Dietary Prickly Ash Seed Meal Suppresses Hepatic Lipogenesis and Improves Fillet Fatty Acid Profiles in Grass Carp ( Ctenopharyngodon idella )

Fishmeal shortage requires replacing fishmeal with plant proteins in aquafeeds. Over-fatness of viscera degrades the efficiency and quality of the fillet in farmed grass carp. Prickly ash seed meal (PASM) provides both high levels of PUFAs and bioactives, but its potential is still unevaluated for lipid regulation and fillet nutrition of this species. We hypothesised that dietary PASM would reduce liver lipid production and enhance fillet fatty acid composition without compromising growth. Five isonitrogenous and isolipidic diets were formulated with PASM at graded inclusion level: PAS 0 (control, 0% PASM), PAS 2.5 (2.5% PASM), PAS 5.0 (5.0% PASM), PAS 7.5 (7.5% PASM), PAS 10 (10% PASM), and fed to juvenile grass carp (9.21 ± 1.20 g) in five groups respectively for a period of 56 days. Growth performance, viscerosomatic index (VSI), relative intestinal length (RIL), intraperitoneal fat index (IPFI), proximate tissue composition, fatty acid profiles, hepatic histology and gene expression of the lipid metabolism genes (pparγ, fas, dgat-1, pparα, cpt-1) were evaluated. Growth and survival were unaffected by PASM inclusion up to 10%. VSI and RIL increased with higher inclusion of PASM (p < 0.05) indicating improved gut development, while IPFI showed a tendency to drop with the higher level of PASM inclusion. PASM supplementation dose-dependently increased crude protein content and decreased lipid content in hepatopancreas and muscle, accompanied by reduced adipocyte size in visceral adipose tissue. Hepatic expression of lipogenic genes (pparγ, fas, and dgat1) was significantly downregulated, while lipolytic genes (pparα and cpt1) exhibited a transient upregulation at 5% inclusion. Fatty acid profiling revealed enhanced deposition of n-3 PUFAs, particularly α-linolenic acid (C18:3n-3) and docosahexaenoic acid (C22:6n-3), in fillets, resulting in an improved n-3/n-6 PUFA ratio. Dietary PASM at 7.5%-10% (PAS 7.5 and PAS 10) effectively suppresses hepatic lipogenesis and improves fillet fatty acid nutritional quality in grass carp, positioning PASM as a promising functional ingredient for sustainable aquaculture feed formulations.