Effect of replacing fish meal with stalked sea squirt (Styela clava) tunic waste on growth performance, health status, and skin pigmentation in juvenile red sea bream (Pagrus major)

This study evaluated the effects of replacing fish meal (FM) with stalked sea squirt ( Styela clava ) tunic (TSC) on growth performance, health status, and skin pigmentation in juvenile red sea bream ( Pagrus major ). Five isonitrogenous and isolipidic diets were formulated, with TSC replacing FM at 0 % (TSC0), 5 % (TSC5), 10 % (TSC10), 15 % (TSC15), and 20 % (TSC20). After an 8-week feeding trial, TSC inclusion up to 10 % did not adversely affect growth, whereas TSC15 and TSC20 significantly reduced weight gain and feed efficiency. Immune response indicators, including lysozyme activity, immunoglobulin M, and interleukin-1, exhibited a linear decline with increasing dietary TSC levels ( P < 0.05), suggesting potential immunosuppressive effects at higher inclusion rates. Skin color analysis revealed linear increases in redness (a*) and yellowness (b*) with higher dietary TSC levels. TSC15 significantly enhanced cranial skin a* and b* values compared to TSC0, although differences from TSC5, TSC10, and TSC20 were not significant. These results indicate that TSC can replace up to 10 % of FM in juvenile P. major diets without compromising growth or health, while higher replacement levels enhance skin pigmentation but may entail trade-offs in immune function and nutritional performance. • Up to 10 % TSC keeps growth and feed efficiency unchanged in red sea bream. • TSC replacement up to 10 % maintains plasma biochemistry and immune function. • Skin redness and yellowness increase linearly with higher TSC inclusion levels.