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                      Abstract
                      Mengistu,  S.  B. (2022).  Closing  the yield gap:  improving  production efficiency in
                      smallholder farms of Nile tilapia through selective breeding.

                      PhD thesis, Wageningen University, the Netherlands

                      The aim of this thesis was to optimise the breeding program of Nile tilapia for a
                      smallholder production system, thereby contributing to closing the yield gap. From
                      Chapter 2 to 4, we generated information that can be used to optimize the current

                      GIFT breeding program for Nile tilapia. Identifying the major factors contributing to
                      the current yield gap in smallholder Nile tilapia farms is crucial and the first step to
                      any intervention.  In Chapter  2,  based on meta-analysis  of literature, dissolved
                      oxygen was identified as one of the major environmental factors contributing to
                      reduced growth and FCR.  In the presence of substantial  dissolved oxygen
                      differences between the environments, genotype by environment (GxE) interaction

                      can be expected. The output from Chapter 2 was used to design the GxE interaction
                      experiment (Chapter 3). In Chapter 3, heritabilities for harvest weight (HW 0.18 and
                      0.23), thermal growth coefficient (TGC, 0.21 and 0.26), survival days (0.03 and 0.04)
                      and size traits (0.08 to 0.45) were estimated, in an attempt to indicate that these
                      traits can be improved by selective breeding. We estimated the genetic correlation
                      for these traits between aerated and non-aerated ponds. The genetic correlations
                      (rg) between the aerated and non-aerated ponds for harvest  weight (0.80±0.30)
                      and growth (0.81±0.27) indicate some GxE interactions. The less than unity genetic
                      correlation implies that the genetic improvements gained in aerated ponds will not
                      be fully  expressed in non-aerated ponds.  Therefore, breeding  programs should
                      consider  GxE interaction.  In Chapter  4, genetic parameters for log-transformed
                      variance of deviations (LnVar),  a  good indicator of resilience, were  estimated.
                      Heritable genetic variation was found for LnVar and it was noted that LnVar is more
                      expressed in the non-aerated pond compared to the aerated pond.  Finally, in
                      Chapter  5,  we estimated genetic parameters for  critical  swimming  speed (Ucrit)
 Mengistu, S. B.      and the genetic correlation between Ucrit and harvest  weight of fish raised in a
 Closing the yield gap: improving production efficiency in smallholder farms of Nile   non-aerated pond.  Substantial  heritable variation was found for Ucrit.  The
 tilapia through selective breeding.   estimated genetic correlations between Ucrit early in life and HW after grow-out in
                      non-aerated pond, and Ucrit and growth were negative, implying that Nile tilapia
 PhD thesis, Wageningen University, the Netherlands (2022)   with higher Ucrit early in life had lower HW and growth later in life. In this Chapter
 With references, with summaries in English   6,  the findings from the  previous chapters combined and discussed their
                      implications for breeding programs.
 ISBN 978-94-6447-322-3
 DOI 10.18174/574231


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