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Environmental impacts
Chapter 14
Environmental impacts Chapter outline 14.1. 14.2. 14.3. 14.4. 14.5. 14.6. 14.7.
Introduction Transfers and introductions Farm effluents and organic enrichments Bioactive compounds Hormones Human health risks Reducing environmental impacts 14.7.1. Effluent management 14.7.1.1. Optimization of feeding regimes 14.7.1.2. Reducing effluent impact with a cagecum-pond integrated system
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14.1 Introduction The aquaculture industry is expanding worldwide at an outstanding rate during the last two decades. As a result, aquaculture production has sharply increased more than 160%, from 41,724,570 t in 2000, representing 30.62% of global fish production (136,284,859 t) to 111,948,623 t in 2017, comprising 54.5% of the fish production (205,582,364 t) (FAO, 2019). This rapid expansion of aquaculture has caused several ecological, human health and socioeconomic impacts. Traditionally, small-scale, semi-intensive aquaculture has been sustainable in many countries, especially in rural areas, with minimum adverse effects on surrounding environments. In recent years, however, there has been a growing trend towards tilapia intensification, which is usually driven by market forces and competitive use of the resources. The use of artificial culture inputs such as prepared feed, drugs, additives, hormones, fuels, etc., has become inevitable in intensive culture practices. This may pose serious environmental and socioeconomic threats. It is also very likely that the high intensity of farmed species will lead to enrichment of aquatic environments. According to Joint Group of Experts on the Scientific Aspects of
Tilapia Culture. https://doi.org/10.1016/B978-0-12-816541-6.00014-3 Copyright © 2020 Elsevier Inc. All rights reserved.
14.7.1.3. Effluent management during harvest 334 14.7.2. Reducing nutrient loading through green water tank culture 334 14.7.3. Removing nutrients through planting rooted plants334 14.7.4. Removing nutrients through aquaponic systems 335 14.7.5. Removing nutrients through integrated multitrophic aquaculture systems 335 14.7.6. Reducing environmental impacts through best aquaculture management practices 336 14.8. Closing remarks 337 References 337
Marine Pollution (GESAMP) (1991), the environmental impacts of coastal aquaculture are listed as follows: 1. Ecological impacts: These include enrichment, interaction with food web, oxygen consumption, disturbance of wildlife, habitat destruction, interaction between escaped farmed stock and wild species, introductions and transfers and bioactive compounds (including pesticides and antibiotics, chemicals introduced via construction materials, hormones and growth promoters). 2. Implications for human health: These include outbreaks of diseases associated with the consumption of shellfish; typhoid fever; infectious hepatitis and other viral diseases; survival of enteric viruses in the marine environment; cholera; influence of fish pathogens on human health, phycotoxins and depuration. 3. Socioeconomic considerations: As mentioned in Chapter 1, tilapia culture is expanding at a very high rate, so that it is now ranked second in terms of global production, only after carps. Tilapia are currently cultured in more than 120 countries around the world. The traditional, small-scale, semi-intensive system of tilapia culture, especially in Asia, is now being replaced with more intensive, large-scale farming systems.
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Intensive tilapia culture can thus pose a possible threat to the environment. This chapter discusses the major environmental impacts of tilapia culture, with emphases on the following impacts: (1) transfers and introductions, (2) farm effluents, (3) bioactive compounds and hormones, (4) human health risks and (5) reducing environmental impacts.
14.2 Transfers and introductions Transfers of aquatic species are movements of that species within its geographical range. Transfers generally take place to support stressed populations, enhance genetic characteristics or re-establish a species that has failed locally (GESAMP, 1991). Introductions are the movements of a species beyond its present geographical range. Introductions are intended to establish new taxa into the flora and fauna of an environment. Transfers and introductions can pose a wide variety of risks to the integrity of ecosystems, existing species, human health, agriculture, aquaculture and related primary industries. In other words, transfers and introductions may change the biodiversity of the receiving ecosystem through interbreeding, predation, competition for food and space and habitat destruction. Tilapia have been introduced as alien species into more than 100 countries worldwide. They are known to thrive and reproduce in most of their new habitats. The introductions of tilapia can be unsuccessful, successful, or successful but with high environmental costs ranging from habitat destruction, hybridization with endemic species, to disappearance of native species. Therefore, a full chapter (Chapter 3) is dedicated to tilapia transfers and introductions. Chapter 3 provides full details on the distribution of tilapia in their natural habitats and the factors affecting this distribution. It also covers the introductions of tilapia into the different geographical regions of the world. The negative impacts, including ecological impacts and genetic pollution of introduced tilapia were also highlighted. The current chapter will, therefore, be dedicated to the following environmental issues.
14.3 Farm effluents and organic enrichments Considerable amounts of uneaten feed along with wastes, including organic matter, nutrients such as phosphorus and nitrogen and suspended solids are released from intensive, land-based, tilapia farms into the water. For example, David et al. (2015) reported that the estimated emission of phosphorus per ton of Nile tilapia produced in cage aquaculture in a large hydroelectrical reservoir in south-eastern Brazil was 14.8 kg, based on farming data of feed composition, whole body composition and feed conversion ratio. These compounds are generally toxic to fish and other
aquatic animals. They may also have varying degrees of environmental impacts, depending on the intensity of the culture operations. In fish ponds, part of these wastes settle to the pond bottom, while the remainder is discharged with farm effluents into the environment. If not removed or treated, they may be subject to oxidation, leading to oxygen depletion and, in turn, producing anaerobic conditions that may stress and/or kill the fish, both in the ponds and in receiving waters. The global expansion of tilapia culture in floating cages is also very likely to pose severe environmental and socioeconomic impacts. The assimilation of wastes and nutrients accumulated underneath the cages requires a very large area of the aquatic system. Berg et al. (1996) reported that the production of Tilapia rendalli, Oreochromis mossambicus and Oreochromis niloticus in 1 m2 of intensively managed lacustrine cages required an ecosystem area of 115 and 160 m2 for phosphorus assimilation and oxygen production. This simply means that the production of tilapia in large cages, which are currently widely used, would require a tremendous ecosystem area for the assimilation of wastes resulting from the cages. Tilapia culture in floating cages in Sampaloc Lake (Philippines) is a good example of the catastrophic environmental and socioeconomic impacts of unmanaged culture practices. Tilapia cage culture in the lake was introduced in 1976 and has been expanding since then, with overuse of commercial feeds. It was reported that about 6000 mt of feed are wasted through the cages and settled on the sediments annually, in addition to the accumulation of fish faeces and other organic wastes underneath the cages. This situation has created severe anoxic conditions and toxic waters, resulting in the progressive depletion of dissolved oxygen (DO), a high biological oxygen demand load and extremely high concentrations of ammonia and total sulphides. In 1990, the operators were warned about the possibility of a fish kill due to the huge volume of anoxic and toxic waters underneath the cages and were also advised to harvest their stocks. The warning was ignored, and, as expected, a massive fish kill of market-size tilapia occurred, causing the farmers to lose their investment (Santiago and Arcilla, 1993; Santiago, 1994). A number of fish kills due to eutrophication caused by cage culture have also been reported in Lake Taal in the Philippines (ADB, 2005). Upwelling of deep deoxygenated and hydrogen sulphide-containing water from the anaerobic degradation of fish wastes generally leads to fish kill in the lake. In addition, about 64% of the nitrogen and 81% of phosphorous of the feed provided to caged fishes is released to the environment causing severe eutrophication (ADB, 2005). Farm wastes can also be a major cause of changes in the structure of benthic communities because uneaten feed may favour certain aquatic species over others. Sedentary animals may also suffer from mass mortality when the water is
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depleted of oxygen, while the mobile population may migrate to other areas. Nile tilapia cage culture has also been widely practiced in many natural freshwater masses (rivers, canals, reservoirs, etc.) in Thailand over the past decade. This has led to a rapid increase in the number of tilapia cages and has also created many social, economic and environmental problems. The FAO (2018) listed the following impacts as a sequence of expanding tilapia cage culture in Thailand: 1. High investment cost especially for feed; 2. High risk of declining profitability because of increased fish mortality and low growth rate of fish; 3. Climate change (air and water temperature) which makes fish more susceptible to disease; 4. Change in water level and quality, especially during the dry season; 5. Increased water pollution; 6. Limited number of waterbodies for operating tilapia cage culture and 7. Carrying capacity of tilapia cage culture in natural waterbodies being exceeded. These impacts indicated the urgent need for environmental carrying capacity assessment (ECCA) tool for tilapia planning in Thailand. This assessment will help to establish the number of cages that would allow the most sustainable farming practice, with highest performance and lowest adverse impacts. In Egypt, cage culture of tilapia in the Nile River, particularly in Damietta and Rosetta branches, has expanded at outstanding rats during the past few years (Fig. 14.1). This has increased public awareness and created a debate among governmental authorities, the academics and environmental protection bodies, with respect to the environmental impacts of these cages. The Egyptian
FIGURE 14.1 Several long arrays of tilapia cages in the Nile River (Egypt) may pose a threat to the environment. Photograph by A.-F.M ElSayed.
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authority removed the cages from the Nile branches by force, after warning the farmers several times, taking into account that most of these cage farms were not licenced. This situation has divided the public opinion into two mainstreams. Those in favour of tilapia cage culture suspect that the cages pollute the Nile and claim that this culture system is practiced all over the globe. They suggest that removing the cages is not the solution, whereas proper management, such as limiting the number of cages km 2, use of high-quality feed with optimum feeding strategies, proper selection of cage sites, farming other fish species such as silver carp with tilapia and continuous governmental monitoring and inspections, would certainly minimize the effects of the cages. Researchers, academics and farmers are in favour of this opinion. On the other side, environmental protection authority and governorate administration claim that cage culture is deteriorating the quality of the Nile and is very likely to cause health hazard, and therefore, removing these cages is the only solution. Unfortunately, removing the cages has caused a catastrophic loss to the farmers. Troell and Berg (1997) suggested that intensive tilapia farming in cages in the tropics can generate severe eutrophication. They found that average flux of particulate nutrients (ammonium and phosphates) under tilapia cages in Lake Kariba (Zimbabwe) was up to 22 times greater than in control areas. The release of the nutrients usually stimulates phytoplankton production and other algal blooms, which may further increase oxygen consumption during the decomposition of uneaten food and organic wastes. These algal blooms may also be toxic. Moreover, when the algae die, they settle to the bottom, decompose and further deplete the oxygen in the water, making the already eutrophic conditions worse. The status of tilapia cage culture in Lake Victoria (Kenya) was evaluated in 2016 and 2017 using geographical information system (GIS), standard water quality monitoring procedures and stakeholder questionnaires and interviews (Aura et al., 2018; Njiru et al., 2018). In addition to the remarkable benefits gained from cage culture, significant adverse impacts were recorded. Dissolved oxygen (DO) was much lower inside the cages than outside them. Ammonia concentration around the cages was also much higher than in cage-free areas, due to the protein in uneaten feed and fish waste that had been broken down into ammonia and nitrite. Waste feed increased eutrophication, decreased DO, enhanced blooming of algae and water hyacinth in the lake and led to a number of fish kills. Frequent occurrence of fish disease was also reported and attributed to poor management practices among cage farmers. On the other hand, no consistent environmental changes were reported in the Tanzanian part of Lake Victoria, as a result of tilapia cage culture (Kashindye et al., 2015).
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However, a significant increase in total nutrient (phosphorus and nitrogen) concentrations was observed in the cage locations after the set of cages, compared to offshore areas. An increase in phytoplankton, bivalves and gastropods communities was also observed after stocking of fish in cages. Regular fish kills due to eutrophication caused by intensive tilapia cage culture have also been recorded in Cirata Reservoir, Indonesia (Abery et al., 2005). Similarly, mass mortalities of tilapia farmed in cages in different rivers in Thailand due to illegal release of untreated effluents from different factories, especially in the Chao Phraya River, has also been reported (Edwards, 2015).
14.4 Bioactive compounds Bioactive compounds, including pesticides, antibiotics and other therapeutic drugs are used in aquaculture for various reasons, including disease control, fertilization, liming, disinfection, oxidation, coagulation, pesticides and adsorption. They are generally added to feed and/or culture water. Some of these compounds, especially drugs, pesticides, and antibiotics may be toxic to aquatic animals and/ or may accumulate in the environment. Aquaculture effluents containing these compounds may therefore produce adverse ecological impacts, in addition to the possible contamination of the flesh of aquatic animals which could pose a hazard to consumers. Rico et al. (2014) found that the maximum concentrations of the most commonly used antibiotics, oxytetracycline and enrofloxacin, in the Tha Chin River (Thailand), in which tilapia cage culture is widely practiced, was much higher than the concentrations recorded in other Asian aquatic environments receiving aquaculture effluents. The peak antibiotic concentrations in the river water occurred during antibiotic administration in the tilapia cages, presumably due to the leaching of antibiotics from fish pelleted feed. Antibiotics accumulated mainly underneath or next to the tilapia cages, suggesting that water-sediment sorption, fish waste and wasted feed are responsible for the transport of antibiotics to the sediment. Accumulated antibiotics were widely distributed in the river and were found to remain in the river sediments for several weeks. These results clearly indicated that tilapia cage aquaculture is an important source of antibiotic pollution.
14.5 Hormones As mentioned in Chapter 8 (Section 8.8.2), steroid hormones are widely used for producing monosex tilapia. Hormones are generally incorporated either with larval feeds or through the immersion of fertilized eggs or sac fry. However, the use of hormones for sex reversal has been under increasing public criticism due to their potential
health and environmental impacts. The hormone residues and metabolites can be a potential environmental contaminant. For example, feeding tilapia with 17-a methyltestosterone (MT)-treated feed has resulted in considerable ‘leakage’ of MT into pond water and sediments (ContrerasSánchez et al., 2001, 2002). MT has been detected in the water during MT treatment, accumulated and remained in the sediments of the ponds for up to 8 weeks. Hormone traces pose a risk of hatchery workers and also of other nontarget aquatic organisms. MT traces remained at 2.8e2.9 ng g 1 in soils for 3 months and in water at 3.6 mg L 1 for 2 weeks, before returning to the base level (up to 0.02 mg L 1), which has been reported in tilapia masculinization ponds (Homklin et al., 2009, 2012; Megbowon and Mojekwu, 2014). MT leakage to the environment can also disrupt the normal sex ratio in wild fish populations. The human food safety and environmental hazards associated with the use of MT and other steroids in production of sex-reversed tilapia have been comprehensively reviewed by Mlalila et al. (2015). Many tilapia farmers and hatchery operators in Egypt use higher amounts of MT hormones than they really need (A.-F.M. El-Sayed, personal survey, 2017), leading to accumulation of hormone residues in the environment. This practice may lead to adverse ecological and human health impacts. For example, the sex ratio of Nile tilapia in the canals and drains adjacent to fish farms and hatcheries in Kafr El-Shaikh Governorate (Egypt) has dramatically changed in favour of males (A.-F.M. El-Sayed, 2018; personal observation). Catching wild females, as broodstock for breeding, from these waterbodies becomes very difficult, since most of the catch is males. The above finding supports the argument that the exposure of untargeted organisms to steroid hormones can result in biased sex ratios. In support, significant masculinization occurred in common carp exposed to water used in MT-impregnated feeding trials (Gomelsky et al., 1994). Abucay et al. (1997) also reported high rates of masculinization and feminization in sexually undifferentiated Nile tilapia fry reared in aquarium water previously used for oral application of hormones used for sex reversal. Also, high proportions of males were found in Nile tilapia fry fed with normal feed in cages adjacent to cages containing MTincorporated feed (Abucay et al., 1997). These findings clearly demonstrate that hormone (or hormone metabolites) leaching from uneaten food can induce sex reversal in exposed untargeted organisms.
14.6 Human health risks As mentioned earlier, many contaminants from land-based run-offs and anthropogenic activities, including organic pesticides, heavy metals, polychlorinated biphenyls,
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hormones, etc., end up in the aquatic environments, causing a wide range of environmental impacts. Farming tilapia in these environments may also pose human health risks, depending on the source, exposure time and concentration of the contaminant. For example, Ling et al. (2009) assessed the heavy metals (arsenic, As; chromium, Cr; copper, Cu; manganese, Mn; nickel, Ni; lead, Pb; selenium, Se and zinc, Zn) in tilapia collected from farms located in industrialized (polluted) areas in Taiwan, compared with samples collected from unpolluted areas. They found that the concentrations of almost all of these heavy metals were higher in the body tissue of tilapia farmed in the polluted areas than in fish grown in relatively clean areas. These results indicated that heavy metals in tilapia muscles may pose a significant risk to residents in the vicinity of industrial zones. Excess lifetime cancer risk and hazard quotients for residents consuming contaminated tilapia located in the vicinity of industrial zones were also higher than those for residents consuming tilapia farmed at the uncontaminated areas. Similarly, the concentrations of heavy metals in Mozambique tilapia (O. mossambicus) collected from ponds receiving domestic effluents in Malaysia were significantly higher than in fish collected from uncontaminated ponds (Yap et al., 2015). However, health risk assessments and the amount of fish required to reach the provisional tolerable weekly intake, estimated daily intake and target hazard quotient revealed that no health risks were associated with heavy metals intake via consumption of tilapia collected from ponds receiving domestic effluents. In addition to their potential environmental impacts, steroid hormones applied in aquaculture may also pose human health risks. However, the effects of these steroids depend mainly on their type, doses, exposure time, pharmacokinetic characteristics and genetic susceptibility. There has been a long debate among societal and scientific communities on whether the MT-treated fish are safe for human consumption. The fish carcass, viscera, blood, and muscle of treated fish have been investigated for MT and its metabolites. Available information and argument concluded that MT-treated fish are safe for human consumption. Pandian and Kirankumar (2003) reported that the estimated residual steroids are less than 5 ng g 1 fish, which is too low to cause any human health risks. In support, Green and Teichert-Coddington (2000) demonstrated that the levels of MT and metabolites in tilapia carcass were less than 100 pg g 1 after 8e40 days of withdrawal, and less than 10 pg g 1 after 6e50 days of withdrawal. Once again, it is very unlikely that these low hormone levels can cause any health problems to humans. In addition, no cases of pharmaceutically active MT and its metabolites from aquaculture effluents to surrounding environments have been reported (Mlalila et al., 2015).
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14.7 Reducing environmental impacts 14.7.1 Effluent management The treatment and management of tilapia culture effluents have been described in details in Chapter 6 (Section 6.8). Boyd (2003) listed the following best aquaculture management practices (BAMPs) for minimizing nutrient loads in aquaculture effluents: 1. Use fertilizers only as needed to maintain phytoplankton blooms. 2. Select stocking and feeding rates that do not exceed the assimilation capacity of ponds. 3. Feeds should be of high quality, water stable and contain no more nitrogen and phosphorus than necessary. 4. Apply feeds conservatively to avoid overfeeding and to assure that as much of the feed is consumed as possible. 5. Do not use water exchange or reduce water exchange rates as much as possible. 6. In intensive aquaculture, apply enough mechanical aeration to prevent chronically low DO concentration and to promote nitrification and other aerobic, natural water purification processes. 7. Provide storage volume for heavy rainfall to minimize storm overflow. 8. Deep water release structures should not be installed in ponds, for they discharge lower-quality water from near pond bottoms. 9. Where possible, seine-harvested fish without partially or completely draining ponds. 10. Where possible, discharge pond draining effluent through a settling basin or a vegetated ditch. 11. Reuse water where possible. While these practices are valid for reducing nutrient loads in tilapia culture, there are certain practices that have been exceptionally successful in effluent management at the farm level. Extensive research was conducted at the Asian Institute of Technology (AIT) in Thailand to mitigate the environmental impact of tilapia culture through reducing nutrient loading in pond culture of Nile tilapia. Special attention was given to (1) optimization of feeding regimes; (2) maximizing nutrient utilization through integration and recycling systems and (3) minimizing waste loading from effluents through appropriate draining techniques during harvest. Lin and Yi (2003) described the results of these practices. It would be useful here to throw light on these results.
14.7.1.1 Optimization of feeding regimes When Nile tilapia reared in fertilized ponds were fed supplemental diets at 50%, 75%, and 100% satiation, they
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produced comparable yields (Lin and Yi, 2003). However, considerable reduction in nutrient loading was achieved at the 50% level. This means that about 50% of the feed can be saved, since the fish are able to supplement their diets with natural food available in the pond. This also indicates that an optimal feeding regime may reduce both feed costs and nutrient loading in the ponds.
14.7.1.2 Reducing effluent impact with a cage-cum-pond integrated system Tilapia culture in a cage-cum-pond integrated system has been described in details in Chapter 5 (Section 5.6.3) and Chapter 6 (Section 6.4.5). The system involves rearing hybrid catfish (Clarias macrocephalus Clarias gariepinus) in cages in ponds and feeding them with formulated feeds, whereas their wastes are used as a natural food for tilapia reared in open pond. This system yields considerable production of catfish and tilapia and also reduces nutrient loading through the use of fish wastes for pond fertilization and natural food production.
14.7.1.3 Effluent management during harvest Seining fish ponds with nets, despite being most common, stirs bottom sediments, leading to the deterioration of water quality and its effluents. Appropriate harvest methods and draining treatments can significantly reduce pollutants from Nile tilapia ponds. In a novel study, Lin et al. (2001) evaluated four treatments to determine the efficiency of fish harvest and effectiveness in reducing nutrient loading in the effluents. The treatments were (1) treating the ponds with tea seed cake (10 ppm) to partially anaesthetize Nile tilapia 1.5 h prior to harvest by seining, (2) liming the ponds (75 g m 3 of calcium hydroxide) 24 h prior to harvest to precipitate phosphorus and organic matter, followed by sequential complete draining with a pump, and Nile tilapia collected from a harvesting pit, (3) draining the ponds by sequential complete draining with a pump and Nile tilapia collected from a harvesting pit and (4) ponds drawn down from 100 to 50 cm with a pump and Nile tilapia harvested by seining twice, followed by complete draining and collection of the remaining Nile tilapia from a harvesting pit. The results of that study revealed that liming the pond 24 h before draining and gradually draining ponds to a 25-cm depth during harvest were most effective.
14.7.2 Reducing nutrient loading through green water tank culture Green water tank culture system can be an efficient method for reducing nutrient loading in fish tanks. In this system, fish wastes and other metabolites are treated in such a way to increase natural food production in the tanks. This
FIGURE 14.2 Growing tilapia in green water system in Virgin Islands, USA. Photo provided by J. Rakocy.
process involves the oxidation of toxic ammonia (NH3) and nitrite (NO2) into relatively nontoxic nitrate (NO3) through nitrifying bacteria grown on suspended organic matter. The bacteria remove the organic matter from the tanks and use it as food, while the bacteria are used as natural food for filter-feeding fishes such as tilapia and carp. However, in green water systems, solid wastes should be removed continuously, and both biofiltration and aeration should be provided. Vigorous aeration is also necessary to support the suspension of the microbial community, maximize contact between bacteria and waste products and, in turn, increase phytoplankton productivity. This system has been successfully used for tilapia culture in various parts of the world, particularly in the Virgin Islands (Fig. 14.2). The advantages of green water culture are summarized in Chapter 6 (Section 6.6).
14.7.3 Removing nutrients through planting rooted plants In semi-intensive tilapia culture, about 80% of the nitrogen and phosphorus added in fertilizers settles to the pond mud (Lin and Yi, 2003). The removal (de-sludging) of pond mud is tedious and labour-demanding. Planting rooted aquatic plants in fish ponds can be an effective way of extracting nutrients from the mud. This approach has been tested with lotus (Nelumbo mucifera) seedlings planted in Nile tilapia ponds fertilized weekly with urea and triple superphosphate at a rate of 4 kg N and 1 kg P ha 1 d 1 (Yi et al., 2002). Lotus effectively removed nutrients from pond mud. The annual nutrient losses from the mud were 2.4 t N and 1 t P ha 1, about 300 kg N and 43 kg of which were taken by lotus. In temperate zones, water temperature drops during winter times and becomes unsuitable for tilapia culture. Generally, tilapia farmers in these regions harvest their fish during SeptembereNovember. During winter season, these ponds are dried and become useless. Growing land winter crops, such as wheat and alfalfa, in these ponds can be an effective method of removing nutrients from pond muds in addition to generating an additional income to the farmer.
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FIGURE 14.4 Growing plants with tilapia in an aquaponic system is an effective way of removing nutrients from culture water. Photo provided by J. Rakocy.
FIGURE 14.3 Growing (top) and harvesting and threshing (bottom) a wheat crop in tilapia ponds in Kafr El-Shaikh Governorate, Egypt. Photograph by A.-F.M. El-Sayed.
This approach has been tested in some areas in Egypt, where alfalfa and wheat have been grown in tilapia ponds during the winter, with promising results (El-Sayed, 2017) (Fig. 14.3).
14.7.4 Removing nutrients through aquaponic systems Aquaponics is the integration of hydroponics with aquaculture in a recirculating system. The wastes and metabolites produced by cultured fish are removed by nitrification and taken up by the plants, while the bacteria living in the gravel and in association with the plant roots play a critical role in nutrient removal (Fig. 14.4). This means that the plants act as a biological filter by removing fish wastes and improving the quality of culture water. Many studies have been conducted on aquaponic systems at the University of Virgin Islands Agricultural Experimental Station and also in other geographical regions, using tilapia and various vegetables, with considerable success. The results of these studies are discussed in Chapter 6 (Section 6.9).
14.7.5 Removing nutrients through integrated multitrophic aquaculture systems Integrated multitrophic aquaculture (IMTA) systems are designed mainly for efficient use of resources. IMTA
systems combine species of different feeding habits and trophic levels in the same aquaculture system. In this combination, the biological and chemical processes are balanced because the waste from one species can be used as food for the other(s). For example, farming tilapia and prawns in polyculture system can be an efficient way of nutrient removal and improving water quality. They have different feeding habits and special distribution and thus can behave synergistically. In a polyculture system, tilapia consume artificial feed and natural foods, while prawns can eat tilapia wastes and unused tilapia feed. In addition, prawn bioturbation at the pond bottom brings the nutrients back to the water column and enhances phytoplankton production, which in turn increases natural food availability for tilapia. In this regard, David et al. (2017a) found that the levels of phosphorus retained from tilapia/prawn IMTA system were higher (20%e28%) than in tilapia and prawn monocultures (10%e20% and 10%e13%, respectively). The same authors (David et al., 2017b) suggested that the integration of Nile tilapia and Amazon River prawn in stagnant ponds may remove significant amounts of nitrogen from nutrient-rich water and could be considered as a mitigation tool. These results clearly indicate that IMTA systems can be used in mitigation programs for aquatic environments receiving high levels of nutrients, especially phosphorus and nitrogen. In-pond raceway system (IPRS) (Fig. 14.5) can be an excellent model for nutrient removal or recovery. In this system, one fish species is raised in raceways installed inside an earthen pond and fed with high-quality feed. Another species (or more) is stocked in the rest of the pond, mainly for water quality improvement, since it depends on the wastes of the raceways for feeding (see Chapter 13, Section 13.10.4 for more details on IPRS). Brown et al. (2012) raised channel catfish (Ictalurus punctatus) in in-pond raceways, while Nile tilapia and paddlefish (Polyodon spathula) were co-cultured in the pond where the raceways were installed. The harvest of Nile tilapia and
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FIGURE 14.5 Growing Nile tilapia in in-pond raceways at the WorldFish Center, Abbassa, Sharkya Governorate, Egypt. Photograph by A.-F.M. El-Sayed.
paddlefish increased phosphorus and nitrogen recovery/ removal by 47.5% and 40%.
14.7.6 Reducing environmental impacts through best aquaculture management practices In addition to the technical and practical measures that should be adopted to reduce the adverse impacts of aquaculture, the BAMPs can play a significant role in preventing these adverse effects. BAMPs in aquaculture can be defined as ‘the application of good management practices to support sustainable aquaculture growth, with minimum risks to the environment, through the adoption of costeffective and continually assessed management measures’ (White et al., 2018). A number of recent documents have been published on the effects of BAMPs on aquaculture performance (tilapia included) including the following: l
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Better management practices for tilapia aquaculture (WWF, 2011); On-farm Feeding and Feed Management in Aquaculture (Hasan and New, 3013). This publication included 6 case studies/reviews on ‘On-farm feed management practices for Nile tilapia (Oreochromis niloticus)’ in six countries (China, Egypt, Ghana, Malaysia, the Philippines and Thailand). Pilot application of selected aquaculture planning and management tools in Indonesia, Thailand and Vietnam (FAO, 2018); Better management practices for feed production and management of Nile tilapia and milkfish in the Philippines (White et al., 2018).
This is in addition to several papers published in different specialized journals on the best management practices
(BMPs) of tilapia culture and their social, economic and environmental effects. A number of examples are provided on the adoption of the BAMPs in tilapia culture and their effects on environmental performance and disease reduction. In Thailand, for example, cage culture of Nile tilapia has been widely practised in many natural freshwater environments over the past decade. This practice has led to a rapid increase in the number of tilapia cages and has also created many social, economic and environmental problems. These problems are listed in Section 14.3. These sequences demonstrate the urgent need for ECCA tool for tilapia planning as a means for BMPs for tilapia cage culture in Thailand. This assessment tool will help to establish the correct number of cages that would allow the most sustainable farming practice, with highest performance and lowest adverse impacts. Another interesting example, also in Thailand, is the spreading of diseases due to the expansion of tilapia cage culture in Chainat Province and the role of BAMPs in reducing their risk impacts. Several irrigation projects have recently been developed in this province for agricultural development, and aquaculture as well, especially cage culture in the Noi River. As a result, the number of tilapia cage farms has rapidly increased, leading to significant adverse impacts, including disease occurrence due to the changes in the environment, water quality, high fish density, inappropriate farm management and the lack of control of fish disease/pathogens (FAO, 2018). To face this problem, a farm biosecurity plan has been developed and put in place to describe the measures and actions that should be followed to protect farms from diseases. These measures should be able to reduce the risks and impacts of disease outbreaks and prevent health issues emerging within the cage farms. To maximize its benefits, the farm-level biosecurity plan should be piloted and disseminated to fish farmers, hatcheries and other concerned stakeholders. A field implementation of the farm-level biosecurity plan tool included the following: (1) providing training for 25 selected farmers; (2) the biosecurity plan for each farm was adopted; (3) the biosecurity plan was adopted by 25 selected cage tilapia farms; (4) on-farm implementation of the biosecurity plan was strictly monitored for 3 months, with continuous data collection and (5) a national evaluation was conducted. The results showed that trained farmers had a better understanding about fish diseases and their prevention. Farmers are now able to check cages and collect dead fish from cages daily and no longer throw the dead fish into the river. They also record the necessary farm information such as date, fish size and number at stocking, the daily amount of feed provided and fish mortality. This would certainly lead to a reduction of overfeeding and minimizing the accumulation of feed waste and losses.
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The adoption of best on-farm feed and feeding management would certainly lead to a reduction in wasted feed and fish waste, better growth and feed utilization, lower organic and inorganic loads in aquaculture effluents and in turn, lower environmental impacts. The following feed and feeding practices should be adopted to improve feed efficiency and reduce the amount of waste entering the environment (see Hasan and New, 2013; White et al., 2018 for details): l
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Relevant practical knowledge and skills in feed and feeding should be gained. The best available ingredients should be used for feed production. Species-specific feeds containing the correct nutrient requirements should be developed, with good pellet integrity, shape, density and water stability. The most appropriate feed type (floating vs. sinking pellets), feeding methods (manual feeding, demand feeder, automatic feeder), feeding levels (percent body weight, ad libitum, satiation) and feeding frequencies should be used. Farmers, farm managers, hatchery operators and feeding staff should be trained on the most appropriate on-farm feed handling, storage and use. Water quality should be monitored continuously. Appropriate initial fish stocking number and size should be used.
14.8 Closing remarks 1. The introductions and/or transfers of tilapia can be unsuccessful, successful or successful but with environmental impacts ranging from habitat destruction, hybridization with endemic species, to disappearance of native species. Full details on the environmental impacts of tilapia introductions are provided in Chapter 3. 2. Varying amounts of uneaten feed, fish wastes, organic matter, nutrients such as phosphorus and nitrogen, hormones and bioactive compounds are released from land-based and cage culture of tilapia. If these wastes are not removed or treated, they will adversely affect the environmental impacts, produce anaerobic conditions that stress and/or kill the fish. 3. The public criticism concerning the use of steroid hormones for sex reversal in tilapia is increasing because hormone residues and metabolites can pose health and environmental impacts. 4. Various techniques, including effluent management, green water tank culture, planting rooted plants, aquaponic systems and IMTA systems have been successfully used for reducing environmental impacts of tilapia culture.
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5. The BAMPs should be adopted for minimizing nutrient loads in tilapia effluents and reducing their environmental impacts. The adoption of best feed and feeding management will lead to a reduction in wasted feed and fish waste, better growth and feed utilization, lower organic and inorganic loads in aquaculture effluents and, in turn, will lower the overall environmental impacts.
References Abery, N.W., Sukadi, F., Budhiman, A.A., Kartamihardja, E.S., Koeshendrajana, S., Buddhiman, et al., 2005. Fisheries and cage culture of three reservoirs in West Java, Indonesia; a case study of ambitious development and resulting interactions. Fisheries Management and Ecology 12, 315e330. Abucay, J.S., Mair, G.C., Skibinski, D.O.F., Beardmore, J.A., 1997. The occurrence of incidental sex reversal in Oreochromis niloticus L. In: Fitzsimmons, K. (Ed.), Tilapia Aquaculture, Proceedings from the Fourth International Symposium on Tilapia in Aquaculture. Orlando, FL, USA, pp. 729e738. ADB (Asian Development Bank), 2005. An Evaluation of Small-Scale Freshwater Rural Aquaculture Development for Poverty Reduction. Operations Evaluation Department, Asian Development Bank, Manila, Philippines, pp. 164. Aura, C.M., Musa, S., Yongo, E., Okechi, J.K., Njiru, J.M., Ogari, Z., et al., 2018. Integration of mapping and socio-economic status of cage culture: towards balancing lake-use and culture fisheries in Lake Victoria, Kenya. Aquaculture Research 49, 532e545. Berg, H., Michélsen, P., Troell, M., Folke, C., Kautsky, N., 1996. Managing aquaculture for sustainability in tropical Lake Kariba, Zimbabwe. Ecological Economics 18, 141e159. Boyd, C.E., 2003. Guidelines for aquaculture effluent management at the farm level. Aquaculture 226, 101e112. Brown, T.W., Boyd, C.E., Chappell, J.A., 2012. Approximate water and chemical budgets for an experimental, in-pond raceway system. Journal of the World Aquaculture Society 43, 526e537. Contreras-Sánchez, W.M., Fitzpatrick, M.S., Schreck, C.B., 2001. Fate of methyltestosterone in the pond environment: detection of MT in pond soil from a CRSP site. In: Gupta, A., McElwee, K., Burke, D., Burright, J., Cummings, X., Egna, H. (Eds.), Eighteenth Annual Technical Report. Pond Dynamics/Aquaculture CRSP, Oregon State University, Corvallis, OR, USA, pp. 79e82. Contreras-Sánchez, W.M., Couturrier, G.M., Schreck, C.B., 2002. Fate of methyltestosterone in the pond environment: use of MT in earthen ponds with no record of hormone usage. In: McElwee, K., Lewis, K., Nidiffer, M., Buitrago, P. (Eds.), Nineteenth Annual Technical Report. Pond Dynamics/Aquaculture CRSP, Oregon State University, Corvallis, Oregon, USA, pp. 103e106. David, G.S., Carvalho, E.D., Lemos, D., Silveira, A.N., Dall’AglioSobrinho, M., 2015. Ecological carrying capacity for intensive tilapia (Oreochromis niloticus) cage aquaculture in a large hydroelectrical reservoir in Southeastern Brazil. Aquacultural Engineering 66, 30e40. David, F.S., Proença, D.C., Valenti, W.C., 2017a. Phosphorus budget in integrated multitrophic aquaculture systems with Nile tilapia,
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Oreochromis niloticus, and Amazon River prawn, Macrobrachium amazonicum. Journal of the World Aquaculture Society 48, 402e414. David, F.S., Proença, D.C., Valenti, W.C., 2017b. Nitrogen budget in integrated aquaculture systems with Nile tilapia and Amazon River prawn. Aquaculture International 25, 1733e1746. Edwards, P., 2015. Aquaculture environment interactions: past, present and likely future trends. Aquaculture 447, 2e14. El-Sayed, A.-F.M., 2017. Social and economic performance of tilapia farming in Egypt. In: Cai, J., Quagrainie, K.K., Hishamunda, N. (Eds.), Social and Economic Performance of Tilapia Farming in Africa. FAO Fisheries and Aquaculture Circular No. 1130. FAO, Rome, pp. 1e48. FAO (Food and Agriculture Organization of the United Nations), 2018. Pilot Application of Selected Aquaculture Planning and Management Tools in Indonesia, Thailand and Viet Nam. Bangkok. pp. 121 Licence: CC BY-NC-SA 3.0 IGO. FAO (Food and Agriculture Organization of the United Nations), 2019. Global Aquaculture Production, pp. 1950e2017. http://www.fao.org/ fishery/statistics/global-aquaculture-production/query/en. GESAMP (IMO/FAO/Unesco/WMO/WHO/IAEA/UN/UNEP (Joint Group of Experts on the Scientific Aspects of Marine Pollution, 1991. Reducing Environmental Impacts of Coastal Aquaculture. Reports and Studies No. 47. GESAMP, Rome, Italy, pp. 35. Gomelsky, B., Cherfas, N.B., Peretz, Y., Ben-Dom, N., Hulata, G., 1994. Hormonal sex inversion in the common carp (Cyprinus carpio L.). Aquaculture 126, 265e270. Green, B.W., Teichert-Coddington, D.R., 2000. Human food safety and environmental assessment of the use of 17a-methyltestosterone to produce male tilapia in the United States. Journal of the World Aquaculture Society 31, 337e357. Hasan, M.R., New, M.B., 2013. On-farm Feeding and Feed Management in Aquaculture. In: FAO Fisheries and Aquaculture Technical Paper No. 583. FAO, Rome, pp. 585. Homklin, S., Wattanodorn, T., Ong, S.K., Limpiyakorn, T., 2009. Biodegradation of 17a- methyltestosterone and isolation of MTdegrading bacterium from sediment of Nile tilapia masculinization pond. Water Science and Technology 59, 261e265. Homklin, S., Ong, S.K., Limpiyakorn, T., 2012. Degradation of 17amethyltestosterone by Rhodococcus sp. and Nocardioides sp. isolated from a masculinizing pond of Nile tilapia fry. Journal of Hazardous Materials 221e222, 35e44. Kashindye, B.B., Nsinda, P., Kayanda, R., Ngupula, G.W., Mashafi, C.A., Ezekiel, C.N., 2015. Environmental impacts of cage culture in Lake Victoria: the case of Shirati Bay-Sota, Tanzania. SpringerPlus 4, 475. https://doi.org/10.1186/s40064-015-1241-y. Lin, C.K., Yi, Y., 2003. Minimizing environmental impacts of freshwater aquaculture and reuse of pond effluents and mud. Aquaculture 226, 57e68. Lin, C.K., Shrestha, M.K., Yi, Y., Diana, J.S., 2001. Management to minimize the environmental impacts of pond effluent: harvest draining
techniques and effluent quality. Aquacultural Engineering 25, 125e135. Ling, M.-P., Hsu, H.-T., Shie, R.-H., Wu, C.-C., Hong, Y.-S., 2009. Health risk of consuming heavy metals in farmed tilapia in central Taiwan. Bulletin of Environmental Contamination and Toxicology 83, 558e564. Megbowon, I., Mojekwu, T.O., 2014. Tilapia sex reversal using methyl testosterone (MT) and its effect on fish, man and environment. Biotechnology 13, 213e216. Mlalila, N., Mahika, C., Kalombo, L., Swai, H., Hilonga, A., 2015. Human food safety and environmental hazards associated with the use of methyltestosterone and other steroids in production of all-male tilapia. Environmental Science and Pollution Research 22, 4922e4931. Njiru, J.M., Aura, C.M., Okechi, J.K., 2018. Cage fish culture in Lake Victoria: a boon or a disaster in waiting? Fisheries Management and Ecology 1e9. https://doi.org/10.1111/fme.12283. Pandian, T., Kirankumar, S., 2003. Recent advances in hormonal induction of sex-reversal in fish. Journal of Applied Aquaculture 13, 205e230. Rico, A., Oliveira, R., McDonough, S., Matser, A., Khatikarn, J., Satapornvanit, K., et al., 2014. Use, fate and ecological risks of antibiotics applied in tilapia cage farming in Thailand. Environmental Pollution 191, 8e16. Santiago, A.E., 1994. The ecological impact of tilapia cage culture in Sampaloc Lake, Philippines. In: Chou, L.M., Munro, A.D., Lam, T.J., Chen, T.W., Cheong, L.K.K., Ding, J.K., et al. (Eds.), The Third Asian Fisheries Forum. Proceedings of the Third Asian Fisheries Forum, 26e30 October 1992, Singapore. Asian Fisheries Society, Manila, Philippines, pp. 413e416. Santiago, A.E., Arcilla, R.P., 1993. Tilapia cage culture and the dissolved oxygen trends in Sampaloc Lake, the Philippines. Environmental Monitoring and Assessment 24, 243e255. Troell, M., Berg, H., 1997. Cage fish farming in the tropical Lake Kariba, Zimbabwe: impact and biogeochemical changes in sediment. Aquaculture Research 28, 527e544. White, P.G., Shipton, T.A., Bueno, P.B., Hasan, M.R., 2018. Better Management Practices for Feed Production and Management of Nile Tilapia and Milkfish in the Philippines. In: FAO Fisheries and Aquaculture Technical Paper No. 614. FAO, Rome, pp. 98. WWF (World Wildlife Fund), 2011. Better Management Practices for Tilapia Aquaculture: A Tool to Assist with Compliance to the International Standards for Responsible Tilapia Aquaculture. World Wildlife Fund, Inc. Version 1, pp. 54. Yap, C.K., Jusoh, A., Leong, W.J., Karami, A., Ong, G.H., 2015. Potential human health risk assessment of heavy metals via the consumption of tilapia Oreochromis mossambicus collected from contaminated and uncontaminated ponds. Environmental Monitoring and Assessment 187, 584. Yi, Y., Lin, C.K., Diana, J.S., 2002. Recycling pond mud nutrients in integrated lotus-fish culture. Aquaculture 212, 213e226.
Environmental impacts Chapter outline 14.1. 14.2. 14.3. 14.4. 14.5. 14.6. 14.7.
Introduction Transfers and introductions Farm effluents and organic enrichments Bioactive compounds Hormones Human health risks Reducing environmental impacts 14.7.1. Effluent management 14.7.1.1. Optimization of feeding regimes 14.7.1.2. Reducing effluent impact with a cagecum-pond integrated system
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14.1 Introduction The aquaculture industry is expanding worldwide at an outstanding rate during the last two decades. As a result, aquaculture production has sharply increased more than 160%, from 41,724,570 t in 2000, representing 30.62% of global fish production (136,284,859 t) to 111,948,623 t in 2017, comprising 54.5% of the fish production (205,582,364 t) (FAO, 2019). This rapid expansion of aquaculture has caused several ecological, human health and socioeconomic impacts. Traditionally, small-scale, semi-intensive aquaculture has been sustainable in many countries, especially in rural areas, with minimum adverse effects on surrounding environments. In recent years, however, there has been a growing trend towards tilapia intensification, which is usually driven by market forces and competitive use of the resources. The use of artificial culture inputs such as prepared feed, drugs, additives, hormones, fuels, etc., has become inevitable in intensive culture practices. This may pose serious environmental and socioeconomic threats. It is also very likely that the high intensity of farmed species will lead to enrichment of aquatic environments. According to Joint Group of Experts on the Scientific Aspects of
Tilapia Culture. https://doi.org/10.1016/B978-0-12-816541-6.00014-3 Copyright © 2020 Elsevier Inc. All rights reserved.
14.7.1.3. Effluent management during harvest 334 14.7.2. Reducing nutrient loading through green water tank culture 334 14.7.3. Removing nutrients through planting rooted plants334 14.7.4. Removing nutrients through aquaponic systems 335 14.7.5. Removing nutrients through integrated multitrophic aquaculture systems 335 14.7.6. Reducing environmental impacts through best aquaculture management practices 336 14.8. Closing remarks 337 References 337
Marine Pollution (GESAMP) (1991), the environmental impacts of coastal aquaculture are listed as follows: 1. Ecological impacts: These include enrichment, interaction with food web, oxygen consumption, disturbance of wildlife, habitat destruction, interaction between escaped farmed stock and wild species, introductions and transfers and bioactive compounds (including pesticides and antibiotics, chemicals introduced via construction materials, hormones and growth promoters). 2. Implications for human health: These include outbreaks of diseases associated with the consumption of shellfish; typhoid fever; infectious hepatitis and other viral diseases; survival of enteric viruses in the marine environment; cholera; influence of fish pathogens on human health, phycotoxins and depuration. 3. Socioeconomic considerations: As mentioned in Chapter 1, tilapia culture is expanding at a very high rate, so that it is now ranked second in terms of global production, only after carps. Tilapia are currently cultured in more than 120 countries around the world. The traditional, small-scale, semi-intensive system of tilapia culture, especially in Asia, is now being replaced with more intensive, large-scale farming systems.
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Intensive tilapia culture can thus pose a possible threat to the environment. This chapter discusses the major environmental impacts of tilapia culture, with emphases on the following impacts: (1) transfers and introductions, (2) farm effluents, (3) bioactive compounds and hormones, (4) human health risks and (5) reducing environmental impacts.
14.2 Transfers and introductions Transfers of aquatic species are movements of that species within its geographical range. Transfers generally take place to support stressed populations, enhance genetic characteristics or re-establish a species that has failed locally (GESAMP, 1991). Introductions are the movements of a species beyond its present geographical range. Introductions are intended to establish new taxa into the flora and fauna of an environment. Transfers and introductions can pose a wide variety of risks to the integrity of ecosystems, existing species, human health, agriculture, aquaculture and related primary industries. In other words, transfers and introductions may change the biodiversity of the receiving ecosystem through interbreeding, predation, competition for food and space and habitat destruction. Tilapia have been introduced as alien species into more than 100 countries worldwide. They are known to thrive and reproduce in most of their new habitats. The introductions of tilapia can be unsuccessful, successful, or successful but with high environmental costs ranging from habitat destruction, hybridization with endemic species, to disappearance of native species. Therefore, a full chapter (Chapter 3) is dedicated to tilapia transfers and introductions. Chapter 3 provides full details on the distribution of tilapia in their natural habitats and the factors affecting this distribution. It also covers the introductions of tilapia into the different geographical regions of the world. The negative impacts, including ecological impacts and genetic pollution of introduced tilapia were also highlighted. The current chapter will, therefore, be dedicated to the following environmental issues.
14.3 Farm effluents and organic enrichments Considerable amounts of uneaten feed along with wastes, including organic matter, nutrients such as phosphorus and nitrogen and suspended solids are released from intensive, land-based, tilapia farms into the water. For example, David et al. (2015) reported that the estimated emission of phosphorus per ton of Nile tilapia produced in cage aquaculture in a large hydroelectrical reservoir in south-eastern Brazil was 14.8 kg, based on farming data of feed composition, whole body composition and feed conversion ratio. These compounds are generally toxic to fish and other
aquatic animals. They may also have varying degrees of environmental impacts, depending on the intensity of the culture operations. In fish ponds, part of these wastes settle to the pond bottom, while the remainder is discharged with farm effluents into the environment. If not removed or treated, they may be subject to oxidation, leading to oxygen depletion and, in turn, producing anaerobic conditions that may stress and/or kill the fish, both in the ponds and in receiving waters. The global expansion of tilapia culture in floating cages is also very likely to pose severe environmental and socioeconomic impacts. The assimilation of wastes and nutrients accumulated underneath the cages requires a very large area of the aquatic system. Berg et al. (1996) reported that the production of Tilapia rendalli, Oreochromis mossambicus and Oreochromis niloticus in 1 m2 of intensively managed lacustrine cages required an ecosystem area of 115 and 160 m2 for phosphorus assimilation and oxygen production. This simply means that the production of tilapia in large cages, which are currently widely used, would require a tremendous ecosystem area for the assimilation of wastes resulting from the cages. Tilapia culture in floating cages in Sampaloc Lake (Philippines) is a good example of the catastrophic environmental and socioeconomic impacts of unmanaged culture practices. Tilapia cage culture in the lake was introduced in 1976 and has been expanding since then, with overuse of commercial feeds. It was reported that about 6000 mt of feed are wasted through the cages and settled on the sediments annually, in addition to the accumulation of fish faeces and other organic wastes underneath the cages. This situation has created severe anoxic conditions and toxic waters, resulting in the progressive depletion of dissolved oxygen (DO), a high biological oxygen demand load and extremely high concentrations of ammonia and total sulphides. In 1990, the operators were warned about the possibility of a fish kill due to the huge volume of anoxic and toxic waters underneath the cages and were also advised to harvest their stocks. The warning was ignored, and, as expected, a massive fish kill of market-size tilapia occurred, causing the farmers to lose their investment (Santiago and Arcilla, 1993; Santiago, 1994). A number of fish kills due to eutrophication caused by cage culture have also been reported in Lake Taal in the Philippines (ADB, 2005). Upwelling of deep deoxygenated and hydrogen sulphide-containing water from the anaerobic degradation of fish wastes generally leads to fish kill in the lake. In addition, about 64% of the nitrogen and 81% of phosphorous of the feed provided to caged fishes is released to the environment causing severe eutrophication (ADB, 2005). Farm wastes can also be a major cause of changes in the structure of benthic communities because uneaten feed may favour certain aquatic species over others. Sedentary animals may also suffer from mass mortality when the water is
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depleted of oxygen, while the mobile population may migrate to other areas. Nile tilapia cage culture has also been widely practiced in many natural freshwater masses (rivers, canals, reservoirs, etc.) in Thailand over the past decade. This has led to a rapid increase in the number of tilapia cages and has also created many social, economic and environmental problems. The FAO (2018) listed the following impacts as a sequence of expanding tilapia cage culture in Thailand: 1. High investment cost especially for feed; 2. High risk of declining profitability because of increased fish mortality and low growth rate of fish; 3. Climate change (air and water temperature) which makes fish more susceptible to disease; 4. Change in water level and quality, especially during the dry season; 5. Increased water pollution; 6. Limited number of waterbodies for operating tilapia cage culture and 7. Carrying capacity of tilapia cage culture in natural waterbodies being exceeded. These impacts indicated the urgent need for environmental carrying capacity assessment (ECCA) tool for tilapia planning in Thailand. This assessment will help to establish the number of cages that would allow the most sustainable farming practice, with highest performance and lowest adverse impacts. In Egypt, cage culture of tilapia in the Nile River, particularly in Damietta and Rosetta branches, has expanded at outstanding rats during the past few years (Fig. 14.1). This has increased public awareness and created a debate among governmental authorities, the academics and environmental protection bodies, with respect to the environmental impacts of these cages. The Egyptian
FIGURE 14.1 Several long arrays of tilapia cages in the Nile River (Egypt) may pose a threat to the environment. Photograph by A.-F.M ElSayed.
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authority removed the cages from the Nile branches by force, after warning the farmers several times, taking into account that most of these cage farms were not licenced. This situation has divided the public opinion into two mainstreams. Those in favour of tilapia cage culture suspect that the cages pollute the Nile and claim that this culture system is practiced all over the globe. They suggest that removing the cages is not the solution, whereas proper management, such as limiting the number of cages km 2, use of high-quality feed with optimum feeding strategies, proper selection of cage sites, farming other fish species such as silver carp with tilapia and continuous governmental monitoring and inspections, would certainly minimize the effects of the cages. Researchers, academics and farmers are in favour of this opinion. On the other side, environmental protection authority and governorate administration claim that cage culture is deteriorating the quality of the Nile and is very likely to cause health hazard, and therefore, removing these cages is the only solution. Unfortunately, removing the cages has caused a catastrophic loss to the farmers. Troell and Berg (1997) suggested that intensive tilapia farming in cages in the tropics can generate severe eutrophication. They found that average flux of particulate nutrients (ammonium and phosphates) under tilapia cages in Lake Kariba (Zimbabwe) was up to 22 times greater than in control areas. The release of the nutrients usually stimulates phytoplankton production and other algal blooms, which may further increase oxygen consumption during the decomposition of uneaten food and organic wastes. These algal blooms may also be toxic. Moreover, when the algae die, they settle to the bottom, decompose and further deplete the oxygen in the water, making the already eutrophic conditions worse. The status of tilapia cage culture in Lake Victoria (Kenya) was evaluated in 2016 and 2017 using geographical information system (GIS), standard water quality monitoring procedures and stakeholder questionnaires and interviews (Aura et al., 2018; Njiru et al., 2018). In addition to the remarkable benefits gained from cage culture, significant adverse impacts were recorded. Dissolved oxygen (DO) was much lower inside the cages than outside them. Ammonia concentration around the cages was also much higher than in cage-free areas, due to the protein in uneaten feed and fish waste that had been broken down into ammonia and nitrite. Waste feed increased eutrophication, decreased DO, enhanced blooming of algae and water hyacinth in the lake and led to a number of fish kills. Frequent occurrence of fish disease was also reported and attributed to poor management practices among cage farmers. On the other hand, no consistent environmental changes were reported in the Tanzanian part of Lake Victoria, as a result of tilapia cage culture (Kashindye et al., 2015).
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However, a significant increase in total nutrient (phosphorus and nitrogen) concentrations was observed in the cage locations after the set of cages, compared to offshore areas. An increase in phytoplankton, bivalves and gastropods communities was also observed after stocking of fish in cages. Regular fish kills due to eutrophication caused by intensive tilapia cage culture have also been recorded in Cirata Reservoir, Indonesia (Abery et al., 2005). Similarly, mass mortalities of tilapia farmed in cages in different rivers in Thailand due to illegal release of untreated effluents from different factories, especially in the Chao Phraya River, has also been reported (Edwards, 2015).
14.4 Bioactive compounds Bioactive compounds, including pesticides, antibiotics and other therapeutic drugs are used in aquaculture for various reasons, including disease control, fertilization, liming, disinfection, oxidation, coagulation, pesticides and adsorption. They are generally added to feed and/or culture water. Some of these compounds, especially drugs, pesticides, and antibiotics may be toxic to aquatic animals and/ or may accumulate in the environment. Aquaculture effluents containing these compounds may therefore produce adverse ecological impacts, in addition to the possible contamination of the flesh of aquatic animals which could pose a hazard to consumers. Rico et al. (2014) found that the maximum concentrations of the most commonly used antibiotics, oxytetracycline and enrofloxacin, in the Tha Chin River (Thailand), in which tilapia cage culture is widely practiced, was much higher than the concentrations recorded in other Asian aquatic environments receiving aquaculture effluents. The peak antibiotic concentrations in the river water occurred during antibiotic administration in the tilapia cages, presumably due to the leaching of antibiotics from fish pelleted feed. Antibiotics accumulated mainly underneath or next to the tilapia cages, suggesting that water-sediment sorption, fish waste and wasted feed are responsible for the transport of antibiotics to the sediment. Accumulated antibiotics were widely distributed in the river and were found to remain in the river sediments for several weeks. These results clearly indicated that tilapia cage aquaculture is an important source of antibiotic pollution.
14.5 Hormones As mentioned in Chapter 8 (Section 8.8.2), steroid hormones are widely used for producing monosex tilapia. Hormones are generally incorporated either with larval feeds or through the immersion of fertilized eggs or sac fry. However, the use of hormones for sex reversal has been under increasing public criticism due to their potential
health and environmental impacts. The hormone residues and metabolites can be a potential environmental contaminant. For example, feeding tilapia with 17-a methyltestosterone (MT)-treated feed has resulted in considerable ‘leakage’ of MT into pond water and sediments (ContrerasSánchez et al., 2001, 2002). MT has been detected in the water during MT treatment, accumulated and remained in the sediments of the ponds for up to 8 weeks. Hormone traces pose a risk of hatchery workers and also of other nontarget aquatic organisms. MT traces remained at 2.8e2.9 ng g 1 in soils for 3 months and in water at 3.6 mg L 1 for 2 weeks, before returning to the base level (up to 0.02 mg L 1), which has been reported in tilapia masculinization ponds (Homklin et al., 2009, 2012; Megbowon and Mojekwu, 2014). MT leakage to the environment can also disrupt the normal sex ratio in wild fish populations. The human food safety and environmental hazards associated with the use of MT and other steroids in production of sex-reversed tilapia have been comprehensively reviewed by Mlalila et al. (2015). Many tilapia farmers and hatchery operators in Egypt use higher amounts of MT hormones than they really need (A.-F.M. El-Sayed, personal survey, 2017), leading to accumulation of hormone residues in the environment. This practice may lead to adverse ecological and human health impacts. For example, the sex ratio of Nile tilapia in the canals and drains adjacent to fish farms and hatcheries in Kafr El-Shaikh Governorate (Egypt) has dramatically changed in favour of males (A.-F.M. El-Sayed, 2018; personal observation). Catching wild females, as broodstock for breeding, from these waterbodies becomes very difficult, since most of the catch is males. The above finding supports the argument that the exposure of untargeted organisms to steroid hormones can result in biased sex ratios. In support, significant masculinization occurred in common carp exposed to water used in MT-impregnated feeding trials (Gomelsky et al., 1994). Abucay et al. (1997) also reported high rates of masculinization and feminization in sexually undifferentiated Nile tilapia fry reared in aquarium water previously used for oral application of hormones used for sex reversal. Also, high proportions of males were found in Nile tilapia fry fed with normal feed in cages adjacent to cages containing MTincorporated feed (Abucay et al., 1997). These findings clearly demonstrate that hormone (or hormone metabolites) leaching from uneaten food can induce sex reversal in exposed untargeted organisms.
14.6 Human health risks As mentioned earlier, many contaminants from land-based run-offs and anthropogenic activities, including organic pesticides, heavy metals, polychlorinated biphenyls,
Environmental impacts Chapter | 14
hormones, etc., end up in the aquatic environments, causing a wide range of environmental impacts. Farming tilapia in these environments may also pose human health risks, depending on the source, exposure time and concentration of the contaminant. For example, Ling et al. (2009) assessed the heavy metals (arsenic, As; chromium, Cr; copper, Cu; manganese, Mn; nickel, Ni; lead, Pb; selenium, Se and zinc, Zn) in tilapia collected from farms located in industrialized (polluted) areas in Taiwan, compared with samples collected from unpolluted areas. They found that the concentrations of almost all of these heavy metals were higher in the body tissue of tilapia farmed in the polluted areas than in fish grown in relatively clean areas. These results indicated that heavy metals in tilapia muscles may pose a significant risk to residents in the vicinity of industrial zones. Excess lifetime cancer risk and hazard quotients for residents consuming contaminated tilapia located in the vicinity of industrial zones were also higher than those for residents consuming tilapia farmed at the uncontaminated areas. Similarly, the concentrations of heavy metals in Mozambique tilapia (O. mossambicus) collected from ponds receiving domestic effluents in Malaysia were significantly higher than in fish collected from uncontaminated ponds (Yap et al., 2015). However, health risk assessments and the amount of fish required to reach the provisional tolerable weekly intake, estimated daily intake and target hazard quotient revealed that no health risks were associated with heavy metals intake via consumption of tilapia collected from ponds receiving domestic effluents. In addition to their potential environmental impacts, steroid hormones applied in aquaculture may also pose human health risks. However, the effects of these steroids depend mainly on their type, doses, exposure time, pharmacokinetic characteristics and genetic susceptibility. There has been a long debate among societal and scientific communities on whether the MT-treated fish are safe for human consumption. The fish carcass, viscera, blood, and muscle of treated fish have been investigated for MT and its metabolites. Available information and argument concluded that MT-treated fish are safe for human consumption. Pandian and Kirankumar (2003) reported that the estimated residual steroids are less than 5 ng g 1 fish, which is too low to cause any human health risks. In support, Green and Teichert-Coddington (2000) demonstrated that the levels of MT and metabolites in tilapia carcass were less than 100 pg g 1 after 8e40 days of withdrawal, and less than 10 pg g 1 after 6e50 days of withdrawal. Once again, it is very unlikely that these low hormone levels can cause any health problems to humans. In addition, no cases of pharmaceutically active MT and its metabolites from aquaculture effluents to surrounding environments have been reported (Mlalila et al., 2015).
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14.7 Reducing environmental impacts 14.7.1 Effluent management The treatment and management of tilapia culture effluents have been described in details in Chapter 6 (Section 6.8). Boyd (2003) listed the following best aquaculture management practices (BAMPs) for minimizing nutrient loads in aquaculture effluents: 1. Use fertilizers only as needed to maintain phytoplankton blooms. 2. Select stocking and feeding rates that do not exceed the assimilation capacity of ponds. 3. Feeds should be of high quality, water stable and contain no more nitrogen and phosphorus than necessary. 4. Apply feeds conservatively to avoid overfeeding and to assure that as much of the feed is consumed as possible. 5. Do not use water exchange or reduce water exchange rates as much as possible. 6. In intensive aquaculture, apply enough mechanical aeration to prevent chronically low DO concentration and to promote nitrification and other aerobic, natural water purification processes. 7. Provide storage volume for heavy rainfall to minimize storm overflow. 8. Deep water release structures should not be installed in ponds, for they discharge lower-quality water from near pond bottoms. 9. Where possible, seine-harvested fish without partially or completely draining ponds. 10. Where possible, discharge pond draining effluent through a settling basin or a vegetated ditch. 11. Reuse water where possible. While these practices are valid for reducing nutrient loads in tilapia culture, there are certain practices that have been exceptionally successful in effluent management at the farm level. Extensive research was conducted at the Asian Institute of Technology (AIT) in Thailand to mitigate the environmental impact of tilapia culture through reducing nutrient loading in pond culture of Nile tilapia. Special attention was given to (1) optimization of feeding regimes; (2) maximizing nutrient utilization through integration and recycling systems and (3) minimizing waste loading from effluents through appropriate draining techniques during harvest. Lin and Yi (2003) described the results of these practices. It would be useful here to throw light on these results.
14.7.1.1 Optimization of feeding regimes When Nile tilapia reared in fertilized ponds were fed supplemental diets at 50%, 75%, and 100% satiation, they
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produced comparable yields (Lin and Yi, 2003). However, considerable reduction in nutrient loading was achieved at the 50% level. This means that about 50% of the feed can be saved, since the fish are able to supplement their diets with natural food available in the pond. This also indicates that an optimal feeding regime may reduce both feed costs and nutrient loading in the ponds.
14.7.1.2 Reducing effluent impact with a cage-cum-pond integrated system Tilapia culture in a cage-cum-pond integrated system has been described in details in Chapter 5 (Section 5.6.3) and Chapter 6 (Section 6.4.5). The system involves rearing hybrid catfish (Clarias macrocephalus Clarias gariepinus) in cages in ponds and feeding them with formulated feeds, whereas their wastes are used as a natural food for tilapia reared in open pond. This system yields considerable production of catfish and tilapia and also reduces nutrient loading through the use of fish wastes for pond fertilization and natural food production.
14.7.1.3 Effluent management during harvest Seining fish ponds with nets, despite being most common, stirs bottom sediments, leading to the deterioration of water quality and its effluents. Appropriate harvest methods and draining treatments can significantly reduce pollutants from Nile tilapia ponds. In a novel study, Lin et al. (2001) evaluated four treatments to determine the efficiency of fish harvest and effectiveness in reducing nutrient loading in the effluents. The treatments were (1) treating the ponds with tea seed cake (10 ppm) to partially anaesthetize Nile tilapia 1.5 h prior to harvest by seining, (2) liming the ponds (75 g m 3 of calcium hydroxide) 24 h prior to harvest to precipitate phosphorus and organic matter, followed by sequential complete draining with a pump, and Nile tilapia collected from a harvesting pit, (3) draining the ponds by sequential complete draining with a pump and Nile tilapia collected from a harvesting pit and (4) ponds drawn down from 100 to 50 cm with a pump and Nile tilapia harvested by seining twice, followed by complete draining and collection of the remaining Nile tilapia from a harvesting pit. The results of that study revealed that liming the pond 24 h before draining and gradually draining ponds to a 25-cm depth during harvest were most effective.
14.7.2 Reducing nutrient loading through green water tank culture Green water tank culture system can be an efficient method for reducing nutrient loading in fish tanks. In this system, fish wastes and other metabolites are treated in such a way to increase natural food production in the tanks. This
FIGURE 14.2 Growing tilapia in green water system in Virgin Islands, USA. Photo provided by J. Rakocy.
process involves the oxidation of toxic ammonia (NH3) and nitrite (NO2) into relatively nontoxic nitrate (NO3) through nitrifying bacteria grown on suspended organic matter. The bacteria remove the organic matter from the tanks and use it as food, while the bacteria are used as natural food for filter-feeding fishes such as tilapia and carp. However, in green water systems, solid wastes should be removed continuously, and both biofiltration and aeration should be provided. Vigorous aeration is also necessary to support the suspension of the microbial community, maximize contact between bacteria and waste products and, in turn, increase phytoplankton productivity. This system has been successfully used for tilapia culture in various parts of the world, particularly in the Virgin Islands (Fig. 14.2). The advantages of green water culture are summarized in Chapter 6 (Section 6.6).
14.7.3 Removing nutrients through planting rooted plants In semi-intensive tilapia culture, about 80% of the nitrogen and phosphorus added in fertilizers settles to the pond mud (Lin and Yi, 2003). The removal (de-sludging) of pond mud is tedious and labour-demanding. Planting rooted aquatic plants in fish ponds can be an effective way of extracting nutrients from the mud. This approach has been tested with lotus (Nelumbo mucifera) seedlings planted in Nile tilapia ponds fertilized weekly with urea and triple superphosphate at a rate of 4 kg N and 1 kg P ha 1 d 1 (Yi et al., 2002). Lotus effectively removed nutrients from pond mud. The annual nutrient losses from the mud were 2.4 t N and 1 t P ha 1, about 300 kg N and 43 kg of which were taken by lotus. In temperate zones, water temperature drops during winter times and becomes unsuitable for tilapia culture. Generally, tilapia farmers in these regions harvest their fish during SeptembereNovember. During winter season, these ponds are dried and become useless. Growing land winter crops, such as wheat and alfalfa, in these ponds can be an effective method of removing nutrients from pond muds in addition to generating an additional income to the farmer.
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FIGURE 14.4 Growing plants with tilapia in an aquaponic system is an effective way of removing nutrients from culture water. Photo provided by J. Rakocy.
FIGURE 14.3 Growing (top) and harvesting and threshing (bottom) a wheat crop in tilapia ponds in Kafr El-Shaikh Governorate, Egypt. Photograph by A.-F.M. El-Sayed.
This approach has been tested in some areas in Egypt, where alfalfa and wheat have been grown in tilapia ponds during the winter, with promising results (El-Sayed, 2017) (Fig. 14.3).
14.7.4 Removing nutrients through aquaponic systems Aquaponics is the integration of hydroponics with aquaculture in a recirculating system. The wastes and metabolites produced by cultured fish are removed by nitrification and taken up by the plants, while the bacteria living in the gravel and in association with the plant roots play a critical role in nutrient removal (Fig. 14.4). This means that the plants act as a biological filter by removing fish wastes and improving the quality of culture water. Many studies have been conducted on aquaponic systems at the University of Virgin Islands Agricultural Experimental Station and also in other geographical regions, using tilapia and various vegetables, with considerable success. The results of these studies are discussed in Chapter 6 (Section 6.9).
14.7.5 Removing nutrients through integrated multitrophic aquaculture systems Integrated multitrophic aquaculture (IMTA) systems are designed mainly for efficient use of resources. IMTA
systems combine species of different feeding habits and trophic levels in the same aquaculture system. In this combination, the biological and chemical processes are balanced because the waste from one species can be used as food for the other(s). For example, farming tilapia and prawns in polyculture system can be an efficient way of nutrient removal and improving water quality. They have different feeding habits and special distribution and thus can behave synergistically. In a polyculture system, tilapia consume artificial feed and natural foods, while prawns can eat tilapia wastes and unused tilapia feed. In addition, prawn bioturbation at the pond bottom brings the nutrients back to the water column and enhances phytoplankton production, which in turn increases natural food availability for tilapia. In this regard, David et al. (2017a) found that the levels of phosphorus retained from tilapia/prawn IMTA system were higher (20%e28%) than in tilapia and prawn monocultures (10%e20% and 10%e13%, respectively). The same authors (David et al., 2017b) suggested that the integration of Nile tilapia and Amazon River prawn in stagnant ponds may remove significant amounts of nitrogen from nutrient-rich water and could be considered as a mitigation tool. These results clearly indicate that IMTA systems can be used in mitigation programs for aquatic environments receiving high levels of nutrients, especially phosphorus and nitrogen. In-pond raceway system (IPRS) (Fig. 14.5) can be an excellent model for nutrient removal or recovery. In this system, one fish species is raised in raceways installed inside an earthen pond and fed with high-quality feed. Another species (or more) is stocked in the rest of the pond, mainly for water quality improvement, since it depends on the wastes of the raceways for feeding (see Chapter 13, Section 13.10.4 for more details on IPRS). Brown et al. (2012) raised channel catfish (Ictalurus punctatus) in in-pond raceways, while Nile tilapia and paddlefish (Polyodon spathula) were co-cultured in the pond where the raceways were installed. The harvest of Nile tilapia and
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FIGURE 14.5 Growing Nile tilapia in in-pond raceways at the WorldFish Center, Abbassa, Sharkya Governorate, Egypt. Photograph by A.-F.M. El-Sayed.
paddlefish increased phosphorus and nitrogen recovery/ removal by 47.5% and 40%.
14.7.6 Reducing environmental impacts through best aquaculture management practices In addition to the technical and practical measures that should be adopted to reduce the adverse impacts of aquaculture, the BAMPs can play a significant role in preventing these adverse effects. BAMPs in aquaculture can be defined as ‘the application of good management practices to support sustainable aquaculture growth, with minimum risks to the environment, through the adoption of costeffective and continually assessed management measures’ (White et al., 2018). A number of recent documents have been published on the effects of BAMPs on aquaculture performance (tilapia included) including the following: l
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Better management practices for tilapia aquaculture (WWF, 2011); On-farm Feeding and Feed Management in Aquaculture (Hasan and New, 3013). This publication included 6 case studies/reviews on ‘On-farm feed management practices for Nile tilapia (Oreochromis niloticus)’ in six countries (China, Egypt, Ghana, Malaysia, the Philippines and Thailand). Pilot application of selected aquaculture planning and management tools in Indonesia, Thailand and Vietnam (FAO, 2018); Better management practices for feed production and management of Nile tilapia and milkfish in the Philippines (White et al., 2018).
This is in addition to several papers published in different specialized journals on the best management practices
(BMPs) of tilapia culture and their social, economic and environmental effects. A number of examples are provided on the adoption of the BAMPs in tilapia culture and their effects on environmental performance and disease reduction. In Thailand, for example, cage culture of Nile tilapia has been widely practised in many natural freshwater environments over the past decade. This practice has led to a rapid increase in the number of tilapia cages and has also created many social, economic and environmental problems. These problems are listed in Section 14.3. These sequences demonstrate the urgent need for ECCA tool for tilapia planning as a means for BMPs for tilapia cage culture in Thailand. This assessment tool will help to establish the correct number of cages that would allow the most sustainable farming practice, with highest performance and lowest adverse impacts. Another interesting example, also in Thailand, is the spreading of diseases due to the expansion of tilapia cage culture in Chainat Province and the role of BAMPs in reducing their risk impacts. Several irrigation projects have recently been developed in this province for agricultural development, and aquaculture as well, especially cage culture in the Noi River. As a result, the number of tilapia cage farms has rapidly increased, leading to significant adverse impacts, including disease occurrence due to the changes in the environment, water quality, high fish density, inappropriate farm management and the lack of control of fish disease/pathogens (FAO, 2018). To face this problem, a farm biosecurity plan has been developed and put in place to describe the measures and actions that should be followed to protect farms from diseases. These measures should be able to reduce the risks and impacts of disease outbreaks and prevent health issues emerging within the cage farms. To maximize its benefits, the farm-level biosecurity plan should be piloted and disseminated to fish farmers, hatcheries and other concerned stakeholders. A field implementation of the farm-level biosecurity plan tool included the following: (1) providing training for 25 selected farmers; (2) the biosecurity plan for each farm was adopted; (3) the biosecurity plan was adopted by 25 selected cage tilapia farms; (4) on-farm implementation of the biosecurity plan was strictly monitored for 3 months, with continuous data collection and (5) a national evaluation was conducted. The results showed that trained farmers had a better understanding about fish diseases and their prevention. Farmers are now able to check cages and collect dead fish from cages daily and no longer throw the dead fish into the river. They also record the necessary farm information such as date, fish size and number at stocking, the daily amount of feed provided and fish mortality. This would certainly lead to a reduction of overfeeding and minimizing the accumulation of feed waste and losses.
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The adoption of best on-farm feed and feeding management would certainly lead to a reduction in wasted feed and fish waste, better growth and feed utilization, lower organic and inorganic loads in aquaculture effluents and in turn, lower environmental impacts. The following feed and feeding practices should be adopted to improve feed efficiency and reduce the amount of waste entering the environment (see Hasan and New, 2013; White et al., 2018 for details): l
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Relevant practical knowledge and skills in feed and feeding should be gained. The best available ingredients should be used for feed production. Species-specific feeds containing the correct nutrient requirements should be developed, with good pellet integrity, shape, density and water stability. The most appropriate feed type (floating vs. sinking pellets), feeding methods (manual feeding, demand feeder, automatic feeder), feeding levels (percent body weight, ad libitum, satiation) and feeding frequencies should be used. Farmers, farm managers, hatchery operators and feeding staff should be trained on the most appropriate on-farm feed handling, storage and use. Water quality should be monitored continuously. Appropriate initial fish stocking number and size should be used.
14.8 Closing remarks 1. The introductions and/or transfers of tilapia can be unsuccessful, successful or successful but with environmental impacts ranging from habitat destruction, hybridization with endemic species, to disappearance of native species. Full details on the environmental impacts of tilapia introductions are provided in Chapter 3. 2. Varying amounts of uneaten feed, fish wastes, organic matter, nutrients such as phosphorus and nitrogen, hormones and bioactive compounds are released from land-based and cage culture of tilapia. If these wastes are not removed or treated, they will adversely affect the environmental impacts, produce anaerobic conditions that stress and/or kill the fish. 3. The public criticism concerning the use of steroid hormones for sex reversal in tilapia is increasing because hormone residues and metabolites can pose health and environmental impacts. 4. Various techniques, including effluent management, green water tank culture, planting rooted plants, aquaponic systems and IMTA systems have been successfully used for reducing environmental impacts of tilapia culture.
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5. The BAMPs should be adopted for minimizing nutrient loads in tilapia effluents and reducing their environmental impacts. The adoption of best feed and feeding management will lead to a reduction in wasted feed and fish waste, better growth and feed utilization, lower organic and inorganic loads in aquaculture effluents and, in turn, will lower the overall environmental impacts.
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