The ovarian mass and gonadosomatic index (I_G) of bluefin tuna Thunnus thynnus, caught in the Strait of Gibraltar (Barbate) during migration to Mediterranean spawning grounds, were several times lower than those found in bluefin tuna from Mediterranean spawning grounds (Balearic Islands). Some of the bluefin tuna from Barbate (8.3%) were classified as immature (the most advanced oocytes present in the ovaries were early vitellogenic), and the majority (the remaining 91.6%) as non‐spawning mature; the ovary contained late vitellogenic oocytes, but there was no sign of spawning activity. Stereological estimation indicated that the ovaries of spawning bluefin tuna from the Balearic Islands contained five‐fold more highly yolked oocytes than bluefin tuna from Barbate. When breeding bluefin tuna cross the Strait of Gibraltar the gonad is at an incipient stage of maturation. The average batch fecundity estimated from stereological quantification of stage 4 (migratory‐nucleus) oocytes in the specimens collected from Balearic was 92.8 oocytes g‐1‘of body mass, and the spawning frequency in this area was calculated to be 1.2 days. In specimens from Barbate a relative batch fecundity of 96.3 oocytes g ‐1 was estimated using stage 3 (late vitellogenic) oocyte counts.