Environmental Impacts of Shrimp Aquaculture and Integrated Multi-trophic Aquaculture (IMTA) as a Solution

Kaitlyn McGarvey – Pre-Veterinary Science

Sean O’Neil – Environmental Science

Spencer Scannell – Natural Resources Conservation

In 1987, Champerico, Guatemala suffered a widespread outbreak of a severe neurological disease called paralytic shellfish poisoning (PSP). What began as six people at health clinics complaining of headaches, dizziness, and weakness, quickly grew into a much larger problem. Within hours, over 100 people sought medical attention for a wide range of symptoms. One child’s symptoms quickly progressed to respiratory paralysis, ultimately causing death. A total of 187 people received medical treatment and of those, 26 died (Rodrigue et al., 1990, p. 267).  Further investigation identified the consumption of clams or clam soup as the common link between the affected individuals (Rodrigue et al., 1990).

Shrimp are the most popular seafood consumed in the United States (US) with an annual per capita consumption of 4 pounds (WWF, 2017; NFI, 2015). Between 1970 and 2000, shrimp farming rapidly increased around the world in response to high demand from the US and other developed countries (Biao & Kaijin, 2007). Despite its popularity in the US, a large of majority of shrimp consumed there comes from farms in Southeast Asia and South America. Over 90% of America’s imported shrimp comes from Ecuador, Indonesia, China, Mexico, Vietnam, Malaysia and India alone (Richardson, 2010, para 11). A large majority of the shrimp that the US imports every year comes from shrimp farms rather than wild stocks. Over 55% of the shrimp produced globally comes from aquaculture sources (WWF, 2017).

In order to keep up with the demand, many shrimp farmers adopted intensive farming practices (Biao & Kaijin, 2007). Intensive shrimp farms are essentially a grid pattern of individual shrimp ponds. The size of each pond varies depending on whether it is a nursery or a grow-out pond. Nursery ponds are smaller and house young shrimp larvae. Once the shrimp reach a certain size, they are transferred to grow-out ponds, which are larger to accommodate the size of the shrimp. However, regardless of the size, each pond is connected on one side to a supply canal and on the other side to a drain canal. The supply canal carries water from a nearby water source, often the ocean or a large river, into the farm. Sluice gates, a type of sliding gate, control the amount and rate at which water enters and exits the ponds. Once the water passes through the pond, it exits through the gate into the drain canal and eventually returns to the original water source. Strategic construction of the ponds with regard to the prevailing wind direction facilitates water movement and thus, aeration, or the mixing of the air and water, of the ponds (FAO.org, 1986).

An intensive shrimp farm.

Shrimp farmers provide large amounts of feed to meet the nutritional requirements and maximize growth of intensively farmed shrimp. Often, the feed is in a pelleted form. Wheat flour, soybean meal, and fishmeal are the three major components of a typical shrimp feed and provide the energy, amino acids, and protein needed in the diet (FAO.org, 2017). Because shrimp nibble rather than eat the whole pellet at once, up to 40% of the added feed settles to the bottom of the ponds and remains uneaten (Boyd, 2004). The accumulation of uneaten feed in shrimp ponds negatively impacts the environment, as feeds are high in nitrogen and phosphorus.

The dissolution of uneaten feed significantly increases nutrient levels in shrimp ponds. Many factors, including pH, temperature, and osmotic pressure among others influence the rate of feed pellet breakdown (Biao & Kaijin, 2007). Not only does the breakdown of feed pellets increase the concentration of suspended solids in the ponds, but also releases the nitrogen (N) and phosphorus (P) from the pellet as it breaks down. Boyd and Queiroz (2001) estimate that shrimp do not absorb 77% of the N and 89% of the P in the feed pellets and as a result, the system gains a significant amount of these two nutrients (p. 84).

High concentrations of dissolved nutrients, especially nitrogen and phosphorus, lead to a type of pollution known as eutrophication (Anh, Kroeze, Bush, & Mol, 2010). These nutrients are important for photosynthesis, a process that occurs in aquatic plants, just like it does in terrestrial plants (Yang, Wu, Hao, & He, 2008). Photosynthesis is the mechanism for plant growth; the ecosystem relies on photosynthetic plants to release oxygen which supports aquatic life. In a healthy ecosystem, the limited availability of nutrients controls the growth of aquatic plants (Bornette & Puijalon, 2011). However, when excess nutrients enter the environment from artificial sources such as shrimp farming, the ecosystem suffers from disproportionate phytoplankton and algal growth (Ryther & Dunstan, 1971). Uncontrolled phytoplankton growth often results in algal blooms, which can negatively affect an ecosystem.

One of the most severe consequences of algal blooms is hypoxia, or the depletion of dissolved oxygen in the water. Just like terrestrial life, sealife depends on oxygen, therefore dissolved oxygen (DO) depletion is detrimental to aquatic organisms (WRI, 2016). The high density of phytoplankton and suspended dissolved feed particles in the water column cloud the water. As a result, less light reaches the deeper parts of the water. Algae grows above and around the plants on the bottom as they compete for light. This causes plants, the major producers of oxygen, to die from lack of light. The absence of these plants means that the amount of oxygen released into the water is drastically reduced.

To make matters worse,  as the plants and phytoplankton die, they are decomposed by bacteria. The decomposition process consumes oxygen and further decreases the DO content of the water (Verdonschot & Verdonschot, 2014). Eventually, the bacteria consume most of the oxygen present in the environment, at which point the ecosystem has become hypoxic. Fish in hypoxic environments experience severe egg malformation, reduced size, and poor respiratory function (Hughes, 1973). Shellfish and shrimp suffer from increased mortality, reduced size, and lethargy (Burnett & Stickle, 2001). If hypoxia reaches high enough levels, aquatic environments can no longer sustain life, becoming a dead zone (NOAA, 2014).

In addition, certain species of algae produce toxic chemicals that can harm other organisms in events called harmful algal blooms (HABs). Under normal circumstances, their concentrations are too small to be poisonous. However, eutrophication allows toxic phytoplankton populations to increase to dangerous levels. In high enough concentrations HABs kill fish, shrimp, shellfish, and most other aquatic animals. Consuming food containing toxic algae can lead to severe illness and even death (NOAA, 2013). Open water aquaculture facilities are vulnerable to HABs because they take in water from the local environment. Red tide can cause huge livestock kills if it enter facilities (Khan, Arakawa, & Onoue, 1997).

Intensive mono-culture, or farming one species alone, can create heavily concentrated wastes that have a huge impact on the environment if released untreated. Biao and Kaijin (2007) state that 43 billion tons of wastewater from shrimp farming is released into Chinese coastal waters every year (p. 543). It is imperative that shrimp farms adapt to methods that reduce the amount of  excess feed and nutrients in discharged waste water, so that it does not imbalance the ecosystem and lead to eutrophication. Shrimp farmers combat the potential negative effects of increased nutrients and hypoxia by implementing aeration systems. These systems help circulate the water in order to increase dissolved oxygen levels. However, the adjacent ecosystem cannot effectively absorb all of the nutrients. As wastewater is discharged, the naturally occurring plants that were growing in the shrimp ponds quickly become over productive.

Because shrimp aquaculture poses great risks to the environment and is predominantly a luxury food for the developed world, some people may think that the best solution is to get rid shrimp farms. However, doing so would have crippling effects on third world economies. Many people rely on aquaculture as a source of income; the shrimp industry employs over one million people in Bangladesh alone (Paul & Vogl, 2013, p. 1). The farmed shrimp industry was worth nearly eleven billion dollars in 2005 (WWF, 2017). 93% of Ecuador’s shrimp exports are devoted to supplying US demand and 90% of that shrimp is farmed (Richardson, 2010, para. 11). Consequently, the solution is not as simple as eliminating shrimp aquaculture altogether. Rather, we must make it more green and sustainable.

To mitigate the negative effects of shrimp aquaculture, the FAO should invest in shrimp farms that implement the integrated multi-trophic aquaculture (IMTA) system. With added biodiversity from different trophic levels, the filtration capacity of IMTA systems are drastically higher than in monoculture systems. IMTA incorporates both bivalves and seaweeds to maximize the recycling and conversion of both organic and inorganic compounds and limits the environmental damage that untreated shrimp effluents cause. It will also increase the shrimp farmers profitable yields by producing more than one harvestable crop (Chopin, Cooper, Reid, Cross, & Moore, 2012).

Bivalves are a classification of mollusks, including clams and oysters, and are filter feeders (US Dept. of Commerce, 2013). These organisms remove suspended particulate organic matter, including uneaten feed, phytoplankton, and bacteria, from the water and use them to build their own body mass (Neori et al., 2004). Bivalves are particularly good at removing fine suspended organic matter from the water column, including the dissolved feed pellets of farmed shrimp, decreasing turbidity and increasing water clarity (Jones, Dennison, & Preston, 2007; Ferreira & Bricker, 2016). This allows more light to reach the lower water column, promoting the production of oxygen by plants and phytoplankton through photosynthesis. As a result, ecosystem can better support life (Ferreira & Bricker, 2016).  The introduction of oysters to a flow-through system, with open inputs from a natural water source and output back into that source, bivalves reduced the amount of suspended particulates by 71% (Jones et al., 2002).

Seaweeds remove the dissolved nitrogen and other inorganic compounds from the water (Chopin et al., 2012). By removing reactive nitrogen, the seaweed improves water quality and decreases the environmental impacts of nitrogen. Nitrogen is a key component in the growth cycle of plants, and as a result, seaweeds use the nitrogen to build biomass. Different species of seaweeds are variably effective at nitrogen removal depending on the environment. Some seaweeds remove up to 6.5 g of ammonium nitrogen per square meter of water per day (Mata, Shuenhoff, & Santos, 2010). Nori is an edible seaweed with high economic value that farmers often grow alongside their shrimp ponds to filter the wastewater. This seaweed removes up to 93% of ammonium nitrogen from the water in intensive systems and over 50% in open-water systems (Wu, Kim, Huo, Zhang, & He, 2017).

We have to act on an international level to reduce the environmental damage caused by shrimp aquaculture. The United Nations (UN) is an international organization that all major shrimp producing nations are party to. The UN organizes agreements between member nations in the form of treaties, resolutions, conventions, and other forms of diplomatic accord. The Food and Agriculture Organization (FAO), a bureau of the UN, strives to promote food security by encouraging the adoption of sustainable, contemporary approaches to food production, including aquaculture (FAO.org, 2016). The Technical Cooperation (TC) department within the FAO promotes investment in sustainable food production in the developing world. They do this by connecting development projects with investors like the World Bank (FAO.org, 2017). The FAO also does numerous things to insure that the projects they help fund are successful. They send representatives to organize development and education. They monitor and analyze the progress of projects they support to insure their success. They also help national governments to create effective policies and regulation to support environmental initiatives (FAO.org, 2017).

In 2016 Angola was losing at least one quarter of its inland fish production a year. Uninformed fishermen try to maximize their catch rates by using blast fishing and catching immature fish. These methods destroy habitat and fish population, significantly decreasing the long term profitability and possibility of fishing. Many of the fish that were caught were spoiled before they ever made it to market because of improper refrigeration. The Angolan government reached out to the FAO for help decreasing the resource loss. The TC secured nearly two million dollars in the form of international funding and a huge investment from the Angolan government. The FAO took local fisherman to sustainable fisheries in Tanzania so they could learn about responsible practices. The farmers saw firsthand how better fishing practices lead to better long-term yields. Upon returning to their country they helped to spread what they had learned among their communities. In developing communities information tends to travel by word of mouth so introducing these ideas is very important. The FAO supported their educational measures by working with the Angolan government and creating a state of-the-art training Center. The training center will teach Angolan farmers how to optimize their fishing practices. Local farmers were encouraged to further improve their businesses by setting up credit accounts and getting social security numbers for their families and employees. The considerable local benefits incurred by the FAO in only a year’s time demonstrate why they are the right group to pursue aquaculture reform (FAO.org, 2016).

The FAO should begin an international initiative to reform open water intensive shrimp aquaculture in South America and Southeast Asia. The TC could procure funding both domestically and internationally. The FAO can send consultants to help educate shrimp farmers about the benefits of utilizing IMTA systems. The FAO can also provide the farmers with access to the bivalves and seaweed needed to utilize IMTA systems. These developments can foster positive change in local communities and can encourage future development. By building public education facilities similar to the one in Angola, the FAO can demonstrate the effectiveness of IMTA systems and teach people how to construct them.

As it stands, the aquaculture industry represents a burgeoning problem. It is actively contributing to the spread of dead zones and HABs which will only increase if the industry continues to grow in its current form. The increase in livestock kills will drive down its profitability as it continues to grow. An FAO initiative in South America and Southeast Asia would help shrimps farmers implement more efficient, sustainable, contemporary practices in their shrimp aquaculture facilities. A properly optimized industry would drastically lower mankind’s impact on wild animal stocks while providing quality employment for many people.


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