Ask a Kiwi fisho what species of tuna we have here, and you'll likely get a list along the lines of yellowfin, bluefin, bigeye, skipjack and albacore — maybe bonito or butterfly tuna thrown in for good measure. Here's the thing: a bonito might look a lot like a skipjack, but it isn't a tuna at all. Despite the name, a butterfly tuna isn't one either. Even the skipjack itself doesn't qualify as a “true tuna”.
So what actually makes a tuna a tuna?
Fifteen Species, One Weird Trick
There are 15 species of tuna, and what sets them apart from every other fish is a mass of red swimming muscle buried deep inside the body, rather than sitting just under the skin like it does in most fish. More interesting still: their blood vessels are plumbed so that the heat coming off those muscles gets recycled back into the body instead of being flushed out through the gills.
Their blood vessels work like a heat exchanger. It's why a tuna can keep itself warmer than the water it's swimming in — and why species like the southern bluefin have become arguably the most successful cold-water predatory fish there is.
Not every tuna regulates its temperature equally well, though. And the fish we lump together as “bluefin” may not be as closely related as the name suggests.
An Absurd Size Range
The 15 species split into 5 genera, and the size range across them is ridiculous. At one end, tiny bullet tuna you can sometimes buy as bait. At the other, the largest Atlantic bluefin ever caught weighed 679 kg.
The “True Tunas”
The “true tunas” are the 8 species in the genus Thunnus. Traditionally this group splits in two. The bluefin group covers albacore, southern bluefin, Pacific bluefin and Atlantic bluefin. The yellowfin group covers yellowfin, blackfin and longtail. Bigeye tuna sits in Thunnus too, fitting about equally into both categories, but it's usually grouped with the bluefins because of how it handles cold water.
Those groupings make sense when you look at the fish and where they're found. The DNA evidence tells a different story about who's actually related to whom — but that's a post for another day.
What's in New Zealand Waters
New Zealand does well for tuna. Around 8 species turn up in our waters, and of those, 4 matter most to anglers: skipjack, albacore, yellowfin and southern bluefin — plus a fifth, Pacific bluefin, that's becoming harder to ignore.
Over the next few posts we'll go through each one — what makes it tick, where to find it, and how to tell the two bluefins apart when it matters.
- Skipjack — NZ's Southern Limit
- Albacore — NZ's “White Meat” Tuna
- Yellowfin — Living on the Cold Edge
- Southern Bluefin — NZ's Comeback Fishery
- Pacific Bluefin & Telling the Two Bluefins Apart
- Caring for a Landed Tuna
Words by Lonny, marine biologist.