Commercial humic substances (HS) and a protease complex (PC) have previously been shown to improve resistance to columnaris disease and demonstrate growth potential in juvenile channel catfish . Although additives enhance fish health, some also have the potential to improve feed efficiency through enhanced digestibility of alternative protein sources. However, it is believed that the duration and period of application may interact with growth or immune enhancement. A feeding trial was designed to optimize the inclusion of HS or PC in a low-protein commercial catfish diet, employing varying feeding regimens to assess the ir impacts on growth performance and health.
An 8-week feeding study was conducted in thirty-six r ecirculating tanks. Channel catfish fingerlings (2.01±0.94g) were fed a control diet ( commercial catfish feed, CTL; 28%) or supplemented diets (PC or HS; 28% ). The control diet was fed in four replicates on a satiation schedule for 8 weeks . The treatment diets were administered through four feeding regimens also in four replicates each: 1) every day for 8 weeks , 2) for the first 4 weeks, 3) for the final 4 weeks , and 4) every other week. Fish were fed the control diet when the supplemented diets were not fed. Following the feeding trial, catfish were sampled for growth performance, blood, and organs for baseline health indicators. Individual additives regimen groups were compared with the control.
The trial demonstrated a high survival rate (98.4±0.05 %) across all treatment groups. Growth results indicated an improved growth pattern in the supplemented diets compared to the basal-fed group. C ontinuous supplementation of HS or PC significantly reduced FCR compared to the CTL group after 8 weeks (P=0.0 04; P<0.001) . Similarly, supplementing PC or HS during the last 4 weeks contributed to a lower FCR than the control (P =0.046; P<0.001) and was indifferent to continuous feeding of any additives . Regarding the percent weight gain (WG%), tank biomass gain (TBG), and thermal growth coefficient (TGC), continuous supplementation of PC or HS also offered the best performance. Feeding PC for the final four weeks significantly improved TBG and higher percent weight gain (WG%) than the control group. In addition , continuous PC treatment enhanced sera lysozyme compared to the control (P=0.041). While no significant trend was observed in sera lysozyme for different HS regimens, skin mucus lysozyme was significantly improved after offering HS in the last four weeks compared to the control (P=0.032) . These findings demonstrated that continuous supplementation of PC or HS in commercial catfish diets best improves growth and enhances the non-specific immune response, indicating immunostimulation properties linked with these additives.