Effects of long-term administration of exogenous dietary protease and humic substance on growth, immunity, and survival of Channel Catfish overwintered in an in-pond raceway system

ABSTRACT Objective This 9-month study aimed to investigate the effects of the prolonged application of a dietary protease complex (PC) or humic substance (HS) on the growth, immunity, and survival of submarket Channel Catfish Ictalurus punctatus reared in an in-pond raceway (IPR) system. Methods A 9-month feeding trial was conducted within nine IPR systems, each with 238 Channel Catfish (body weight [mean ± SE] = 206 ± 3 g). Raceways were randomly assigned to one of three 28% crude protein diet treatments in triplicate: a control diet (C28), a PC-supplemented diet (C28+PC), or an HS-supplemented diet (C28+HS). Growth sampling was performed every 30 d, and the final growth metrics, survival, immune responses, and gene expression were evaluated poststudy. Results Monthly sampling showed that growth performance in all groups declined with temperature reduction and was lowest during winter. Both C28+PC and C28+HS groups showed the best percent biomass gain and feed conversion ratio until the sixth month of the study. However, as temperatures fell below 20°C, growth performance and survival were reduced in both treatment groups. Although the feed conversion ratio was high across groups (>3.0), it was significantly lower in the C28+PC group than in the C28 group. Conversely, the C28+HS group had lower biomass gain than the C28 group. Fulton’s condition factor was higher in the C28+HS group than in the C28+PC and C28 groups. At harvest, the C28 group had significantly higher survival (85%) than both the C28+PC (41%) and C28+HS (34%) groups. Serum lysozyme activity was higher in the C28+PC group than in the C28+HS or C28 group after 9 months. Additionally, the C28+HS group showed increased serum lysozyme activity compared to the C28 group. Conclusions Although continuous administration of PC or HS in diets may enhance the nonspecific immune response and PC contributes to improved growth in catfish raised in IPR systems, prolonged exposure may compromise catfish immunity, increase susceptibility to environmental stress, and reduce survival.