Maintaining an optimal level of body fat is essential for the survival in Atlantic salmon, but higher levels can result in costly harvest waste and economic losses. Breeding programs could in principle, incorporate whole body fat as a trait and thus contribute to more sustainable and cost-efficient fish farming. However, high throughput and cost-effective methods for phenotyping whole-body fat in thousands of fish are currently lacking. The reference Folch lipid extraction method (Folch et al., 1957) is destructive, highly laborious, and expensive. Two rapid non-invasive methods, namely the Distell fat meter which is a highly portable dielectric spectrometer (DS) and a new semi-automated Near-infrared (NIR) system offer the possibility to change this but require further validation. To address this issue, we conducted a large-scale genetic validation study on thousands of Atlantic salmon with the goal of 1) comparing the accuracy and reliability of digital NIR and DS methods for estimating total body fat; and 2) examining the pattern of total body fat across three different life phases in Atlantic salmon.
The large-scale genetic validation study is done on ~3000 Atlantic salmon belonging to 35 fish families of 2017-year class at MOWI Genetics, Norway . From fertilization the families were reared in separate trays, while from the eyed egg stage families were pooled and reared in a common garden tank. At an average body weight of 40 grams fish were PIT tagged with and a fin clip taken for genotyping with MOWI’s customized single nucleotide polymorphisms SNP array which contains 55735 SNP markers.
In August 2020 at an average body weight of 48.9 (SD= 7.9) grams, the fish were transported to the freshwater facility of Nofima Research Station for Sustainable Aquaculture, located at Sunndalsøra (62°40N 8°31E), Norway. After a three-week acclimation period, the fish were sorted into 3 phase (Parr, Pre-Smolt and Post-Smolt) groups as shown in figure 1.
The Whole-body fat traits were recorded on different numbers of participating fish in different phases through reference Folch lipid extraction, DS and NIR spectroscopy. Genetic, Biometric, spectroscopic and chemical measurements were taken at all life stages. The DMU software and Rdmu package were used for genetic estimates and multivariate analysis (Madsen et al., 2014).
Both the NIR and DS methods showed strong agreement with the reference method (rp= 0.80 -0.88). Moderate to high genetic estimates were obtained for whole body fat estimation through NIR and DS as presented in the table 1 below.
We found substantial variation in total body fat across the different life phases, with an average of 11.32 ±1.23 % in parr, 33.80 ±2.6 % in pre-smolt, and 14.98 ±1.27 % in post-smolt fish. The genetic and phenotypic agreement between whole-body fat in different phases are presented in table 2. A significant decrease in phenotypic and genetic correlation was observed between fresh water and sea water stages.
Conclusion
For the first time, genetic parameters are presented for whole-body fat in Atlantic salmon in multiple life phases . The substantial genetic estimates demonstrate significant potential in selective breeding for whole-body fat, but cognizance must be taken of the life phase. The g enetic validation proves the capability of non-invasive DS and NIR methods for measuring whole body fat at different life stages without killing or filleting , and their usefulness depends on the user’s specific needs and preferences.
Reference(s):
Madsen, P., J. Jensen, R. Labouriau, O. F. Christensen, and G. Sahana. "DMU-a package for analyzing multivariate mixed models in quantitative genetics and genomics." In Proceedings of the 10th world congress of genetics applied to livestock production, pp. 18-22. 2014.
Folch J, Lees M, Sloane SG. A simple method for the isolation and purification of total lipides from animal tissues. J Biol Chem. 1957; 226:497–509"