Fish Protein Hydrolysates Mitigate the Adverse Effects of No-Fishmeal Diets in Gilthead Seabream Juveniles

The aquaculture industry must continue to reduce its reliance on finite marine ingredients and promote biocircularity to enhance sustainability. This study evaluated the effects of no-fishmeal (FM) diets and fish protein hydrolysates (FPHs) on the growth performance, antioxidant status, and immune responses of gilthead seabream (Sparus aurata). Following established conditions, two FPHs were prepared from the enzymatic hydrolysis of discards from whole-body blue whiting (Micromesistius poutassou) and gurnard heads (Trigla spp.); the former contained a higher proportion of large peptides (LPs), while the latter had more small peptides (SPs). Four isoproteic (48%) and isolipic (16%) diets were tested: a commercial (COM)-like diet with 35% FM, 10% poultry meal, and 5% soy protein concentrate; a FUTURE (FUT) diet, without FM or soy protein concentrate, containing 25% poultry meal; and two FUT diets supplemented with FPH (FUTLP and FUTSP). Gilthead seabream (initial weight ± 8.0 g) was distributed into 500 L tanks at an initial density of 1.4 kg m−3 and fed the experimental diets to apparent satiety for 8 weeks. Sampling was performed at the end of the growth trial, followed by a digestibility trial. Nutrient and energy digestibilities were significantly lower in the FUT than in the COM diet, with protein and energy digestibilities being 7% and 16% lower, respectively, in the FUT treatment, leading to higher nitrogen losses. Growth performance and feed utilization were negatively impacted in the FUT treatment, with weight gain of only 310 ± 33% compared to 482 ± 22% in the COM treatment. Additionally, immune responses in plasma and antioxidant status in the liver were slightly impaired in the FUT treatment. Supplementation of FPH to the FUT diet mitigated or even reversed these negative effects. The results confirmed that including small- and medium-sized peptides in no-FM diets is more beneficial than using larger ones.