It’s a dish, as the old joke goes, to die for. Fugu, also known as pufferfish, is a Japanese delicacy that comes seasoned with an outside chance of death.

In reality, you’re extremely unlikely to be poisoned—dodgy oysters kill significantly more people each year. Ordering fugu is glamorous, a little dangerous, and very expensive, with a high-season price for the fish alone of $265 a kilogram.

Now, there’s yet another reason to pause before tucking in. Climate change—specifically the warming seas around the Japanese archipelago—are causing pufferfish species to interbreed, creating mutant hybrid fish.

As Reuters reports, these hybrid pufferfish aren’t necessarily more dangerous than either of their parents. The risk comes in, however, in understanding the precise locus for the fish’s toxicity. Ordinarily, the deadly neurotoxin reserves can be found in the liver and reproductive organs, which can be excised with relative ease. When if it’s elsewhere, such as in the skin or muscle, there’s a far greater risk, especially if the preparer has mistaken them for the ordinary sort.

Here’s an obvious solution: Don’t eat hybrid pufferfish. (The Japanese government agrees.) But it’s harder to execute than you might think—even veteran experts struggle to differentiate hybrids with the naked eye. “Quarters,” or the second-generation offspring of hybrid fish, look all but identical to their non-hybrid parent. What’s more, these mutant fish are increasingly very common, making slip-ups even more likely. On one day at the end of June, according to Reuters, hybrids made up more than 20% of all pufferfish caught off the Pacific coast of Miyagi prefecture, 460 kilometers (285 miles) northeast of Tokyo.

The Japanese government is now taking steps to investigate the problem, tasking the health ministry division responsible for food safety with collecting information since September. Industry groups are applying pressure on the government to standardize chefs’ licenses to prepare fugu across Japanese provinces. A failure to do so could be deadly—no matter how tasty.