Despite the name, southern bluefin and Pacific bluefin (PBT) may not be particularly closely related. Recent DNA analysis suggests southern bluefin actually sits closer to the tropical tuna group than to the other two bluefin species. Pacific bluefin mitochondrial DNA, meanwhile, is almost identical to albacore — most likely the result of the two species hybridising in the deep past.
A Different Migration Story
Historically, PBT were found off the west coast of the South Island, but reports have recently been coming in from the North Island as well. Unlike SBT, Pacific bluefin has several known spawning locations near Japan and Taiwan. And unlike SBT, it will cross the entire Pacific basin — from Japan to the Americas and down to New Zealand.
Bigger and Faster-Growing
PBT is a substantially larger animal, reaching 400 kg against the SBT's 170 kg. It also grows faster, reaching maturity at age 5, and lives to around 26 years, though the average is closer to 15.
Along with SBT, PBT is one of the coldest-water tuna species, and can run its swimming muscle 10°C above the surrounding water — letting it push further into cold water than most tuna. That ability builds gradually though, and smaller fish are much worse at it than larger ones. Interestingly, a bluefin doesn't warm its own heart, so its thermal range may be dictated less by how well its muscle warms its blood and more by how cold its heart can tolerate getting.
Telling the Two Apart
PBT has traditionally been much rarer in our waters, but recent catches and sightings make telling the two species apart worth knowing.
Size. The easiest check. If the fish is over 220 cm or over 200 kg, it's almost certainly a PBT. A 100 kg fish is more of a problem — at that size the two are difficult to separate on size alone.
Caudal keel. The big central ridge at the base of the tail. Yellow or pale means SBT. Dark or black means PBT. Large SBT can develop a darker keel, so a yellow keel confirms SBT, but a dark one is inconclusive either way.
Body cavity bulge. Once the gills and guts are out, SBT shows a prominent bulge at the top front of the body cavity. PBT shows a small, narrow bulge in the same spot.
Gill rakers. If you're willing to count the spiky structures inside the gills (visible from the fish's mouth), SBT has 31 to 34, PBT has 34 to 43. A count in the high 30s confirms a PBT.
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Words by Lonny, marine biologist.