GENETIC POPULATION-STRUCTURE OF SOUTHERN BLUEFIN TUNA (THUNNUS-MACCOYII)

Samples of southern bluefin tuna, Thunnus maccoyii (Castelnau), taken from off the coasts of South Africa, Western Australia, South Australi a and Tasmania from 1992 to 1994 were analysed for six polymorphic all ozyme loci (ADA, GDA* GPI-A*, MPI*, PGDH* and PGM-1*, n = 595 to 733 per locus) and for mitochondrial DNA variants revealed by three restri ction enzymes (BamHI, BclI and EcoRI) detecting polymorphic cut sites (n = 555). No significant spatial heterogeneity was detected. There we re no sex-related differences in allele or mtDNA haplotype frequencies . Juveniles (30 to 35 cm and 46 to 54 cm) from what are thought to be two temporally-separated spawning peaks showed no significant genetic differentiation. There were also no significant differences in allele or haplotype frequencies between fish smaller than 70 cm and those lar ger than 70 cm. These data are consistent with the null hypothesis of a single unit stock of southern bluefin tuna, with a single spawning a rea. This is located to the south of Java and off the north-west coast of Australia.