Muscle-Associated Lipid Deposition in Fish: Biological Foundations, Genetic Architecture, and Emerging Frontiers for Precision Improvement

Muscle-associated lipids influence fish flesh composition, nutritional value, and sensory quality, but inconsistent definitions obscure comparisons among studies. This review distinguishes adipose tissue between anatomically distinct muscles, adipocytes within muscle connective tissue, lipid droplets within myofibers, and chemically measured muscle lipid content. In salmonids, myoseptal adipose tissue occupies the partitions between adjacent myomeres, whereas perimysial adipocytes lie within myomeres. We examine how these compartments develop, how their lipid storage is regulated, and how genetic and nutritional approaches can improve flesh quality. Salmonid precursor cultures support adipogenic differentiation involving PPARγ and C/EBPs, while evidence for depot-specific progenitor lineages remains limited. Genetic studies identify heritable variation and associated genomic regions for muscle lipid traits, but association, expression, and functional evidence frequently address different phenotypes or tissues. Translating these findings requires standardized sampling, spatially resolved measurements, and functional testing of candidate mechanisms. Research priorities include distinguishing adipocyte recruitment from lipid loading, separating cell-composition changes from intracellular regulation, and connecting causal variants with depot-specific phenotypes. Breeding objectives should integrate lipid distribution and fatty acid composition with processing quality, growth, feed efficiency, and environmental robustness. Imaging and multi-omics can support this integration when calibrated against defined biological traits and validated across populations and production environments.