Demographic patterns of southern bluefin tuna, Thunnus maccoyii, as inferred from direct age data

Abstract

Otolith-based age estimates were obtained for 2769 southern bluefin tuna (SBT) ( Thunnus maccoyii) caught on feeding grounds in the southern oceans and their spawning ground in the north-east Indian Ocean between 1985 and 1998. Ages ranged from 0 to 41 years, and males were on average larger at age than females after age 6 for fish sampled in the southern oceans. This sexual dimorphism in growth contributed to the sex ratio being biased toward males for length classes ≥170 cm FL. The estimated age composition of catches, derived from age-length keys applied to length frequency data, showed clear differences between fisheries and fishing grounds that are consistent with seasonal and ontogenetic changes in migration patterns. Specific information that can be gained on the distribution and migration of juveniles from their relative abundance in catches is limited because of unknown targeting or discarding practices on some fishing grounds. However, the occurrence of 2–4-year-old SBT on opposite sides of the Indian Ocean during the austral summer confirms that juveniles are not restricted to the southern coastal waters of Australia, and that a divergent migration path must exist possibly near the southern west coast of Australia. During the winter months, SBT aged 2–4 years were caught on all fishing grounds south of the spawning ground, confirming that they are capable of extensive migrations. By age 5, SBT were rarely caught north of about 35°S, except in waters adjacent to Australia's west coast where adults were caught migrating to/from the spawning ground. The commercial catch of SBT in the southern oceans was dominated by juveniles and sub-adults on most fishing grounds examined. Off north-east New Zealand, however, the majority of fish caught were of spawning age (61% were >10-year old) suggesting that the Tasman Sea may form the eastern boundary for juvenile migration. SBT caught on the spawning ground ranged in age from 8 to 34 years with the majority being between 15- and 25-years old.