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General introduction   |   15
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 All  the Nile tilapia  selective breeding  is undertaken in optimal  environmental   used as an indicator of  resilience and animals with better resilience  show lower
 conditions. However, there are large differences in productivity between the best   LnVar values (Berghof et al., 2019b). More resilient animals are expected to be less
 performing farms and low performing farms. A mismatch between genotype and   disturbed by perturbation and could grow  more uniformly.  More uniform  fish
 environment  could lead to such differences.  The  GIFT  strain has  been  selected   growth could improve biomass  estimation, feeding  management,  welfare and
 under an optimal  dissolved oxygen environment  while smallholder production is   avoid size grading. More accurate biomass estimation could reduce feed wastage   1
 undertaken in non-aerated earthen ponds.  The dissolved oxygen (hereafter DO)   resulting from exaggerated biomass estimation. Resilience is favorably correlated
 differences between the  selection and the smallholder  production environment   with the survival  and health of animals.  Mulder  et  al.  (2015) found a  favorable
 could lead to genotype-by-environment (GxE) interaction. That means the genetic   genetic correlation between residual variance of piglet birth weight, an indicator of
 gain achieved in the selection environment  may not  be fully  achieved in the   resilience, and survival at birth. The genetic correlation between LnVar of milk yield
 production environment.   and traits such as  health,  ketosis, fertility and longevity were negative and
                      favorable (Elgersma et al., 2018; Poppe et al., 2021). Therefore, selective breeding
 Many studies have investigated GxE interaction in Nile tilapia  between different   of Nile tilapia for lower LnVar could improve health, welfare and survival.
 environments: different fertilization and different feeding supplement (Eknath  et
 al., 2007),  fertilized pond with/without  feed supplement, cage culture with feed   Different studies in dairy cattle, sheep, pigs and chicken found heritable variation
 supplement/commercial pellet feed and rice-fish culture (Bentsen et al., 2012) cage   for resilience based on repeated measurements (Elgersma et al., 2018; Berghof et
 and pond environments (Khaw  et  al., 2012),  low  and high  input  environments   al., 2019a; Putz  et al., 2019; Dobrzański  et al., 2020; Poppe  et al., 2020; Garcia-
 (Trọng  et  al., 2013),  monosex and mixed sex (Omasaki  et  al.,  2016). In  total 17   Baccino et al., 2021; Moncur et al., 2021; Poppe et al., 2021), indicating resilience
 studies investigated GxE interaction between different environments (reviewed by   can be improved by selective breeding. However, we have not found any studies on
 Sae-Lim  et  al., 2016). Most  of these  studies concluded that genetic  correlations   resilience in Nile tilapia.
 were less than unity. However, none of these GxE studies investigated the effect of
 aeration.            Recurrent hypoxia tolerance, one of the aspects of resilience, is also an important
                      trait.  Aerobic  metabolism  is  30  times more efficient  than anaerobic  metabolism
 DO is one of the limiting  factors of productivity.  Under climate  change, DO is   and, therefore, hypoxia tolerance is an advantage (Richards, 2009). Better hypoxia
 expected  to decrease  with increasing  water temperature (Lennox  et  al., 2019).   tolerance could help  fish to maintain aerobic  metabolism and improve feed
 Increasing water temperature and dropping DO levels are expected to cause more   efficiency. In fish critical swimming speed may reflect the oxygen uptake efficiency.
 stress and diseases are projected to become more frequent (Alders et al., 2021).   Vandeputte  et  al.  (2016) found heritable genetic  variance  for  swimming
 Hypoxia  can suppress growth and immunity in fish (Abdel-Tawwab  et  al., 2019).   performance in European  sea  bass (Dicentrarchus  labrax). Genetic  parameter
 Therefore, optimal Nile tilapia farming requires appropriate levels of DO. However,   estimates for swimming performance and genetic correlation between swimming
 most  smallholder Nile tilapia  farms do not  use aerator  which  leads to recurrent   performance and production traits in Nile tilapia are not investigated so far.
 hypoxia. Recurrent hypoxia could lead to poor performance of Nile tilapia. Genetic
 improvement  of Nile tilapia has  been carried out  in an aerated environment   1.4.  Motivation and objectives of the study
 (normoxic  environment), mainly focussing  on improving harvest  weight.  Given   Currently, many small- and medium-sized tilapia farms in developing countries still
 these challenges, Nile tilapia  breeding  programs need to improve Nile tilapia   underperform in terms of feed efficiency, despite the use of genetically improved
 resilience to stressors such as diseases and tolerance to DO fluctuation.    strains of tilapia  such as  GIFT.  Productivity differences among  many small-  and
                      medium-sized tilapia farms lead to a yield gap between the best performing and
 Resilience is defined as  the capacity of an animal  to  be minimally affected  by   low  performing  farms.  In aquaculture, growth, feed conversion ratio (FCR)  and
 perturbations or to quickly  recover to the state  it  had before the perturbation   survival  are the  main determinants of productivity (Kankainen  et  al.,  2012; de
 (Colditz and Hine, 2016). A Log-transformed variance of deviations (LnVar) can be   Verdal et al., 2018).


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