Partial Replacement of Fish Meal with Fish Scale Meal Enhances Body Color Brightness and Iridophore Development in Koi (Cyprinus carpio var. koi)

Body color brightness is a key determinant of ornamental value in koi, yet nutritional strategies targeting structural coloration mediated by iridophores remain underexplored. Fish scale meal (FSM), an abundant fishery by-product, contains guanine and collagen, which may contribute to structural coloration and body brightness. This study evaluated the effects of partial replacement of fish meal with FSM on growth performance, body color parameters, pigment accumulation, iridophore density, and iridophore-related gene expression in koi. A total of 384 koi with an initial body weight of 19.02 ± 0.18 g were randomly assigned to six dietary treatments for 8 weeks, with FSM inclusion levels ranging from 0% to 9.55%, corresponding to 0–100% replacement of dietary fish meal. Growth performance, including final body weight, weight gain rate, and specific growth rate, decreased with increasing FSM inclusion. Body lightness (L*) and scale guanine content increased progressively with FSM level and reached their highest values at 9.55% FSM, whereas redness (a*), yellowness (b*), and total carotenoid content showed the opposite trend. Iridophore density was significantly higher in the 3.82–5.73% FSM groups than in the other treatments. The expression of iridophore-related genes, including sox10, pax3, ltk, alx4a, and foxd3, showed non-linear responses and generally reached higher levels at moderate FSM inclusion, whereas pka showed an opposite transcript pattern. Overall, dietary FSM inclusion elicited trait-dependent responses in koi. Low or no FSM inclusion was preferable for maintaining growth, intermediate inclusion was more favorable for iridophore development, whereas higher FSM inclusion enhanced brightness-related traits. These findings suggest that FSM should be used according to the intended production objective, with inclusion levels adjusted for growth maintenance, iridophore-related coloration improvement, or brightness enhancement.