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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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