Age and Growth of Juvenile Southern Bluefin Tuna Thunnus maccoyii based on Otolith Microstructure

Abstract

We examined the possibility that growth increments in otolith of southern bluefin tuna Thunnus maccoyii are deposited daily and investigated the growth in the juvenile stage of this species. Among samples collected from off the west and southwest coast of Australia between 1990 and 1993, otoliths of 122 fish from 247 to 820 mm FL were observed under SEM. The number of increments of these otoliths ranged from 78 to 877. The following findings support the daily deposition of increments: 1) The features of increments in the nucleus region were the same as those of larvae which were verified as daily increments, 2) The back-calculated date of first increment deposition corresponded well with the spawning period, and 3) Growth of tag-recapture fish agreed with that from otolith examination assuming daily deposition. Growth of this species in the juvenile stage was linear and 0.761 mm/day. Estimated sizes were 508 mm FL at age 1 and 786 mm FL at age 2, which was much larger than previously estimated.