Introduction
Ongoing climate change is likely to profoundly alter aquaculture production conditions, particularly for freshwater species such as rainbow trout. Resilience, defined as an animal's ability to be minimally affected by disturbances or to rapidly return to its pre-disturbance state (Colditz and Hine, 2016), could represent a major focus in breeding objectives to help the sector cope with global warming. In addition, sterile triploid trout intended for smoked fillet production currently account for a large proportion of French production. The aim of this study was to evaluate the genetic parameters of resilience indicators between corresponding diploid and triploid trout families.
Material and methods
Three batches of 1,200 progenies each were derived from a single semi-factorial mating design of 100 females and 90 males from a commercial line of rainbow trout (Oncorhynchus mykiss) of the breeding company Les Sources de l'Avance. The first diploid batch (2n_breed) was reared in the breeding farm at stable temperature of 14°C. The two additional batches, one diploid (2n_prod) and the other triploid (3n_prod), were reared in a production farm characterized by a variable thermal profile with chronic episodes of elevated temperatures. Each individual was electronically tagged to monitor body weight four to five times until reaching the average body weight of 2.5 kg. Abiotic parameters (temperature, pH, etc.) were recorded throughout the experiment. At 2.5kg, individuals were slaughtered and measured for various processing traits. Fish were genotyped for parentage assignment using 1,000 SNP markers (Roche et al., 2024). Animal models using pedigree kinship adapted to ploidy level were used for additive variance estimations. Genetic parameters were then estimated, including within-group genetic correlations and pairwise between-group genetic correlations across the two environments and the two ploidy levels.
Results
Growth was faster at the breeding site than at the production site, allowing fish to be slaughtered at an average body weight of 2,127 g after 325 days post-hatching (dph) at the breeding site, whereas processing was carried out at 2,705 g for diploids and 2,575 g for triploids after 420 dph at the production site. Mortality reached 13.5% at the breeding site, compared with 27.8% for diploids and 34.2% for triploids at the production site. The difference in survival between 3n_prod and 2n_prod batches was significant during the early life stages but disappeared from 170 g (169 dph) onwards.
Headless gutted carcass yield, which is a proxy of fillet yield, at processing was 0.79 [0.01] for each batch, although the head yield is slightly higher at the production site, particularly for triploids (0.12 vs 0.11 vs 0.10, for 3n_prod, 2n_prod and 2n_breed respectively).
Heritability estimates for body weight ranged from 0.21 [0.05] to 0.45 [0.08], depending on the age at measurement, the rearing site, and ploidy level. They tended to decrease throughout fish growth. Genetic correlations for weight gain between ploidy levels at the production site were very high (>0.90), only estimation for weight gain during the autumn period was slightly lower (0.81 [0.28]). At slaughter, the genetic correlation between ploidy levels for body weight at slaughter was 0.73 [0.20], whereas that for headless gutted carcass yield reached 0.91 [0.06].
Table : Heritability of weights and headless gutted carcass yield according to batch (2n_breed in green, 2n_prod in yellow and 3n_prod in red)
Comparison of weight gain and body weight for diploids across the two sites showed more moderate genetic correlations, ranging from 0.45 to 0.55 and 0.51 to 0.66 on comparable periods. The genetic correlation between sites for body weight of diploid fish at slaughter was 0.76 [0.17], and 0.87 [0.07] for headless gutted carcass yield. Genetic correlations between 2n_breed et 3n_prod were slightly lower.
Discussion
The results of this study highlight moderate genetic correlations for growth between the two rearing sites, suggesting that breeding strategies should be adapted according to the targeted breeding objectives for growth improvement. These differences may be partly explained by contrasting rearing conditions, particularly with regard to the thermal profiles of the farms. They become even more pronounced when the ploidy level of the breeding nucleus differs from that of the fish in production. However, genetic correlations for processing yields remained very high, suggesting no re-ranking of families for these traits according to ploidy or environment. This study provides a foundation for the development of breeding programs for diploid or triploid trout reared under contrasting environmental conditions.
Acknowledgment
This work was supported by the European Maritime, Fisheries and Aquaculture Fund (FEAMPA) under the HyperSelect project.
References
Colditz, I.G., Hine, B.C., 2016. Resilience in farm animals: biology, management, breeding and implications for animal welfare. Animal Production Science 56, 1961–1983. https://doi.org/10.1071/AN15297
Roche, J., Griot, R., Allal, F., Besson, M., Haffray, P., Patrice, P., Phocas, F., Vandeputte, M., 2024. APIS: an updated parentage assignment software managing triploids induced from diploid parents. G3: Genes, Genomes, Genetics 14, jkae143. https://doi.org/10.1093/g3journal/jkae143