Red tilapia (Oreochromis spp.) is a cornerstone of global aquaculture, cultivated in a wide range of systems, from traditional freshwater ponds to river cages. However, the progression of saline intrusion in deltas like the Mekong is driving a growing interest in its cultivation in saltwater environments. This diversification raises a critical question for genetic improvement programs: will a fish selected for genetic superiority in freshwater maintain the same level of performance in a saline environment?
A first-of-its-kind study for this species, published by researchers from the Research Institute for Aquaculture No. 2 and the University of the Sunshine Coast, addresses this question by investigating the genotype-by-environment (GxE) interaction. It analyzed how two distinct cultivation environments—freshwater and saltwater ponds—influence economically important traits such as growth, coloration, and survival. The findings are fundamental for designing and optimizing future selective breeding programs for red tilapia.
Key findings
- The study found that GxE interaction is biologically significant for body length and survival in red tilapia, suggesting that the top-performing genotypes in one environment may not be the best in another.
- Heritability estimates for growth traits (weight and length) were higher in freshwater ponds compared to saltwater ponds.
- Selection for higher body weight in freshwater led to favorable genetic improvements in length, color (reduced dark blotches), and survival across both environments.
- Despite the GxE effects, the genetic correlations between the different traits studied (growth, color, survival) showed similar patterns and magnitudes in both environments.
How was genetic performance evaluated across water types?
To conduct the research, scientists used data from a long-term selective breeding program at the Research Institute for Aquaculture No. 2 (RIA2) in Vietnam.
- Fish Origin: Data was collected from 75,950 fish, which were the offspring of 970 full-sibling families (produced from 970 dams and 486 sires).
- Testing environments: After an initial nursing period, fingerlings from each family were marked with a Passive Integrated Transponder (PIT) tag and distributed into two cultivation environments for evaluation.
- Freshwater: A 2000 m² earthen pond with a depth of 1.5 m.
- Saltwater: Similarly sized ponds with an average salinity of 10-15‰.
- Traits measured: After a 223-day grow-out period, individual data were collected for four key commercial traits: body weight, total length, color, and survival. Color was visually assessed, classifying fish on a “pass” or “fail” basis depending on whether dark blotches covered less than 5% of the body.
Using a multi-trait mixed model, the researchers estimated genetic parameters and the response to selection in both environments.
The influence of the environment on genetics
Growth and survival: A disparate performance
The study revealed that fish raised in freshwater ponds reached a significantly higher average weight than those in saltwater (470.8 g vs. 436.3 g). Interestingly, the individual survival rate was slightly higher in the saltwater ponds.
From a genetic standpoint, heritability for weight and length was higher in freshwater than in saltwater, primarily due to greater genetic variance in the former environment. This suggests that under freshwater conditions, the potential to achieve genetic gains through selection for growth is greater.
Color and its link to survival
Aesthetic quality, such as the absence of dark blotches, is an important trait for the marketability of red tilapia. The study found that the proportion of fish with dark blotches was lower in freshwater (27.2%) compared to saltwater (38.2%). Furthermore, a negative genetic correlation was observed between color (presence of blotches) and survival, indicating that fish with dark blotches tended to have lower survival rates.
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The core finding: Genotype-by-environment (GxE) interaction
The concept of GxE refers to how different genotypes respond differently to various environments. A strong interaction means that the “best” genotype in one setting may not be the best in another.
The results showed a significant GxE interaction, especially for quality traits (color) and survival. The genetic correlation for body weight between the two environments was moderate (rg=0.70), but for total length, it was low (rg=0.17), and for survival, it was not significantly different from zero. These values, being less than one, confirm the presence of GxE and suggest that its effects on length and survival are biologically important.
Implications for producers and breeding programs
These findings have practical and strategic consequences for red tilapia aquaculture.
- Current selection programs may not be optimal: Since selection is commonly performed in freshwater ponds, the full genetic potential for saltwater production systems may not be captured.
- The need for tailored selection strategies: The study underscores the importance of incorporating GxE interaction into breeding programs. Future strategies could include developing environment-specific breeding programs or, alternatively, selecting families that demonstrate consistently high performance across both systems.
- Response to selection: Despite the GxE interaction, the selection program for increased body weight in freshwater resulted in favorable genetic improvements for all traits studied in both environments. Gains were observed in total length and survival, along with a reduction in the proportion of fish with black blotches, although the latter was more pronounced in freshwater.
Conclusion
This study conclusively demonstrates that genotype-by-environment interaction is a real and significant factor in the cultivation of red tilapia. Although growth, color, and survival traits appear to be under similar quantitative genetic control in both freshwater and saltwater, the differences in performance and heritability highlight the need to refine selection strategies.
To optimize genetic improvement programs and maximize productivity as saltwater cultivation expands, it is crucial to consider the final production environment. Testing genotype performance in multiple settings will be essential for selecting superior animals and ensuring the sustainable future of red tilapia aquaculture.
Reference (open access)
Phuc, T. H., Khoa, P. D., Dang, N. T., Huong, T. T., Lien, H. T., Tham, V. T., Duy, N. H., & Nguyen, N. H. (2025). Genotype-by-Environment Interaction in Red Tilapia (Oreochromis spp.): Implications for Genetic Parameters and Trait Performance. Genes, 16(8), 966. https://doi.org/10.3390/genes16080966
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
