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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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