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Livestock, Climate Change and the Environment |
There is an increasing demand to develop sustainable and environmentally friendly aquaculture systems, including a better integration between agriculture and aquaculture, introduction of new fish species and using alternative feed resources. Wild fish use in aqua feeds, in the form of fish meal and fish oil, must be reduced if aquaculture production is to maintain its contribution to human fish supplies. In addition, the feed utilization has to be improved in order to minimize its negative environmental impact in terms of nitrogen and phosphorus discharge. Moreover, the regular use of chemicals and anti-microbial additives in intensive aquaculture has to stop.
In order to reduce poverty and hunger, and to feed a growing human population, there is an obvious need to increase global food production. However, this has to be accomplished within sustainable and environmentally safe food production systems.
Three out of four poor people in developing countries live in rural areas and most depend on agriculture (crops, livestock, agro-forestry and aquaculture) for their livelihoods. More than two billion people in developing countries live on less than 2 US $ a day (World Development Report 2008). The solution to improve the livelihood for these resource-poor people is to develop sustainable small-scale food production systems. Gross domestic product (GDP) growth originating in agriculture is considered at least twice as effective in reducing poverty as GDP growth originating outside agriculture (World Development Report 2008).
Recently, a total of ten planetary boundaries has been identified that must not be transgressed in order to prevent unacceptable global environmental changes (Rockström et al 2009). All planetary boundaries are tightly coupled, and each one more or less directly affected by human activities. Three out of ten (i.e. climate change, rate of biodiversity loss and the nitrogen cycle) boundaries have already been transgressed, while others (i.e. the phosphorus cycle, stratospheric ozone depletion, ocean acidification, global freshwater use and change in land use) are coming close to being transgressed if the current global development continue unchanged. Thus, in order to save the global environment for future generations, the development of food production has to take place within defined planetary boundaries.
The worldwide decline of ocean fisheries stocks has provided impetus for rapid growth in fish and shellfish farming (FAO, 2009). However, farming of fish, in particular carnivorous species such as the Salmonidae, requires large inputs of wild fish for production of feed (Naylor et al 2009). This also applies to the rapidly growing farming of striped catfish (Pangasianodon hypopthalamus) in the Mekong delta, where the use of fish meal and fish oil in the feed is extensive (Phan et al 2009).
If the growing aquaculture industry is to maintain its contribution to world fish supplies, it must reduce wild fish inputs in feed and adopt a more sustainable fish production (Naylor et al 2000; 2009). Therefore, there is an increasing demand for sustainable alternative fish culturing systems, including new fish species and the use of alternative feed sources. At present, nutrients from plant sources receive most attention as an alternative to fishmeal and fish oil in aqua feeds (FAO 2009). However, there may also be other protein sources, based on microorganisms, that could have potential to be used in fish feed (Naylor et al 2009; Kiessling 2009). This calls for better knowledge of the nutritive value of non-conventional feedstuffs that could potentially be used to replace fish meal (Edwards 2004), in particular in low-cost fish feed suitable for small-scale farmers in developing countries. In recent years, research on the use of locally available feed resources, such as agricultural by-products, industrial waste and animal processing by-products, in fish feed has increased (Alan et al 2000; Sklan et al 2004; Gatlin et al 2007).
Intensive fish culture systems are dependent on a continuous supply of complete feeds that should provide energy and all required nutrients to support a high growth rate and maintain a high health status. The provision of an inadequate diet imposes potential risks for an improper energy and nutrient supply as well as an improper balance of essential nutrients (Edwards 2004; Hung 2004). This will have a negative impact on growth rate and could result in the occurrence of various diseases, depending on the specific conditions and the fish species involved (Roberts 2002).
The nutrition of larvae and fingerlings comprise different challenges, and both areas need to be addressed in order to improve the productivity of small-scale fish production (Edwards 2004). Fish larvae eat live food (zooplankton) and are dependent on a high availability of live food with the right properties (size and nutrient content) in order to survive and grow. Zooplankton lives on algae and therefore the production of high quality zoo plankton for fish larvae requires a production of algae with suitable properties. There is a great need to develop techniques and feeding systems that allow an efficient nutrient supply to fish larvae from different fish species and thereby also make the production of fingerlings more successful and resource efficient.
Fingerlings require a high and continuous supply of organic matter with the correct energy and nutrient content to grow at their potential capacity. In current small-scale aquaculture systems there is an abundant use of animal manure and effluent from biogas production as feed for fish. This practice will not utilize the full growth potential of the fish due to incorrect supply of energy and nutrients, and improper balance of essential nutrients (Allan 2004; Edwards 2004; Hung 2004). Thus, current feeding practices are often inefficient and the potential aquaculture production can not be realised due to the use of poor dietary strategies in small-holder farms. This has a negative impact on the livelihood of small-holder farmers. By introducing supplemental feeding in small-scale fish cultures the potential productive capacity can be realised, which will contribute to the long-term sustainability of these systems (Choulamany 2004; Heng et al 2004).
There is no demonstrated dietary requirement for carbohydrates in fish (Wilson 1994). However, if no carbohydrates are provided in the diet protein and lipids will be catabolized to provide metabolic intermediates for the synthesis of important cell components. Thus, carbohydrates may have a protein and lipid saving effect and it may therefore be important to provide carbohydrates in the diet of cultured fish to support vital metabolic processes.
The utilization of dietary carbohydrates will depend on several factors, such as carbohydrate origin, inclusion level, physical state and molecular complexity (Stone 2003). Certain fish species utilize simple sugars as well as starch, while others do not utilize simple sugars as an energy source (Wilson 1994; Stone 2003). The ability to digest starch varies and appears to be linked to the nutritional habits of the respective fish species (Hildago et al 1999; Stone 2003). In general, cooked starch (gelatinized) appears to be better utilized than raw starch by many fish species (Wilson 1994; Peres and Oliva-Teles 2002). Moreover, it should in theory be possible to increase the availability of monomer sugars from various plant carbohydrate polymers by enzyme treatment. However, this may have detrimental effects on fish performance and health due to low tolerance for some sugars (Stone 2003). Starch over-feeding (0 to 30 % starch included in the diet) in southern catfish (Silurus meridionalis) resulted in increased fasting metabolic rate, interpreted as an attempt to oxidize unwanted assimilated carbohydrate (Fu and Xie 2004).
In general, warm-water omnivorous fish have a greater potential to utilize dietary carbohydrates as an energy source than do cold-water and carnivorous fish species (Stone 2003). However, some cold-water and carnivorous fish will utilize dietary carbohydrates poorly or not at all (Wilson 1994; Stone 2003).
The intestines of carnivorous fish have evolved for processing highly digestible diets with high protein content and low carbohydrate content (Buddington et al 1997). The omnivorous and herbivorous fish species on the other hand have high capacity to utilize carbohydrates of different origin and complexity (Buddington et al 1997). The uptake capacity of the intestine for glucose is much higher in herbivorous fish than in carnivorous fish, while there was much less variation in the uptake of proline (Buddington et al 1987). As the fish were fed the same diet, this was interpreted as a genetic adaptation to different natural diets.
In addition to protein, most plant protein sources contain a large fraction of carbohydrates, which due to different properties will vary in digestibility (Gatlin et al 2007). Some fibre carbohydrates may be classified as anti-nutritional, as they may have marked physiological effects on the animal, often resulting in poor performance and health issues. In particular, the Salmonidae appear to be sensitive to a number of these substances and in addition also high levels of carbohydrates. It has been shown that whole soy causes everything from reduced protein and mineral digestion to severe inflammation of the hindgut in salmon, resulting in diarrhoea and possibly compromised welfare (Baeverfjord and Krogdahl 1996).
The use of inorganic fertilizer is not a common practice in aquaculture, except for nursing stages. During this period fertilizers are applied for stimulation of algae growth. This leads to the growth of zooplankton, which are the preferred natural food for fish larvae and juveniles. However, the use of inorganic fertilizer only to stimulate natural food in aquaculture systems is not recommended without manure to stabilize the biomass production. In the nursing period, farmers often use soybean meal and fish meal as nutrient inputs to fish ponds but also as direct feed for larvae which will also stimulate zooplankton development.
Animal manure is frequently used in integrated systems in south-east Asia to maximize the use of on-farm resources. In some places, farmers collect manure from their neighbours to fertilize their poly-culture ponds. Others construct a pigsty or chicken house on fishponds or nearby fishponds to collect manure dropping into fishponds. The application rate for animal manure is vague, especially in integrated systems. Chicken, pig and duck manure have high nitrogen content and are most frequently applied in aquaculture systems. However, direct application of manure to fish ponds is not a common practice in all countries in south-east Asia due to the competition of using manure for cropping.
Besides animal manure, some green fodders can also be composted to form another type of fertilizer. In aquaculture, green fodders are usually composted within fish ponds. The collected plants are placed in a pond corner as a means of composting. The nutrients released then stimulate natural foods to develop which are utilized by fish in poly-culture systems.
The south-east Asian region is characterised by a rapid urbanisation, industrial development and uncontrolled utilisation of natural resources. Environmental toxicity has so far been neglected in most countries in the region. However, primary investigations have revealed that many water resources are contaminated by anthropogenic pollutants that reduce the production of aquatic animals, and eventually become a public hazard. The environmental situation in south-east Asia is very complex. A large variety of pesticides are in use for pest control. High concentrations of DDT and metabolites with known oestrogen-like activities have been recorded both in fish and in crocodiles in major fresh water systems in the area. Spreading of environmental toxicants and their impact in receiving recipients is poorly investigated in countries in the region.
The use of sewage and manure in aquaculture systems involves a potential risk for spreading of pathogens to fish and humans. In addition, use of raw non-cleaned water, which could be contaminated with various chemical compounds, could impose other risks both to fish and humans.
Aquaculture production can play a key role in providing food with high nutritional value to a growing human population. However, this has to be accomplished within sustainable and environmentally safe production systems. This calls for re-designing existing large-scale industrialized aquaculture systems, largely based on wild fish inputs in feed and with regular use of chemicals and anti-microbial additives, which cause environmental problems.
Research is needed to find alternative sources of protein and oil to be used in diets for fish and other cultivated aquatic animals, as well as to identify and explore fish species that are suited to utilize alternative feed resources. Moreover, to secure high food safety in aquaculture-based food products more focus has to be placed on water sanitation and pollution.
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