Comparative histological and stereological analysis of gonads from longline- and purse seine-caught Atlantic bluefin tuna Thunnus thynnus was undertaken to assess the reproductive potential of the population in the western Mediterranean breeding area. Bluefin tuna sampled by purse seine clearly had higher gonadosomatic index values (GSI) than those caught by longline. While no remarkable histological differences were found between purse seine- and longline-caught males, the ovarian histology indicated a higher level of maturity in females from purse-seine samples, where most specimens were found to be in active spawning (AS). Within the longline sample the proportion of active spawning females was considerably higher at the end of the fishing season (mid-June to mid-July) than earlier in May to mid-June (52.8 vs. 24%). Stereological analyses revealed that the samples collected by the 2 sampling gears differed primarily in the numbers of migratory-nucleus oocytes, which were overwhelmingly more abundant in purse-seine catches, with average relative fecundity: 59 oocytes g(-1) vs. 1.2 oocytes g(-1) (purse-seine vs, longline). The frequency of ovaries with postovulatory follicles (POFs) in purse seine-caught bluefin tuna was 83.7%, equivalent to a spawning interval of 1.2 d; in contrast, only 32.6% of the females in longline catches had POFs, representing a spawning interval of 3.1 d. Assuming an average individual duration of the spawning process of 14 d, and considering the estimates of fecundity and spawning interval obtained from purse-seine samples, the average annual fecundity of bluefin tuna in the Balearic spawning ground would be approximately 77 x 10(6) eggs (relative fecundity of 702 eggs g(-1) BW [body weight]).