…[and] the extent of competition with agriculture is relatively less because coastal land is not suitable for agriculture” (FAO, 1987). Additionally, aquaculture is often utilized in tandem with rice fields in Asia, with the fish being incorporated within rice fields, since rice fields are flooded for portions of the growing season.  This allows farmers to diversify their crops, as well as utilize fish excrements as a fertilizer for the growing rice (FAO, 1987). Aquaculture is not limited to the outdoors however, advancements have been made that allow fish farming to occur in a closed loop indoor system, referred to as aquaponics.

Aquaponics is an emerging branch of aquaculture that involves a recirculatory system where water is treated and recirculated continuously for reuse within the system. This process includes various types of filters, including pebbled filters, honeycomb shaped filters, and natural filters utilizing phytoplankton and zooplankton to “…[remove] faecal matter and denitrify catabolic wastes through bacterial action” (FAO, 1987).  These systems are often energy intensive, due to the need to maintain consistent temperatures, but new technology with heat exchange systems has proven to reduce energy costs while continuing to save water through recirculation (FAO, 1987). Clearly, these systems can be instituted in different ways, but this only scratches the surface of the different types of aquaculture.

Aside from pond and enclosed systems, aquaculture can also occur in cages, pens or other enclosures, in both saltwater and freshwater.  These cages or pens can be used in bays, estuaries, or in open water (FAO, 1987).  The types of cages and their location can vary significantly.  Some cages are placed on the sea floor, often for shellfish farming, in order to utilize the substrate available naturally for them to grow and mature (FAO, 1987).  Floating cages or net pens may be utilized offshore for large finfish, such as bluefin tuna in the Mediterranean Sea (FAO, 1987).  These cages and nets vary in size and structure, but many have similar benefits for fish farmers.  Utilizing pens or cages in open water allows for new water to continually enter the cages, moving away excrements and bacteria, which would normally be the responsibility of the fish farmer to filter and remove if this was an indoor system.  Additionally, they do not need water replacement like a pond would, it reduces threats of pesticide contamination from agriculture, and the initial investments for farmers are relatively small, compared to intensive indoor systems (FAO, 1987). Aquaculture in its current form can be harmful to the environment and wild fish stocks, we should reduce pollution through utilizing better feeds, collecting pollution more efficiently, and reducing overcrowding.

When aquaculture farmers feed their fish pelleted feeds, they often contain fishmeal from wild fish, in addition to additives such as corn and other grains.  This fishmeal is derived from low trophic level fish, such as anchovies, that are caught in the wild (FAO, 1987).  Trophic levels refer to an animal’s place on the food chain, with predators, such as sharks, being high trophic level animals, and lower trophic level fish being those that are eaten by the higher trophic predators (Trophic levels).  According to the FAO, “…over 70% of the world’s fish species are either fully exploited or depleted” (UN).  This decline is attributed to the growing human population, advances in fishing technology, and aquaculture needs (UN).  Therefore, it is important to note that the utilization of wild fish stocks as feed for aquaculture operations can cause catastrophic effects on fisheries that are already depleted (Naylor et al., 2007. p. 1017).  

Over time, the trend in wild fish capture has shifted from fishing for large and valuable fish species such as salmon, to smaller trophic level fish, which are used to feed salmon on fish farms.  As their wild stocks declined, they began to be grown on fish farms,  (UN). However, the author points out that “…most ocean fisheries stocks are recognized as over or fully fished” (Naylor et al., 2007. p. 1018).  This includes lower trophic level fishes that are used as feed for salmon and other fish in aquaculture operations.  While some might believe that aquaculture will help relieve the pressure on wild fish stocks, it is clear that farmers still rely on vast amounts of wild fish to produce farmed fish.

Additionally, omnivorous farmed fish, such as tilapia, are fed about 15% fish meal, which exceeds their required levels.  This over-formulation of feeds is often due to lack of dietary information about certain species (Naylor et al., 2007. p. 1019). It is important to note that there are varying estimates of the feed efficiency of aquaculture systems.  One estimate determined that in an aquaculture system, it takes 5 kilograms of wild fish in feed to produce one kilogram of a carnivorous fish, such as a salmon (Naylor et al., 2007. p. 1019).  While this feed conversion ratio is more efficient than cattle or pigs, it still relies on wild fish stocks, and much more feed than a lower trophic level fish would.  In one study, tilapia were found to have a feed conversion ratio of 1.19 kilograms of soy based feed to produce 1 kilogram of tilapia (UN).  Clearly, tilapia are more efficient at putting on weight, and their ability to grow from soy based feeds means that wild fish stocks are not needed to feed them.  As wild fish stocks continue to decline, becoming more rare, their price will go up as a function of supply and demand.  When this happens, the prices may be too high for farmers to include these wild fish in their aquaculture feed (Ellingsen & Aanondsen. 2006. p. 64).  This leads many to the conclusion that terrestrial feeds a better choice for growing farmed fish.  However, Ellingsen and Aanondsen (2006) found that “By substituting the marine-based salmon feed with grain products, the energy intensity in salmon farming is reduced, but it remains to see if this is a sound environmental strategy if other impacts like land use are introduced” (p. 65).  Other authors note there are several issues with using terrestrial feeds, due to detrimental effects on the environment, increased pollution, and limited land availability for growing crops (Pahlow et al., 2015).

After farmers switch to terrestrial feeds, scientists studied the natural environment surrounding the aquaculture farms and found that increased terrestrial feeds lead to pollution in the surrounding water (Pahlow, van Oel, Mekonnen, & Hoekstra, 2015).When looking at how pollution from aquaculture affects the surrounding environment, scientists look at the system’s water footprint, which is divided into 3 categories:  green, blue and gray. “The green water footprint refers to consumption of rainwater, the blue water footprint refers to consumption of surface and groundwater and the gray water footprint is the volume of freshwater that is required to assimilate the load of pollutants [into the natural environment]” (Pahlow, et al. 2015). A study by the Food and Agriculture Organization of the United Nations compared the natural diets of 6 different fish with artificial diets made of different percentages of terrestrial feeds.  They found that in 5 of the 6 fish studied, when the terrestrial feed percentage increased, the total water footprint (green, blue and gray water footprints together) increased accordingly.  The most shocking example of this jump in pollution from terrestrial feeds is that of the European seabass. When the fish’s diet was changed from a 5% terrestrial diet to a 52% terrestrial diet, the pollution to the surrounding environment increased by 577% (Pahlow et al. 2015). This pollution can spread downstream to contaminate other ecosystems.

There are two types of pollution produced by almost every fish farm: phosphorous and nitrogen. The main difficulty with handling this waste is the form it comes in.  It was found that 60-90% of nitrogenous waste (ammonia) is in the dissolved form, while 25-85% of phosphorous waste is excreted as a solid in the fecal matter (van Rijn, p. 50, 2015). Nitrogen and phosphorous are necessary for plant and fish growth, but an overabundance of these nutrients can cause some serious problems. Too much nitrogen and phosphorus causes excess growth of aquatic plants and algae (Mueller & Helsel, 2015). This is problematic because surface algae is unsightly, which causes any person or animal who would normally use the water for drinking or recreation to avoid the area. These algae plumes can also clog water intake pipes, and can hinder the effectiveness of the filtration systems on aquaculture farms. Too much phosphorus causes accelerated eutrophication which “…is a natural process that results from accumulation of nutrients in lakes or other bodies of water” (Mueller et al. 2015). If this process isn’t slowed down, it can cause noxious tastes and odors in fish. This process also removes much needed oxygen from the water that if left unchecked could cause mass fish kills (Mueller et al. 2015).  Clearly, proper pollution disposal is an important aspect of aquaculture farms.

Additionally, a major concern for consumers should be the bioaccumulation of mercury and other contaminants in predatory fish. Biomagnification is “…bioaccumulation of a pesticide through an ecological food chain by transfer of residues from the diet into body tissues. The tissue concentration increases at each trophic level in the food web when there is efficient uptake and slow elimination” (USGS, 2015).  Basically, this means that if you consume fish higher up on the food chain, the fish is more concentrated with mercury and other toxins, creating greater risks to the consumer. For example, the Food and Drug Administration recommends that women who are pregnant or wish to become pregnant do not eat certain types of fish: swordfish, tilefish, king mackerel, shark and tuna, and that other consumers eat them in moderation (FDA, 2014). Additionally, mercury in fish is not uncommon.  A study by the Environmental Protection Agency found that “Mercury concentrations in fish fillet samples exceeded EPA’s recommended tissue-based water quality criterion of 0.3 ppm at 49% of the sampled population of 76,559 lakes” (EPA, 2015).  Bioaccumulation can negatively affect the health of consumers, and it is important that they are well-informed when selecting the type of fish they are going to consume, whether it is from an aquaculture system, or a wild caught fish.

Aquaculture systems that are confined in small spaces keep fish close together, causing detrimental effects that decrease their well-being, growth and taste. Fish create large quantities of pollution in the form of excrements, especially when they are in such close quarters. If they are constantly swimming around in their excrement, it can be absorbed into their skin, creating a bad tasting fish, rendering it undesirable to consumers (Barber, 2010). According to Robin et al. (2006), “Off-flavors represent one of the most significant economic problems encountered in aquaculture. The appearance of repulsive odor or taste in fish may cause a major reduction in the consumption of the products, or make them unsuitable for sale.”  Additionally, Robin et al. (2006) states that the amount of uneaten pellets creates a large organic load that pollutes the water and then gets absorbed into the fish skin.  Large amounts of pollution can also hinder fish growth, undermining the efficiency of the operation. Also with fish kept so close together, diseases can spread quickly among them, as well as to the wild fish stocks around them (Gamble, 2012). “Pathogen exchange between farmed and wild fish populations is inevitable” (Murray & Peeler, 2005). Murray and Peeler also state that:

High farm stocking densities leads to rapid infection transmission and predisposes to clinical disease. Pathogens can spread in the water column between cages or ponds within a farm and between farms. Thus, aquaculture provides a suitable environment for the emergence, establishment and transmission or new pathogens. (Murray & Peeler, 2006)

Fish in open aquaculture nets can and do escape into the surrounding water, which can pose threats to wild fish.  Additionally, the concentration of fish in nets and pens will produce a lot of excrements, and can spread diseases.  These excrements and diseases also threaten wild fish stocks that may inhabit the same area (FAO, 1987). Aquaculture systems in the ocean are enclosed fish nets, which does not prevent their effluents from being spread out throughout the ocean. The excretions from the fish pile up on the bottom of the ocean and contain nutrients such as nitrogen that can increase algal blooms in the water. These algal blooms remove oxygen from the water and create dead zones (Gamble, 2012). Serious contamination problems result from using aquaculture systems in the environment that are polluting the fish stocks and everything around it.

In order to stop pollution, you must start at the source of it all: the fish feed. As stated before, fish farmers are switching to terrestrial feed ingredients to decrease costs, but these new feeds have drastically increased the water footprint of the aquaculture farms. As farmers move towards these feeds, pollution increases on a yearly basis, and when this increase is combined with the natural growth of the industry, local environments around these aquaculture farms could be devastated. If the “‘business as usual scenario’ [continues,] the increasing production will put further pressure on freshwater resources” (Pahlow et al. 2015). Breaking away from ‘business as usual’ will not be easy for many farmers, but it is not impossible. One simple solution to the problem is farming more efficient fish. Pahlow et al. (2015) studied the monetary value of 37 different fish, taking the amount of revenue each fish generates over the year and dividing it by the cubic meters of pollution it creates each year. In this situation, the higher the ratio is, the more economically efficient the fish is. The most efficient fish were lower trophic level fish, the gilthead seabream followed closely by the japanese amberjack, both of which generate about $12 for every cubic meter of pollution (Pahlow et al. 2015). If farmers are concerned about making money and saving the environment, they can switch their farms to grow fish that have a better revenue-to-pollution ratio.

An easy way to increase the economic efficiency ratio is to decrease the amount of pollution coming out of the farm. This can be done by switching back to more aquatic feeds. It has been proven that terrestrial feeds increase pollution, so if farmers can feed their fish a more natural diet, they will increase their economic efficiency ratio. If farmers do not want to front the cost of the slightly more expensive feed, they can look for other options, such as creating a polyculture system, like the one on the Guadalquivir River in Spain, which grows the feed naturally on site. Many fish feed on algae which grows naturally in water. Algae can be grown more abundantly by increasing the nitrogen and phosphorus in the system but this should be done with caution. As mentioned above, too much nitrogen and phosphorus can cause the fish to taste worse, algae to overtake the system, and possibly enable mass kills (Mueller et al. 2015).

Farming high efficiency fish gets the farmers more money, and switching to aquatic feeds reduces the pollution, but what about fully removing aquaculture based pollution from the natural environment? In order to remove pollution from aquaculture farms, farmers will have to implement filtration systems. Filtration can be accomplished through several different processes, such as utilizing UV light, ozonation, nitrification, or bacterial removal (van Rijn, Jaap, 2013). The simplest application of this process is through a recirculating aquaculture system (RAS). in a RAS, “…water is recirculated between the culture and water treatment stages” (van Rijn, 2012). As mentioned before, nitrogen and phosphorus are the two main pollutants that require removal. These two types of pollution require two different stages to remove them fully from the environment. Removal of these compounds is essential so the fish can grow healthy and so algae does not overtake the fish farm.

In indoor aquaculture systems, nitrification can significantly reduce the waste water pollution. In this process, bacteria fermentation converts most of the organic carbons into carbon dioxide, which is released as a gas from the system. “Waste discharge for nitrogen and organic solids could be reduced by 81% and 60% respectively” (van Rijn, 2012). A similar process can take place with a different filtration system that turns the waste into a sludge. The sludge is moved to an additional reactor where the denitrification takes place. In a normal RAS, phosphorus is discharged with the wastewater, but in these advanced systems, the phosphorus gets trapped in the sludge and removed from there (van Rijn, 2012). These complex system are currently the best way to increase the health of the fish while simultaneously decreasing the amount of pollution the system expels. It is clear that the best way to reduce pollution coming from aquaculture farms is to change feeds to more natural ones and to thoroughly filter the systems.

One way to offset the health and environmental impacts of eating fish is to consume lower trophic level fish.  One study found that the herbivorous fish grass carp was safer for human consumption and had lower levels of mercury than higher trophic level fish (Cheng et al. 2015).  Cheng et al. (2015) focused on aquaculture raised carp, which were fed using plants, planktons, and the waste of other fish species. This study highlighted that lower trophic level fish can be safer for consumers to eat. Additionally, eating lower trophic level fish can be better for the environment.  In the documentary Fish Meat, the filmmakers explored different types of aquaculture farms, trying to determine what type of farm was the most environmentally friendly.  Both of the filmmakers, Ted Caplow, an environmental engineer, and Andy Danylchuk, a fish ecologist, are considered experts in their respective fields.  Through their exploration of fish farms, they concluded that the most environmentally friendly was a farm in Turkey.  This farm produces carp, a low trophic level fish, that feeds off of fish wastes, and a minimal amount of pelleted feed (Caplow et al., 2012).  The waste water from the fish is used to produce vegetables for the farmer and his family, and the farm is close to its suppliers of feed, and the markets where it sells its fish.  There is minimal travel, and therefore less carbon emissions, which makes for healthier people consuming the fish, and a healthier planet.  If consumers began to choose these lower trophic level fish, creating a demand for them, there would be vast environmental benefits, and it would begin to relieve some of the pressure on wild fish stocks.

A multilevel approach to developing a solution to aquaculture systems is aquaponics, the combination of aquaculture with hydroponics. Hydroponics is the process of growing plants in sand, gravel, or liquid, with added nutrients but without soil (Veludo, Hughes & Le Blan, 2012). According to Rakocy et al. (2003), aquaponics is the combined culture of fish and plants in closed recirculating systems. Waste nutrients in the aquaculture effluent are used to produce crops, and then the effluent is treated by the plant component and returned to the fish tank (Rakocy et al. 2003). In an aquaponic system, the continual generation of nutrients from fish waste prevents any nutrient depletion, while the uptake of nutrients by the plants prevents nutrient accumulation within the tank. One of the best examples of aquaponics systems is the Colorado Aquaponics. This 3,200 square foot farm produces fruits, vegetables, and fish in the same facility. It works by “…recirculating water from a fish tank through a vegetable grow bed. Nutrients from the fish waste feed the plants, and the plants filter the water to keep the fish healthy” (Sawyer, 2015). There are many benefits of this system: it reduces water use by 90% compared to traditional farming, it eliminates the need for pesticides, and does not create wastewater runoff (Sawyer, 2015). Additionally, aquaponics systems use low amounts of energy, and produce a variety of fish, fruits and vegetables, providing an efficient means of producing a well-rounded diet.

The benefits of incorporating aquaponic systems opposed to aquaculture systems include food security, food health and safety, reduced water use, reduced carbon footprint, reduced chemical use, reduced erosion of soils, and reduced need for land (Veludo, Hughes, and Le Blan, 2012).  Aquaponics can create new sustainable methods for landscapes, urban agriculture, and cities by transforming waste into resources. These systems promote better-tasting, healthier, and bigger fish by reducing wasteful pollution and recycling effluent and excess feed throughout the contained ecosystems. A new technique called polyculture can be included within the aquaponic system by producing a variety of seafood products for consumption (Martan, 2008). For example, lobsters are grown with tilapia in a polyculture system, in areas where the plants are grown on the surface of the water; creating a layer for each species (Martan, 2008). Aquaculture systems may produce mass quantities of fish, but if the fish taste bad and production creates pollution in the environment then new methods need to be implemented before all fish stocks (wild and farmed) are lost.

Although aquaculture systems in their current form can be harmful to the environment, there are promising solutions that can improve the sustainability of aquaculture in the future. Currently the world’s oceans are being devastated by overfishing. Rising prices in fish food have forced farmers to switch to highly polluting feeds that disrupt the natural ecosystems. On top of this farmers are overstocking their facilities causing disease and mass kills. This big of a problem can’t be fixed by one easy solution. Farmers need to invest in polyculture systems which create their own food and give back to the local environment. They also need to invest in more efficient feeds and filter the pollution from them properly. As for what we, the customers can do it’s simple; buy more low trophic level fish and demand those fish to be healthy and tasty.  As our population continues to grow, it is important to address these issues facing aquaculture in order to feed the most people possible, while reducing the negative impacts to the planet.

 

References

 

Abend, Lisa. (2009). Sustainable Aquaculture: Net Profits. Time Magazine, Volume 173 (23). http://content.time.com/time/subscriber/article/0,33009,1902751,00.html

 

Barber, D.  (February 2010). How I Fell in Love With a Fish. Retrieved from https://www.ted.com/talks/dan_barber_how_i_fell_in_love_with_a_fish?language=en#t-656866 on November 11, 2015.

 

Beveridge, M. C. M., Phillips, M. J., & Macintosh, D. J. (1997). Aquaculture and the environment: The supply of and demand for environmental goods and services by Asian aquaculture and the implications for sustainability. Aquaculture Research, 28(10), doi: 797-807.

 

Caplow, T. (Producer), Curry, S. (Associate Producer), Danylchuk, A. J. (Host & Scientific Adviser). (2012).  Fish Meat. [Documentary]. United States:  Fish Navy Films.

 

Cheng, Z., Mo, W. Y., Man, Y. B., Lam, C. L., Choi, W. M., Nie, X. P.,…Wong, M. H.  Environmental mercury concentrations in cultured low-trophic-level fish using food waste-based diets. Environmental Science and Pollution Research. 22 (1). p. 495-507. doi:   10.1007/s11356-014-3333-6

 

Ellingsen, H. & Aanondsen, S. A. (2006). Environmental impacts of wild caught cod and farmed salmon: A comparison with chicken. The International Journal of Life Cycle Assessment 11 (1). p. 60-65.  doi: http://dx.doi.org/10.1065/lca2006.01.236

 

Environmental Protection Agency [EPA]. Fish tissue data collected by the EPA. (2015, October 13). Retrieved from http://www2.epa.gov/fish-tech/fish-tissue-data-collected-epa

 

Food and Agriculture Organization [FAO].  Introduction to aquaculture. (1987, April). Retrieved from http://www.fao.org/docrep/field/003/ac169e/ac169e00.htm#ch5.

 

Food and Drug Administration [FDA]. What you need to know about mercury in fish and shellfish (brochure). (2014, June 10). Retrieved from http://www.fda.gov/food/resourcesforyou/consumers/ucm110591.htm

 

Gamble, M. (2012). Talking Fish. (2012, October 16). Retrieved from http://www.talkingfish.org/did-you-know/all-about-aquaculture-environmental-risks-and-benefits

 

Martan, E. (2008). Polyculture of fishes in aquaponics and recirculating aquaculture. Aquaponics         Journal, 48, 28-33. Retreived from http://mail.aquaponics.com/media/docs/articles/Polyculture-of-Fishes-in-Aquaponics.pdf

Murray, A. G., & Peeler, E. J. (2005). A framework for understanding the potential for emerging diseases in aquaculture. Preventive veterinary medicine, 67(2), 223-235. Retrieved from http://www.aquacultureassociation.ca/slmndb/content/framework-understanding-potential-emerging-diseases-aquaculture

Mueller, David. & Helsel, Dennis. (2015). Nutrients in the Nation’s Waters–Too Much of a Good Thing? USGS, National Water-Quality Assessment Program. http://pubs.usgs.gov/circ/circ1136/circ1136.html#NIT

Naylor, R. L., Goldburg, R. J., Primavera, J. H., Kautsky, N., Beveridge, M. C. M., Clay, J., . . . Troell, M. (2000). Effect of aquaculture on world fish supplies. Nature 405, 1017-1024. doi 10.1016/j.aquaeng.2012.11.010

 

National Oceanographic and Atmospheric Administration [NOAA]. What is aquaculture? Retrieved from http://www.nmfs.noaa.gov/aquaculture/what_is_aquaculture.html

 

National Oceanographic and Atmospheric Administration [NOAA]. Basic questions about aquaculture. Retrieved from http://www.nmfs.noaa.gov/aquaculture/faqs/faq_aq_101.html

Pahlow, M., van Oel, P.R., Mekonnen, M.M. & Hoekstra, A.Y. (2015). Increasing pressure on freshwater resources due to terrestrial feed ingredients for aquaculture production. Science of The Total Environment 536, 847-857. doi: 10.1016/j.scitotenv.2015.07.124

 

Rakocy, J., Shultz, R. C., Bailey, D. S., & Thoman, E. S. (2003, February). Aquaponic production of tilapia and basil: comparing a batch and staggered cropping system. In South Pacific Soilless Culture Conference-SPSCC 648 (pp. 63-69). Retrieved from https://ag.arizona.edu/azaqua/ista/ista6/…/676.pdf

 

Robin, J., Cravedi, J. P., Hillenweck, A., Deshayes, C., & Vallod, D. (2006). Off flavor characterization and origin in French trout farming. Aquaculture, 260(1), 128-138. Retrieved from http://www.sciencedirect.com/science/article/pii/S0044848606004480

 

Sawyer, J. (2015). Colorado Aquaponics. Retrieved November 11, 2015, Retrieved from
http://www.coloradoaquaponics.com/

Trophic levels. Retrieved from http://www.botany.uwc.ac.za/sci_ed/grade10/ecology/trophics/troph.htm

United Nations. Overfishing: A threat to marine biodiversity [UN].  Retrieved from http://www.un.org/events/tenstories/06/story.asp?storyID=800

United States Geological Survey [USGS]. Biomagnification. (2015, August 4). Retrieved from http://toxics.usgs.gov/definitions/biomagnification.html

 

van Rijn, Jaap (2013). Waste treatment in recirculating aquaculture systems. Elsevier 53, 49-56, doi:10.1016/j.aquaeng.2012.11.010

 

Veludo, M., Hughes, A., & Le Blan, B. Introduction to Aquaponics: A Key to Sustainable Food Production. Retrieved from http://www.iked.org/pdf/Aquaponics.pdf