Upcycled Animal Protein Hydrolysates for Gilthead Seabream: Impact on Growth Performance, Nutrient Utilisation, Intestinal Structure and Skeletal Muscle Gene Expression

The development of local circular feed systems has increased interest in protein hydrolysates (PHs) derived from processed animal proteins (PAPs), which can upcycle protein-rich streams into functional aquafeed ingredients. This study evaluated novel PHs sourced from insect meal (Hermetia illucens), fish by-products and swine by-products as replacements for a high-quality fish PH (CPSP90) in diets for gilthead seabream (Sparus aurata). A control diet, containing 20% fish meal, 22% plant proteins, 23% PAPs and 3% CPSP90, was compared with three experimental diets in which CPSP90 was replaced by 3% of each alternative PH, resulting in the INSECT, FISH and SWINE diets. Seabream juveniles (11 g) were reared in triplicate tanks and fed the experimental diets three times per day over 88 days. A parallel digestibility trial was also conducted. All diets were well accepted across treatments (voluntary feed intake [VFI] = 1.7–1.8 g 100 g⁻¹ average body weight day⁻¹). Nutrient digestibility, fish final body weight (FBW; 60–65 g), feed conversion ratio (FCR; 1.1–1.2) and anterior intestine histomorphology remained similar across treatments. Fish fed the SWINE diet showed upregulation of mrf4, mlc2a and mlc2b, suggesting modulation of myogenic differentiation and muscle structural pathways, although this was not reflected in improved growth. Fish fed the INSECT diet showed reduced faecal lipid losses, together with downregulation of capn3, which may reflect metabolic adjustments related to lipid utilisation. Fish fed the FISH diet exhibited muscle gene expression patterns and growth performance similar to those of the control group. Alternative PHs showed potential to replace CPSP90 in gilthead seabream diets, supporting circular aquafeed formulations while sustaining feed efficiency and fish growth.