Skeletal Development and Enzymes Activity are influenced by Protein and Hydrolysates Origin in European Seabass Fry

Fish protein hydrolysates (FPH) are known to improve nutrient availability and immune function in early-stage fish. Their high content and mix of peptides of different sizes and single amino acids, although widely variable depending on the commercial source, makes them particularly suited for fry, whose gastrointestinal tracts are still maturing.*This study explored how dietary inclusion of FPH in combination with fishmeal (FM) levels influence growth performance, gastrointestinal (GIT) enzyme activities, and skeletal development (specifically haemal lordosis) in European seabass (Dicentrarchus labrax) fry.*

The study was conducted as a collaboration between BioMar, IRTA, and the University of Crete, testing four experimental diets combining two levels of fishmeal (FM; 53% and 65%) and two levels of FPH (0% and 13%). European seabass fry (0.2 g initial weight) were fed the diets for 70 days under controlled conditions. When fish reached 1.5 g, a 7-day swimming challenge was introduced to induce mechanical stress and assess susceptibility to haemal lordosis*.The skeletal condition was evaluated pre- and post-challenge, with growth and enzyme activities also assessed along development and at the end of the trial. Nutrient-response patterns were analysed by means of two-way ANOVA and Partial Least Square (PLS) regression.

Results

Growth Performance

All diets supported normal growth throughout the trial. Final weight and overall performance did not differ significantly across treatments, showing that FPH inclusion up to 13% does not remarkably impact growth in post-larval stages.

Enzyme Activity

The GIT enzyme activities were significantly affected by dietary composition. Activities of trypsin, pepsin, and leucine-alanine aminopeptidase (LAP) varied with FM and FPH levels, suggesting that protein source, peptides size distribution and amino acid compositions influence digestive capacity in seabass fry.##### Skeletal Development

The incidence of haemal lordosis was significantly influenced by dietary composition. A notable interaction between fishmeal (FM) and fish protein hydrolysate (FPH) levels was detected, with certain combinations mitigating skeletal deformities following mechanical stress exposure. This effect appears to involve multiple nutrients, including water-soluble proteins, peptide size distribution, polar lipids, vitamin D₃, and specific minerals. These findings suggest that early dietary modulation plays a critical role in promoting skeletal robustness in seabass fry, as part of a broader multifactorial process, particularly under conditions of mechanical stress.

These findings confirm the need to 1) properly characterize raw materials (i.e., FM and FPH levels) in our formulations to overcome the wide nutritional variability among batches, and 2) define nutrient requirements based on the appropriate targeted endpoint. Regardless of the negligible effect on fish growth, we show that fish protein hydrolysatesneed to be strategically combined with fishmeal to support optimal digestive enzyme activity and skeletal integrity in European seabass fry. The nutritional benefits extend beyond amino acid and peptides supply, highlighting the importance of co-nutrients such as phospholipids, vitamins, and trace elements.

*All procedures were approved by the Ethics Committee for Animal Experimentation at IRTA, in accordance with European and Spanish Directives on the protection of animals used for scientific purposes. Fish were monitored daily, and no adverse effects on health or welfare were observed during the trial.