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1.1 Introduction
                                                                                                                       1.1.  World aquaculture
                                                                                                                       From 2000 to 2018, total world aquaculture production increased by 42%, reaching
                                                                                                                       an all-time high (FAO, 2020b). From 1995 to 2018, the trend of aquaculture fish
                                                                                                                       production by percentage of world total was increasing in Africa and the Americas,
                                                                                                                       and the trend was  stable in Asia;  however, the trend was decreasing  in Europe
                                                                                                                       (FAO, 2020a). In 2018, the value of aquaculture production was 139.7 billion USD
                                                                                                                       and aquaculture employed 20.5  million people (FAO, 2020a).  Aquaculture is the
                                                                                                                       fastest-growing livestock production sector (Garlock et al., 2020). Freshwater fish
                                                                                                                       aquaculture production increased by 131% between 2000 and 2018 (FAO, 2020b).
                                                                                                                       Most  aquaculture production comes from Asian countries, dominated by China,
                                                                                                                       Indonesia and India (FAO, 2020b). Carps and tilapia are the dominant freshwater
                                                                                                                       species groups.

                                                                                                                       1.2.  Nile tilapia Aquaculture
                                                                                                                       Nile tilapia  is the most  important  commercial fish  species in tropical  freshwater
                                                                                                                       aquaculture, with an estimated global production of 4.8 million tons in 2018 (FAO,
                                                                                                                       2020a). Nile tilapia constitutes 8.3% of the total finfish produced in 2018, which
                                                                                                                       makes it  the third most-produced finfish next to Grass  carp (Ctenopharyngodon
                                                                                                                       idellus) (10.3%) and Silver carp (Hypophthalmichthys molitrix) (8.4%) (FAO, 2020a).
                                                                                                                       Global Nile tilapia production increased by 19.6 times in 2019 compared to 1990
                                                                                                                       (Figure 1.1) (FAO, 2021). Nile tilapia production in 2019 increased from about 201
                                                                                                                       thousand tons to 2.9 million tones in Asia, from about 4.7 thousand tons to 436
                                                                                                                       thousand tons in Latin America and from about 27 thousand tons to 1.26 million
                                                                                                                       tons compared to 1990 (Fig 1).

                                                                                                                       The feeding  habit, high  growth rate  and disease tolerance of Nile tilapia  have
                                                                                                                       helped the production of  this species to become  widespread.  Tilapias are
                                                                                                                       herbivorous/omnivorous  fish (El-Sayed, 2006), therefore,  less  wild fish is used in
                                                                                                                       tilapia feed than in carnivorous fish feed (Naylor et al., 2021); and feeding tilapia is
                                                                                                                       less costly than carnivorous fish. Nile tilapia is also hardy, resistant to disease and
                                                                                                                       tolerates poor water quality (Bhujel, 2014), and is farmed under diverse production
                                                                                                                       systems: extensive, semi-intensive, or intensive production systems. Extensive and
                                                                                                                       semi-intensive Nile tilapia  farms are managed by smallholder farms,  while the
                                                                                                                       intensive Nile tilapia farms are managed by big commercial companies.

                                                                                                                       Extensive production systems are characterized by the use of low stocking density,
                                                                                                                       no or limited use of supplementary feed or fertilization as Nile tilapia can rely on


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