Reproductive parameters of wild and hatchery‐reared sea trout ( <scp> <i>Salmo trutta</i> m. </scp> <i>trutta</i> <scp>L</scp> .) females from the <scp>Ł</scp> eba River (southern <scp>B</scp> altic Sea)

Sea trout Salmo trutta m. trutta L. has high socioeconomic and ecological importance in the Atlantic region; therefore, stocking hatchery-reared fish is widely practiced for stock enhancement and recovery of wild populations. In this study, fecundity of wild and stocked hatchery-reared sea trout sampled from the Łeba River (southern Baltic Sea basin) was evaluated. The average absolute fecundity (F_a) was 5311 and 5338 eggs in the A.1+ age group, and 7897 and 9796 eggs in the A.2+ age group for wild and hatchery-reared fish, respectively. Significant difference in the average F_a values between wild and hatchery-reared female sea trout within the age groups was not observed. The relationships between F_a and fish length (FL) (log₁₀ F_a = -1.6238 + 3.0544 × log₁₀ FL, R² = 0.809) and between F_a and fish body weight (log₁₀ F_a = 0.4845 + 0.9787 × log₁₀ W_total, R² = 0.874) were strongly positive. The relative fecundity of the analysed females was, on average, 2527 and 2652 eggs per 1 kg of total body weight for one- and two-sea-winter sea trout, respectively, and did not differ significantly regardless of sea age and fish origin. This study demonstrated that egg size was strongly related to fish size, but fish origin did not influence this relationship. Furthermore, no significant differences were found between the mean gonadosomatic index (GSI) values of hatchery-reared and wild sea trout within the A.1+ and A.2+ age groups. Higher GSI values were observed, suggesting that Baltic sea trout invest more in gonad development than populations from other regions. Our results demonstrated that sea age only indirectly explained the absolute fecundity of sea trout, whereas the parameters that directly influenced fecundity were body length and weight. This study provides new insights for future stock recruitment analyses of Baltic sea trout and fills a gap in our knowledge of its reproductive parameters.