Abstract:The study examined the impacts of four kinds of photoperiod on the gastrointestinal contents of two sizes of juvenile flounder (Paralichthys olivaceus) after being fed to satiation. It also determined the optimal photoperiods, feeding times, and the mathematical model for the optimal gastric emptying rate for the juvenile flounder feeding. The results showed that the instantaneous gastric emptying rate of the juvenile flounder increased over time post-feeding, eventually reaching 100%. For 70 d of juvenile flounder, the 24L∶0D photoperiods group had the highest gastrointestinal content weight ratio, followed by the 18L∶6D group, and the control group had the lowest ratio. The fastest gastric emptying rate was observed in the 24L∶0D group, followed by the 18L∶6D group, and the slowest rate in the control group. For 100 d of juvenile flounder, the 18L∶6D group had the highest gastrointestinal content weight, followed by the 12L∶12D group, and the lowest weight in the 24L∶0D group. Meanwhile, stomach emptying rate was fastest in the 18L∶6D group, followed by the 24L∶0D group, and the slowest in the control group. The experimental data from both 70 d and 100 d of juvenile flounder under different photoperiods were well-fitted by three models, with varying correlation coefficients (R2). The linear model was more suitable for the 70 d control group, 24L∶0D group, and the 100 d 12L∶12D group; the exponential model was more appropriate for the 70 d 12L∶12D group and the 18L∶6D group, and the square root model was better for the 100 d control, 18L∶6D, and 24L∶0D groups. Overall, the photoperiods had a significant effect on the food intake in 70 d and 100 d of juvenile flounder. The photoperiods of 70∶d and 100 d of juvenile flounder should be 18L∶6D, and the second feeding should be carried out at 18.5h for 70 d of juvenile flounder and 22.5 h for 100 d of juvenile flounder. [Chinese Fishery Quality and Standards, 2024, 14(4): 14-19]