Visceral lipid percentage rather than relative visceral weight is genetically associated with feed efficiency in saltwater Atlantic salmon

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Abstract
Feed efficiency (FE) is a major determinant of economic and environmental sustainability in Atlantic salmon aquaculture, yet the biological drivers of FE during the saltwater (SW) phase remain poorly understood. This study aimed to develop and validate methods for predicting visceral fat percentage (VFP) and estimate its genetic relationships with growth, fillet lipid deposition, and feed-efficiency traits. A population of 836 fish from 30 families was phenotyped for growth, feed intake, and body composition traits. VFP was predicted with high accuracy using near-infrared spectroscopy on homogenized viscera (cross-validated R2 = 0.91), while fillet fat percentage (FFP) was predicted using Raman spectroscopy (cross-validated R2 = 0.58). Genetic parameters were estimated using animal models based on genomic relationships. VFP was moderately heritable (h2 = 0.28 ± 0.06) and showed a strong positive genetic correlation with feed conversion ratio (FCR; rg = 0.90), indicating that lower visceral lipid was genetically associated with improved FE. When relative visceral weight was partitioned into lipid and non-lipid components, decreased visceral lipid was specifically associated with improved FCR, whereas increased relative non-lipid organ visceral mass was associated with higher specific growth rate. FFP was only moderately genetically correlated with FCR traits (rg = 0.34-0.53). A negative partial genetic correlation between VFP and FFP conditional on body weight (rg = -0.36 ± 0.15) suggested a small allocation tradeoff between lipid compartments. Overall, visceral lipid content is a heritable and informative indicator trait for improving FE in SW Atlantic salmon.

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