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Marine biologists say they've discovered a kind of living ghost in the Atlantic, a fish they didn't' believe existed. It's a giant predator with a sword-like bill and a tail so blue it seems to glow. As NPR's John Nielson reports, it's a finding that could lead to tighter fish-protection rules.

JOHN NIELSON: It's known as the roundscale spearfish, and to the untrained eye it looks exactly like a white marlin. That's a fish you'll find in lots of Internet videos depicting epic battles between anglers and big sports fish.

(Soundbite of fisherman)

JASON SCHRATWIESER: White marlins do a lot of acrobatics when hooked. For many, they're the epitome of salt-water angling achievement.

NIELSON: Jason Schratwieser is a marlin expert with the International Game Fish Association. He says the white marlin is one of the most popular sport fish in the Atlantic and one of the most endangered. A while ago, conservation groups tried to get white marlin added to the federal endanger species list. Government biologists rejected that proposal in 2002, arguing in part that landing records provided by fishermen proved that white marlin weren't headed toward extinction.

But what if the white marlin caught by all those fishermen weren't really white marlin? What if they were roundscale spearfish that just look like white marlin? These are questions fishing experts and federal regulators have been asking themselves since a team of scientists used DNA testing to prove that some of the fish that look like white marlin aren't.

Mr. MAHMOUD SIVJI (Nova Southeastern University): There's absolutely no doubt that the roundscale spearfish is completely different from the white marlin.

NIELSON: Mahmoud Sivji, the lead author of the new study, runs the Guy Harvey Research Institute at Nova Southeastern University in Florida. Until a few years ago, he didn't believe the roundscale spearfish even exited. Then a billfish expert in a tuna fishing boat took a close look at a group of live white marlin hauled in by mistake.

Mr. SIVJI: He found four animals that looked a bit odd to him, and being a billfish taxonomist, he looked at them more closely…

NIELSON: And realized that he was staring at a case of mistaken identity. For example, while white marlin has extremely sharp and pointed scales, the fish in front of him had round and smooth scales. When one of these not-quite-marlins died, it was immediately shipped to Sivji's lab for DNA testing.

In the current issue of the Bulletin of Marine Science, Sivji and several co-authors confirm that this fish wasn't a white marlin but a roundscale spearfish. Sivji says his paper raises serious new questions about how many white marlins are still left in the Atlantic.

SIVJI: Nobody knows what proportion of fish that have been called white marlin for all these years have actually been a roundscale spearfish, and it could be a very low proportion, or it could be something substantially higher.

NIELSON: If it is higher, conservationists say the federal government may have to reconsider its decision to deny the white marlin a spot on the endangered species list. But one billfish expert says this paper should also make people pause and think about the fish that's just been found here. That's Jason Schratwieser of the International Game Fish Association. After all, he says, when was the last time you saw and eight-foot-long ghost?

Mr. SCHRATWIESER: It just goes to show that it's a bid ocean out there, and we're still, you know, confirming or describing new species at, you know, this late age, and it's pretty amazing.

NIELSON: Scientists are already planning to launch follow-up studies that will separate the marlin from the roundscales. John Nielson, NPR News, Washington.

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