'Hybrid' pufferfish with unpredictable toxins emerge in Korean waters as sea temperatures soar

"Not all pufferfish are the same."

A single pufferfish carries enough toxin to kill roughly 30 adults — yet Koreans have long eaten the fish safely, on the understanding that removing the toxic organs and preparing it properly makes it safe to consume.

Now, however, a new "mutant" variety is upending that assumption in Korean waters.

The culprit: hybrid pufferfish whose toxic organs cannot be reliably identified.

Rapidly shifting sea surface temperatures have pushed pufferfish into new habitats, where species that never previously encountered each other are now interbreeding.

Korean waters are warming faster than almost anywhere else on Earth, and sightings of previously unseen, highly venomous fish are becoming increasingly common.

According to the National Institute of Fisheries Science, the average annual sea surface temperature in Korean waters rose about 1.6 degrees Celsius over the 58 years from 1968 to 2025. Over the same period, the global average sea surface temperature increased by 0.76 degrees — meaning Korean waters are warming more than twice as fast as the world average.

The rate of warming has been particularly steep over the past decade. From 2016 to 2025, temperatures rose at roughly 0.09 degrees Celsius per year — about three times faster than the long-term average.

The marine environment is changing visibly as a result. One of the most striking shifts is a turnover in fish species. Many cold-water species — pollock and squid among the most familiar — have been disappearing from Korean waters. But as one species vanishes, another moves in to take its place.

Hybrid pufferfish are one example of this phenomenon. As pufferfish migrate northward in search of suitable temperatures, they encounter and breed with species they would not normally meet. The Ministry of Food and Drug Safety published a "Pufferfish Field Guide" on June 19 and said that hybrid pufferfish have been found in South Korea and Japan as rising sea surface temperatures alter pufferfish habitats.

Korea has long been home to multiple pufferfish species. Though highly toxic, they can be eaten safely when properly prepared according to species. Hybrid pufferfish are a different matter. They may look similar to familiar species, but it is extremely difficult to predict which parts of the fish contain toxins, or in what concentrations.

The ministry said hybrid pufferfish may carry toxin levels in their skin and other parts that exceed safe limits, classifying them as unfit for human consumption and urging the public to exercise caution.

The toxin in pufferfish is tetrodotoxin, which blocks nerve signal transmission and can be fatal. Deaths from pufferfish poisoning continue to occur. With hybrid pufferfish now appearing as a result of rising ocean temperatures, warnings are growing that pufferfish should only be prepared and consumed through licensed specialists.

Beyond hybrid pufferfish, highly venomous fish capable of directly harming humans are appearing more frequently of late, as subtropical species migrate northward with rising sea surface temperatures.

A subtropical filefish species has been caught repeatedly by anglers along the southern coast of Jeju Island. Its flesh and bones contain palytoxin, a poison more potent than pufferfish toxin. The toxin can be transferred through bare-skin contact alone, making it a serious hazard.

The National Institute of Fisheries Science has found that since the 2000s, highly venomous creatures from tropical and subtropical waters around Taiwan and Okinawa have been entering the waters off Jeju Island along the path of the Tsushima Warm Current. Blue-ringed octopuses and broad-banded sea snakes have been confirmed along the southern coast and around Jeju.

Venomous jellyfish also pose a growing threat. The institute recorded eight species of venomous jellyfish in 2025. The subtropical Portuguese man-of-war was first spotted in Jeju waters on May 9 of that year, appeared in large numbers in mid-to-late July, and spread along the East Sea coast through September.

Some analysts say "subtropicalization" is already underway. According to a 2024 climate change impact report published by the National Institute of Fisheries Science, subtropical fish species accounted for 29.4 percent of catches in coastal fixed-net surveys off Jeju, 13.2 percent off the East Sea coast, and 12.6 percent off the South Sea coast. In some coastal areas of Jeju, subtropical species made up more than half of all fish caught.

The changes in the ocean are also reshaping the fishing industry. Korea's coastal and offshore fishery output fell from roughly 1.51 million tons per year in the 1980s to about 920,000 tons in the 2020s — a decline of approximately 40 percent. Climate change alone does not account for all of this, but experts say shifts in the distribution and abundance of certain species are making it increasingly difficult to predict when and where to fish.

The effects of rising sea surface temperatures extend well beyond the fishing industry. The temperature of the seas surrounding the Korean Peninsula directly influences the weather Koreans experience. As sea surface temperatures rise, they affect temperatures on the Korean Peninsula and alter precipitation patterns. The changes unfolding in the ocean are, in effect, an early warning sign of climate change itself.

Meanwhile, climate change is raising the overall danger posed by the ocean beyond the emergence of venomous species. Ciguatera — a form of fish poisoning caused by toxic plankton — is becoming a growing problem overseas. As the ocean warms, the range of waters where ciguatera-producing plankton can survive expands.

The danger lies in the fact that fish accumulate the toxin after eating the plankton. When people eat grouper, sea bream or other species that have built up ciguatoxin in their bodies, they can suffer symptoms including numbness in the hands and feet, vomiting and diarrhea. Simply avoiding visibly toxic fish is not enough to eliminate the risk.

“한 마리에 1000만원인데” 이걸 다 버린다니 ‘충격’…많이 잡으면 손해, 대체 왜? [지구, 뭐래?]

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