Reproductive assessment of farmed and wild Atlantic bluefin tuna (Thunnus thynnus) in the eastern Adriatic Sea during the spawning season

The role of the Adriatic Sea in the reproductive dynamics of Atlantic bluefin tuna (ABT, Thunnus thynnus ) remains poorly resolved, despite its importance as a feeding and farming area. This study provides a comprehensive assessment of reproductive traits in farmed and wild ABT sampled in the Croatian Adriatic Sea during the 2024 spawning season. A total of 317 farmed and 16 wild individuals were examined to quantify sex ratio, gonad development, oocyte dynamics, batch fecundity, spawning frequency, and condition indices. Histological analyses confirmed asynchronous batch spawning, with spawning-capable and actively spawning females present from mid-May to mid-June. Gonadosomatic index patterns, oocyte size distributions, and ovarian development stages were broadly consistent between farmed and wild fish, and aligned with values reported for the Eastern Atlantic and Mediterranean stock. Mean batch fecundity and relative batch fecundity were within known biological ranges, while spawning frequency estimates indicated intervals of approximately 1.7 days in farmed and 1.4 days in wild individuals. Observed reproductive parameters reflected the generally smaller and likely younger size structure of Adriatic ABT compared with other Mediterranean spawning areas. The combined evidence demonstrates that ABT in the Adriatic Sea are reproductively active both in the wild and within farming cages. Together with previous observations of early life stages in the region, these findings indicate that the Adriatic Sea may function not only as a feeding and farming area, but also as a complementary or seasonally important spawning habitat. This information is relevant for refining stock assessments and spatial management measures under the ICCAT.