Size and age at 50% maturity in southern bluefin tuna. An integrated view from published information and new data from the spawning ground.

Jessica Farley, Jessica Farley and John Gunn

CSIRO Marine Research, HOBART, AUSTRALIA

THEME: ASFB

The size and age at which southern bluefin tuna (SBT) reach maturity is of critical importance within the SBT stock assessment. Several methods have been used to assess SBT maturity in the past, giving a range of results from 130 cm to 157 cm. At the 1994 CCSBT Scientific Workshop, an estimate of age 8 yrs was adopted as the 'mean size at first maturity' (size at which 50% of fish are mature) for SBT based on the conversion of length (130 cm) to age using available growth curves. If SBT matured at age 8, we would expect significant numbers of 8 year-olds to be caught on the spawning ground. However, there are very few fish less than 12 years-old in the Indonesian longline catch on the spawning ground. Questions have been raised, however, over the whether data from the Indonesian fishery are representative of the spawning population. This has been clarified to some extent by recently work on the size partitioning by depth of SBT on the spawning ground. However, there remains some uncertainty regarding the relative abundance of smaller/younger fish at different depths. We use data from CSIRO monitoring of the Indonesian fishery to estimate the size and age at which 50% of the population has recruited onto the spawning ground. These provide a reasonably consistent estimate of size at maturity between 158.4 and 163.1 cm, which translates to an age in the order of 11 or 12 years.