**Background:** The common carp (Cyprinus carpio L.) is a globally important aquaculture species, with global production reaching 3,821,611 tonnes in 2019 and Polish production estimated at ~21,000 tonnes in 2020. Controlled reproduction is essential for synchronized spawning, typically achieved using carp pituitary homogenate or GnRH analogs combined with dopamine antagonists. Previous research at the Gołysz Institute (Poland) demonstrated that reproductive outcomes vary significantly between different carp breeding lines, even under identical conditions. Luteinizing hormone (LH) stimulates ovarian follicles to produce 17α,20β-dihydroxyprogesterone (17α,20β-DHP), the maturation-inducing hormone that promotes oocyte maturation and ovulation. This study aimed to characterize and compare reproductive effectiveness and hormonal profiles (LH and 17α,20β-DHP) in two genetically distant carp lines (Polish line 6 and Lithuanian line B) during Ovopel-induced ovulation.
**Methods:** The experiment was conducted at the Institute of Ichthyobiology and Aquaculture in Gołysz (Polish Academy of Sciences) during the natural spawning period in May. Twenty-eight females (10–11 years old, 7.1–11.1 kg) were selected: 13 from Lithuanian line B and 15 from Polish line 6. After 2 days of acclimatization at 20–21°C, females received a priming dose (1/5 pellet/kg BW) of Ovopel (containing 18–20 µg D-Ala6,Pro9NEt-mGnRH-a and 8–10 mg metoclopramide per pellet), followed 12 hours later by a resolving dose (1 pellet/kg BW). Blood samples were collected at 0 h (before priming), 12 h (at resolving dose), and 24 h (at ovulation check). LH and 17α,20β-DHP were assayed by ELISA. Eggs were stripped, weighed, fertilized with pooled sperm from the same line, and incubated at 21±1°C. Living embryos were counted at 24, 48, and 70 h. Statistical analyses included least-squares ANOVA, F-tests, chi-square tests, Mann-Whitney U-tests, and Friedman tests with Dunn's post-hoc.
**Key Results:** Ovulation occurred at two time points per line: line B at 13 h (30.7% of females) and 15 h (46.2%); line 6 at 15 h (33.3%) and 18 h (20%). Ovulation ratio was 10/13 for line B and 8/15 for line 6 (not statistically significant by chi-square test). Egg weight was higher in line 6 (by 303 g and 2.95% BW) but not statistically significant. Egg quality was significantly higher in line B (p ≤ 0.05) at all incubation time points; after 70 h, living embryos were 14.5% higher in line B. Total egg number was higher in line 6 (by 209,000), but mean living embryos at 70 h were similar (~500,000) for both lines. LH levels increased significantly over time in both lines: line 6 from 2.798 to 200.3 ng/mL (p ≤ 0.05); line B from 2.669 to 242.7 ng/mL (p ≤ 0.05). No significant differences in LH were found between lines at any time point, or between ovulated and non-ovulated females within lines. 17α,20β-DHP also increased significantly: line 6 from 0.1918 to 0.5481 ng/mL; line B from 0.189 to 1.359 ng/mL (p ≤ 0.05). At 24 h, 17α,20β-DHP was significantly higher in ovulated vs. non-ovulated females in line 6 (0.664 vs. 0.416 ng/mL, p ≤ 0.05), but not in line B (1.639 vs. 0.423 ng/mL, not significant). Multiple regression showed better predictability of living embryos for line B (R² = 0.98) than line 6.
**Clinical Implications:** This study demonstrates that Lithuanian line B carp exhibit superior reproductive effectiveness compared to Polish line 6 when induced with Ovopel, characterized by higher egg quality, higher ovulation rate, and shorter latency periods despite lower absolute egg weight. The finding that LH and 17α,20β-DHP levels at 12 h post-resolving dose were higher in the line with better reproductive outcomes (line B) suggests that endocrine responsiveness to GnRH-analog treatment may be a marker of reproductive potential. These results have practical implications for aquaculture breeding programs, as line B requires lower costs per viable embryo produced. The lack of statistically significant hormonal differences between lines, despite clear reproductive performance differences, indicates that factors beyond circulating LH and 17α,20β-DHP levels—potentially including receptor sensitivity, gene expression differences, or other molecular mechanisms—contribute to differential breeding success. Further research at the molecular level, including mRNA transcript analysis of reproduction-related genes, is needed to elucidate these mechanisms.