Effects of dietary inclusion of kelp residue on growth performance, blood biochemistry, hepatic metabolism, digestive enzymes and intestinal health of turbot Scophthalmus maximus

Abstract

This study investigated the effects of dietary kelp residue (KR) inclusion on growth performance, nutrient utilization, and physiological responses in juvenile turbot (Scophthalmus maximus; initial body weight 35.17 ± 0.25 g) through an 8-week feeding trial. Six isonitrogenous and isolipidic diets containing 0, 3, 6, 9, 12, and 15 % KR were randomly assigned to triplicate tanks. Results revealed growth metrics (weight gain, specific growth rate) and nutrient digestibility coefficients (dry matter, crude lipid, energy) significantly decreased with KR inclusion > 9 %, accompanied by elevated feed conversion ratio compared with the control group (P < 0.05). Protein digestibility declined markedly at KR levels > 12 % (P < 0.05). Serum lipid profiles (triglycerides, low-density lipoprotein) and intestinal tight junction proteins (zona, occludens-1) decreased, while hepatic carnitine palmitoyltransferase-1 activity increased significantly with KR inclusion > 6 % compared with the control group (P < 0.05). KR inclusion > 9 % suppressed hepatic metabolism (alanine aminotransferase, hexokinase) and intestinal antioxidant capacity (superoxide dismutase), concurrently reducing serum albumin and glucose levels. High KR inclusion (12–15 %) induced metabolic dysregulation, evidenced by elevated serum aminotransferases and upregulated intestinal pro-inflammatory cytokines (IL-1β, TNF-α), coupled with diminished metabolic and antioxidant enzyme activities as compared with the control group (P < 0.05). Through quadratic regression analysis, the optimal KR inclusion level was determined as 6.57–6.72 % for maximal growth performance. These findings demonstrate that excessive KR inclusion (>9 %) compromises nutrient assimilation and induces metabolic stress in juvenile turbot, providing critical thresholds for sustainable aquafeed formulation.