According to Foreman (1980), for the Pacific tuna, and Bard (1981) for the Atlantic tuna, before sexual maturity is reached, there is a 1:1 sex ratio.
However, a higher presence of males in the larger length classes has been reported for Thunnus alalunga as well as for other species. As size increases the proportion of males also increases up to a size where no females at all are found.
Bard (1981) concluded that this is due to differential growth and mortality. When sexual maturity is reached, the percentage of females per size decreases strongly, with a clear male prevalence in individuals longer than 85 cm. There are almost no females in sizes longer than 100 cm FL (Postel 1964; Bard 1981).
Megalofonou (1990), in the Aegean sea, found a sex ratio of females to males of 1:2.1 for a size range of 54 to 89 cm FL.