<![CDATA[This study aimed to evaluate the effect of the timing of a 40°C heat shock applied at 5, 7, 9, and 11 min post-fertilization on the hatching rate of common carp (Cyprinus carpio) eggs. The experiment was conducted at the Fish Seed Center of Binjai City from April to June 2026 using a completely randomized design with four treatments and three replicates. The experimental unit was the incubation container, with three containers per treatment, 20 eggs per container, and 240 eggs in total, all obtained from a single broodstock pair. Data were analyzed using analysis of variance, polynomial contrasts, and the Least Significant Difference (LSD) test, and the findings were further verified using binomial logistic regression. Heat-shock timing had a highly significant effect on hatching rate (F(3,8) = 35.41; p < 0.001; ω² = 0.90). Mean hatching rates increased monotonically with later heat-shock application, reaching 35.00 ± 5.00%, 50.00 ± 10.00%, 76.67 ± 5.77%, and 86.67 ± 5.77% at 5, 7, 9, and 11 min post-fertilization, respectively. The linear component accounted for 97.3% of the treatment variation (p < 0.001). However, the hatching rate at 11 min did not differ significantly from that at 9 min (difference = 10.00%; LSD₀.₀₅ = 13.03). Therefore, the 11-min treatment produced the highest mean hatching rate among the treatments tested, but it cannot be considered the optimal timing. Because the experiment did not include an unshocked control group, the observed differences reflect the effect of heat-shock timing rather than the effect of heat shock relative to untreated eggs. These findings suggest that earlier heat shock may be more detrimental to embryonic development and should be further evaluated using an untreated control and a broader range of post-fertilization times.]]>
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