Short‐Term Effect of Fishmeal‐Free Feed Formulation on Growth Performance, Feed Efficiency and Transcriptomic Profile of Hypothalamus and Liver in Atlantic Salmon

Atlantic salmon ( Salmo salar ) aquaculture is increasingly seeking alternatives to marine‐derived ingredients, but the physiological and molecular consequences of fishmeal (FM) replacement remain incompletely understood. This study assessed the effects of two nutritionally equivalent summer diets for Atlantic salmon under simulated Tasmanian summer conditions (18°C): FM and FM‐free (FMF). Growth performance, feed efficiency and transcriptomic responses in the hypothalamus and liver were evaluated to assess the impacts of FM replacement. After 40 days, FM showed significantly higher final body weight and condition factor. The transcriptome showed diet‐specific responses. Genes linked to neuropeptide melanin‐concentrating hormone (MCH) activity and fatty acid nutrient sensing were significantly upregulated in the hypothalamus of FM‐fed fish relative to FMF, suggesting a stimulatory effect on feed intake and energy regulation. In the liver, the FM treatment induced significant lipid activation, signalling and remodelling pathways, indicating a higher availability of essential fatty acids. The FMF treatment significantly upregulated genes involved in hepatic de novo lipogenesis, implying a limited supply of fatty acids. The study identifies diet‐dependent hypothalamic gene expression patterns consistent with altered feed intake regulation, particularly within the MCH pathway, alongside compensatory hepatic lipogenic responses in FMF‐fed fish. These transcriptomic findings advance our understanding of the molecular consequences of complete FM replacement in Atlantic salmon under summer temperature conditions.