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General introduction   |   13
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 natural food.  Semi-intensive production systems are characterized by moderate   for example, Genetically Improved Farmed Tilapia (GIFT) (Pullin et al., 1991; Eknath
 input use, pond fertilization using manures or inorganic fertilizers to boost natural   et  al.,  1993; Bentsen  et  al., 1998)  and FaST(Bolivar, 1998), GenoMar Supreme
 food production,  use of supplementary homemade or commercial feed, higher   Tilapia  (Zimmermann and Natividad, 2004) and GET-EXCEL  (Tayamen, 2004).
 stocking density than extensive production systems, and Nile tilapia can be farmed   Among the Nile tilapia selective breeding programs, the GIFT breeding program is
 in monoculture or polyculture system. Earthen ponds without aeration are most   the  most important non-commercial breeding  program. The  GIFT project  was   1
 commonly used in both extensive and semi-intensive production systems. Absence   started in 1988 in the Philippines and executed by the International  Center  for
 of aeration results in recurrent high dissolved oxygen fluctuation during the day.   Living Aquatic Resources Management (ICLARM), now WorldFish, in collaboration
 Nile tilapia requires a normoxic environment (dissolved oxygen above 5 mg/L) for   with partner organizations (Pullin  et al., 1991). The base population was formed
 optimal  growth.  However, tilapia  are known to tolerate  dissolved oxygen levels,   from four wild stocks from Egypt, Ghana, Senegal and Kenya and four commercial
 which often drop below 3 mg/L during nights.    Nile tilapia strains farmed in the Philippines (Pullin et al., 1991; Gjedrem, 2012).
                      After six generations of selective breeding in the Philippines, WorldFish continued
 Intensive production systems are characterized by high  input  use, high  stocking   the selective breeding of the GIFT strain in Kedah State, Malaysia, from generation
 density, use of a nutritionally complete pelleted diet, use of continuous aeration for   six after receiving 63 full sib groups of 35 fish each from the Philippines towards the
 an optimized environment. Intensive Nile tilapia  production is practiced in   end of 2000 and the beginning of 2001 (Ponzoni et al., 2011b). Currently, the GIFT
 raceways, cages and ponds. Extensive and semi-intensive production systems are   strain has been selected for 20 generations in Malaysia and disseminated to more
 probably the dominant production systems, but exact numbers are not available.   than 16 countries worldwide (Agha et al., 2018; WorldFish, 2021).
 Hereafter, both extensive  production systems and  semi-intensive production
 systems are referred to as smallholder production systems.   The Genetically Improved Farmed Tilapia (GIFT) breeding program benefited from
                      the experiences gained in salmon breeding  that started in 1971 in Norway
                      (Gjedrem, 2012).  Genetic gains in harvest  weight ranging  from 7-11%  per
                      generation over four to six  generations of  GIFT breeding  program  have been
                      published (Khaw et al., 2008; Ponzoni et al., 2011a; Thodesen et al., 2011). After six
                      generations of selection, the GIFT strain grew up to 85% faster than the fish used in
                      the base population (WorldFish, 2021).  The estimated selection response ranges
                      from 10  to 15%  in the last ten generations (Khaw,  2015).  Recently estimated
                      average selection response for GIFT  strain was 7% for 17 generations, indicating
                      continued response to selection (Benzie et al., 2021). Data from breeding programs
                      conducted with other Nile tilapia strains by WorldFish in Abbassa, Egypt, indicate
                      that on average 3%  selection responses have been achieved for 12  generations
                      (Benzie et al., 2021).

                      Other Nile tilapia selective breeding programs also reported considerable selection
                      responses. In China, on average 11.4% (range 7.4 to 18.7%) selection response was
 Figure 1.1 World tilapia production from 1990 to 2019 (FAO, 2021).   recorded for Progift Nile tilapia body weight (Thodesen et al., 2011). For GenoMar
                      Supreme  Tilapia  the average selection response over  11 generations  was  17%
 1.3.  Nile tilapia selective breeding programs   (Gulzari, 2017).
 Many selective breeding  programs have been established for Nile tilapia.  Until
 2005, seventeen breeding programs were established for Nile tilapia (Neira, 2010),



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