Animal Waste and Agro-by-Products: Valuable Resources for Producing Fish at Low Costs in Sub-Saharan Countries

Animal and crop production throughout the world generate high amounts of wastes or by-products annually that may possess added value compounds with high functionality. These wastes and by-products may cause negative environmental impacts and significant expenses if not well managed and or controlled. Much of these wastes and by-products is valuable and cheaper source of potentially functional compounds such as proteins, lipids, starch, micronutrients, bioactive compounds, and dietary fibbers. In aquaculture, feed is expensive, and the existing body of literature has shown that animal manure and its extracts can be successfully incorporated into fishpond to increase fish production at a low cost. In addition, crop residues such as rice bran, maize bran, and seed cakes are commonly used as pond inputs in small-scale aquaculture. Animal waste and crop residues are added in a fishpond that filter-feeding fish can use directly as feed, and these may form a major proportion of the detritus in the pond. These resources also stimulate the growth of phytoplankton that are rich in protein and are the basis of the food web that can support the growth of a range of herbivorous and omnivorous fish. Therefore, technically, wastes are used as direct feed, a source of minerals for autotrophic production and a source of organic matter for heterotrophic production. In this context, animal manure and crop residues have been used to provide great opportunities to improve food security. The purpose of this review is to project the potential of animal waste and agro-by-products as a sustainable alternative as aquaculture inputs to reduce poverty, malnutrition, and hunger in developing countries.


Introduction
Aquaculture is one of the world's fastest growing food production sectors with great potential for food supply, poverty alleviation, and enhanced trade and economic benefits, as targeted by sustainable development goals SDGs. The contribution of aquaculture to global fish supply increased from 3.9 percent in 1970 to over 41.3 percent in 2011 amounting to 63.7 million metric tonnes valued over USD 119 billion [1]. Its average growth rate of 8.8 percent has outpaced capture fisheries (1.2%) and terrestrial farmed meat production (2.8%) [1]. Aquaculture accounts for around 50 percent of seafood supply globally [2]. This quantity is expected to increase substantially as population increases (Figure 1). Aquaculture has gained much importance globally due to a decline in wild stock from natural water bodies; thus, aquaculture plays a key role in augmenting dwindling catch capture fisheries. It is well known that among other challenges facing the aquaculture sector, availability and quality of feeds affect its growth particularly in sub-Saharan (see for example [4][5][6][7][8][9][10]). Despite this challenge, aquaculture has been considered as one of the economic activities that contribute to poverty reduction, food security, and nutrition in the sub-Saharan Africa [4,11,12] and Asian countries [1,[13][14][15][16][17].
In order to realize the contribution of aquaculture in the alleviation of poverty and improvement of food security, development agencies should broaden their focus beyond poor/subsistence producers to include small and medium enterprises adopting a value chain perspective [18]. Bangladesh, which is among developing countries, has proven that aquaculture intervention in resource poor and marginalized group marked an increase in income savings and frequency of fish consumption [19]. Although small-scale fish farmers play a big role in poverty reduction and food security, the intensification from extensive to semi-intensive is essential [20]. However, for the intensification to take place, there should be an increase in investment in technological innovation and transfer through (i) Nutrition, feeds and feeding management, and (ii) low-impact production systems. This paper discusses  the valorization of animal waste/by products and plants/crops-by-products to produce fish at low cost in order to increase nutrition and reduce food insecurity.

Aquaculture production in Africa
Africa's fisheries output is dominated by capture fisheries, but the contribution of aquaculture to the total amount of fish produced in the region has grown at a steady pace over the past decade (Figure 1). In these countries, fish is produced from capture fisheries and aquaculture. However, fish catches from wild sources have been declining, due to multiple anthropogenic pressures including climate change, overfishing, habitat destruction, invasion of non-native species, illegal, and unregulated fishing, and poor governance [21]. For example, consumption in the Eastern Africa region was projected to increase from 4.80 kg in 2013 to 5.49 kg by 2022 [22]. This implies that in order to meet the gap between fish production and the increasing demand for food fish, aquaculture production must double by 2050 to satisfy the Africa's fast-growing human population [23]. An appropriate way of keeping this sector growing constantly is the development of new researches aimed at determining the benefits of using different and cheap resources of feeds and determining how these strategies influence economic and productive parameters.

Country
Total production Percent In the aquaculture sector, Africa produced about 1400 tonnes of fish from freshwater aquaculture in 2018, but most of this came from Egypt, which contributed more than 70 percent of the total production (Figure 1, Data obtained in [3]). Major aquaculture producers in 2018 with more than 10,000 tonnes include Egypt, Nigeria, Ghana, Uganda, Zambia, Kenya, Tanzania, and Zimbabwe. Production has increased three times for the past ten tears from 563,000 in 2008 to 1,440,000 in 2018 (Figure 1, [3]). In general, African aquaculture production is overwhelmingly dominated by finfishes (99.3%), with only a small fraction of production from marine shrimps and mollusks [23]. Among the freshwater cultured finfishes, tilapia farming is the main product, which is also the most popular fish from a consumer perspective. Aquaculture production in Africa is also increasing as presented in Table 1.

Aquaculture production systems
In Africa, aquaculture systems are made up of extensive and semi-intensive systems. Small-scale earthen ponds (extensive systems) are characterized by low inputs and low yields. However, semi-intensive systems are characterized by human intervention where by fertilization is done to improve feed availability, hence, improved fish yield. In East Africa, semi intensive mainly used to produce Oreochromis niloticus and Clarias gariepinus in either monoculture or polyculture [24]. They consist mostly of earthen ponds, liner ponds and concrete ponds.
Other systems include cage particularly in areas with large water bodies including East Africa great lakes. Cage culture involves holding organisms under captivity within an enclosed space while maintaining free exchange of water. Cages use the existing water bodies, therefore, require comparatively low capital outlay and use simple technology, they can be used not only as a method for producing cheaply and high-quality protein but also for cleaning up eutrophicated waters through the culture and harvesting of caged planktivorous species. Although fish farming in cages in the existing water body is considered inexpensive relative to pond construction and its associated infrastructures [25], the feasibility and profitability of fish cage culture is influenced by the cost of input invested and revenue collected from output.
Although not common, re-circulating aquaculture system (RAS) has been used in some countries particularly South Africa. RAS refers to a fish farming technology that reuse wastewater from tanks/rearing premises [26]. Water reuse in RAS is supported by both inline and end pipe treatment using a series of mechanical filter for solid waste removal, bio-filter for dissolved nitrogenous waste removal and sludge pond to settle suspended solid [27]. RAS technology is termed as sustainable advanced production system that provides constant and independent production conditions and reduces water consumption compared with semiintensive pond aquaculture, RAS technology provides high fish productivity with better effluent control of environmental conservation [28,29]. Some of the sub Saharan countries have benefited from high temperature to which RAS performs efficiently [30]. The adoption of the system is low due to high cost of initial capital investment in tanks and high cost of electricity required in running the system and feeds. This has therefore called for sustainable aquaculture by integration of fish with livestock. Such integration involves the recycling of livestock wastes and processing by-products as manure and/or direct food for fish. Today, aquaculture in developing countries is mostly a small-scale activity and is usually not practiced as a stand-alone economic activity, but rather as subsistence farming integrated with agricultural activities such as horticulture and rearing of livestock.

Organic manure and fish growth
The production volume and market share of aquaculture products are advancing extremely rapidly. However, feed is usually recognized as the single largest cost to producers, hence, the best way of reducing the cost of fish production is using organic manure and supplementary feed when available. Animal manure is widely used in developing countries in fish production in earthen ponds. The quality of manure as a fertilizer varies depending on the source of animal and the quality of feed fed to the animal [31,32]. Research showed that pig, chicken and duck manures increase fish production more than cow and sheep manure. For example, in Asia, fish farming is probably the only branch of animal husbandry in which the use of manures is a traditional management tool. In Sub-Saharan Africa, ponds are fertilized using organic manures such as cow dung, sheep, poultry or rabbit manure [33]. The use of animal manure to fertilize ponds has been widely practiced in many countries in order to increase plankton so that there is more natural food for fish to eat, hence, high fish production. Manuring is therefore considered a cheap and preferred source of nutrient to increase fish production.
Pond fertilization with animal manure stimulates production of bacteria, phytoplankton, zooplankton, and benthic organisms [34]. The use of animal waste (livestock) has been studied under integration systems in Africa [35][36][37] and extensively in Asian countries [38,39]. Benefits of integrated Agro-aquaculture systems have been reported in resource poor areas particularly in developing countries [38,40,41]. Studies conducted in sub-Saharan countries on the integrated aquaculture and agricultural systems are presented in Table 2.
Several studies showed that organic supplements contributed to fish yields by supplying P, N and C for algal growth and by stimulating detritus production and heterotrophic utilization. It is well known that high fish yields can be achieved through abundance of plankton in the cultural system [46]. Africa has vast resources of livestock and poultry, which play a vital role in pond fertilization. Livestock wastes including animal manure and poultry by-products are valuable resources in fish farming [47]

Fish feed availability and the concept of valorization
One of the solutions of fish feed availability is to entice animal feeds producing industries to consider also the production of fish feed [49]. However, the big issue here will be affordability; of these industrial feed products; most of our farmers belong to subsistence income bracket, hence, they might not afford these feed products. The use of floating pellets needs higher investment [50], which in most cases is lacking among smallholders; and unless the government intervenes in addressing the problems through either credit facilities or the provision of subsidies, the situation is not likely to get any better. It has been established that profitability in aquaculture is influenced by the cost of feed [51]. In Sub-Saharan countries, justification for industrial scale production of fish feed is not a priority despite the availability of raw materials [4]. Therefore, in order to feed fish, farm-made feeds can be made using locally available ingredients including animal by-products and plant residues.
In Tanzania, more than 80 percent of fish farmers relied on locally available feed ingredients as a major feed supplement for their cultured fish [43]. These local feed ingredients are categorized into four groups, (i) animal by-products, (ii) agricultural by-products, (iii) plant leaves and weed, and (iv) industrial by-products. It has been reported that the early growth phase of tilapia in 1991-2000 was significantly contributed by the use of alternative sources of protein including fishery by-products, terrestrial animal by-products, and a wide range of plant by-products [52]. In this chapter, discussion is cantered on the valorization of two broad categories of ingredients, plant and animal based ingredients.

Plant based ingredients and by-products
In addition to fertilization, feeding in ponds is done using supplementary feeds formulated on farm or purchased from cottage fish feed production industries. In some cases, cereal bran such as grains as energy source (Figure 2) and soybeans as  source of protein (Figure 3) are used in aquafeeds to increase pond productivity. The production from this system ranges from 1000 to 2500 kg/ha/year [33]. Most farmers prefer this system since it is less expensive in terms of feed inputs. Ten edible plant leaves were evaluated (see in [54]) as potential feed ingredients for aquatic animal, the results suggested that some of the plant leaves used contributed on growth performance, immune system, and disease resistance for the fish. Other important plant leaves which have been subjected to experiments to see whether they can be used as ingredients for fish feed formulation includes cassava leaves [55] and Moringa leaf [56]. In another study results showed that the integration of vegetables (Brassica oleracea) as pond inputs increased fish production and net yield than those reared under non-integrated systems [57]. In general, the amount of grain and soybean required in the four East African countries is given in Figures 2 and 3. Another experiment (see [58]), showed that when wheat bran, rice bran, and groundnut bran were used as agro-industrial by-products to examine their economic effectiveness in fish production, there were variability in growth rate and economic benefits, suggesting that variability of agro-by products reflects the growth rate of fish.

Animal based ingredients and by-products
According to the circular economy approach which focuses on the "reduce, reuse and recycle" of resources, waste from animal and food can be valorized leading to the production of proteins and other valuable compounds [59,60]. For example, chicken, pig and cattle manures are substrates for production of housefly (Musca domestica) maggots which are in turn used as fish feed, or as supplement to fish meal in fish feed formulation [61]. Maggots are readily available and are accredited for having high nutritional value with an amino acid profile with biological value Figure 3. Amount of soybean required for fish feed coumpound in four countries of East Africa. Source [53]. Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products exceeding that of soybean and groundnut. Maggots can be harvested, processed into a meal that can be used to substitute or replace fish meal [61,62]. Maggot grown on a mixture of cattle blood and wheat bran contained 92.7% dry matter, 47.6% crude protein, 25.3% fat, 7.5% crude fiber, 6.25% ash, and an amino acid profile comparable to fish meal [59] suggesting that animal wastes utilization can be used to produce insects which can be utilized as fish feed hence, reduce feed cost significantly, thus leading to a viable and sustainable aquaculture industry. The replacement of 25 percent fishmeal in catfish feed, culture with maggot gave high growth performance and profitability than fishmeal based diet [63,64]. Several researches [65,66] have been reporting on the use of red worms, black soldier fly, common housefly, and yellow mealworm as a source of protein to replace fishmeal. It is envisaged that the valorization of animal and animal by-products such as animal blood, offal of poultry, residues of traditional brewery waste, animal manure and fish wastes may contribute significantly on fish production hence, food nutrition and security.

Conclusion
It is clear that fish consumption in Sub-Saharan Africa is increasing. In order to maintain the present amount of fish consumption, considerable additional quantities of fish are required through aquaculture. In turn, aquaculture requires feed as a major input for increasing production. Since commercial fish feed production in most of the sub-Saharan countries is limited, considerable investments are required in local and low costs feed manufacturing. Raw materials of plant and animal origin are sufficiently available in the region albeit the possible competition from livestock and human consumption. Therefore, valorisation of animal and agro-products in the Sub-Saharan countries is imperative/inevitable for increasing fish production at low cost.

Recommendations
In order to increase food nutrition from aquaculture production through valorization of agro-by-products in the sub-Saharan countries, the following are recommended: • Strengthen the use of Public Private partnering by putting more emphasis in services related to the collection of feed ingredients and preservation