Allozyme and mitochondrial DNA separation of Pacific northern bluefin tuna, Thunnus thynnus orientalis (Temminck and Schlegel), from southern bluefin tuna, Thunnus maccoyii (Castelnau)

Northern and southern bluefin tunas are morphologically similar and can be misidentified, posing problems for fishery management and marketing. Allozyme variation and restriction-site variation in mitochondrial DNA (mtDNA) were used to distinguish between the two species. A survey of 36 allozyme loci active in white muscle and liver tissue showed that the genetic identity between the species was high (Nei's I = 0.907). One diagnostic locus (sAH*) and two nearly diagnostic loci (ADA* and GDA*) were found, and four loci showed highly significant allele frequency differences (FH*, GPI-A*, PGDH* and sSOD*). A survey of the mtDNA genome, using 15 restriction enzymes and southern blotting, revealed five restriction enzymes that gave species-diagnostic restriction digest profiles (Ban I, Bcl I, Dra I, Pvu II, Xba I) and a further three enzymes (Pst I, Bam HI and Nco I) with large haplotype frequency differences. Mitochondrial DNA analysis provided more reliable discrimination of specimens than did allozyme analysis, although the more rapid allozyme identification will be accurate for most specimens. The two biochemical genetic methods were then used to identify Australian-caught fish of uncertain identity. Six of 12 tuna originally considered to be northern bluefin tuna were confirmed as northern bluefin and six were identified as southern bluefin. The presence of northern bluefin tuna as far south as south-western Tasmania was confirmed.