The Atlantic northern bluefin tuna is one of the most commercially valuable wild animals in the world. In the last two decades this species has been subject to intense over-fishing, which has caused a decline in both the eastern and western populations because of lowered recruitment. A good understanding of the reproductive parameters (especially sexual maturation, fecundity and spawning) of tunas is of paramount importance for population dynamics studies and the management of their fisheries. Nevertheless, a very limited amount of scientifically useful information is available on the reproductive biology for most tunas. Recent work has increased our knowledge on the reproductive biology of female Thunnus thynnus in the eastern Atlantic and the Mediterranean, but many questions remain still to be answered regarding male reproductive activity in this and other tuna species.

Gonad histology and ultrastructure, as well as steroid hormones levels, were studied in bluefin tuna from the eastern Atlantic and Mediterranean during the reproductive season. Gonad maturation was investigated by comparing samples from bluefin caught on their eastward reproductive migration off Barbate (Strait of Gibraltar area) with samples of bluefin fished in spawning grounds around the Balearic Islands.

Gametogenesis in both sexes is described by light and electron microscopy, showing similarities and differences with other teleosts studied so far. In the case of females, several reproductive parameters were estimated, like batch fecundity (through stereological methods), spawning frequency or cost of spawning.

Regression between steroid hormones (testosterone, ketotestosterone and estradiol, measured through ELISA) concentration and sex or maturational status (assessed through morphometric indices or stereological estimations, in the case of females) aimed to develop a tool for determining sex or reproductive status through byopsies, with the evident interest in a species of such economic importance.


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