In 1980 and 1981, divers in Oura Bay, near the town of Shimoda at the tip of Japan’s Izu Peninsula, cleared small squares of seafloor at five to ten metres deep and weighed the kajime kelp (Ecklonia cava) they collected. At the seasonal peak, it came to about 2.37 kilograms of dried kelp per square metre. When researchers went back to the same bay in 2023 and 2024, they didn’t find a single plant. A new paper in Global Change Biology, led by Ben Harvey of the University of Tsukuba’s Shimoda Marine Research Center with colleagues in Japan, France and the UK (including the University of Plymouth and the Marine Biological Association), tries to explain how a forest that lasted for decades disappeared. Its answer is two pressures working together: heat that the kelp can’t tolerate, and plant-eating fish that now get far longer to feed.
The decline was gradual at first. Peak biomass in the bay was about a third lower by the mid-1990s and more than 80 percent lower by 2015–16. A national long-term monitoring program, Monitoring Site 1000, then recorded the final collapse: kajime cover went from 75 to 80 percent in 2009–2010 to under 5 percent by 2020, and the kelp was gone entirely by 2022–2023. The team combined these biomass and cover records with satellite sea temperatures, fish surveys and lab experiments, covering about 200 kilometres of the Izu coast. Across that stretch, reefs have been turning into isoyake, the Japanese term for barren rocky ground where seaweed used to grow.
Most of the heat arrived recently. From 1982 to 2016, satellite records show the waters off Izu warming by about 0.15°C per decade. Then the Kuroshio, the big warm current running along Japan’s Pacific coast, entered a prolonged “Large Meander” from August 2017 to April 2025. Over 2017–2024 the warming trend jumped to nearly 3°C per decade, and the region had roughly 34 more days at or above 29°C than in the eight years before. In the lab, young kajime held up well until about 29.5°C, after which its photosynthesis fell sharply, and plants were no longer viable at around 32 to 33°C. Even below that limit, warmth slowed them down: over 35 days, juveniles kept at 22°C grew about 45 percent in surface area, while those at 28°C grew only about 12 percent.
The fish surveys, run roughly monthly from May 2021 to March 2022, compared three nearby reefs: one still forested, one patchy, one bare. Underwater video showed that a single species, the Japanese parrotfish (Calotomus japonicus), took most of the bites out of the kelp: about 69 percent at the forested reef, 85 percent at the barren one and 95 percent at the patchy one. Parrotfish and the other subtropical and temperate grazers start feeding in earnest once water reaches about 15 to 16°C. During the meander years those temperatures arrived about 71 days earlier, and the grazing season grew by an estimated 86 days a year overall. The rabbitfish Siganus fuscescens gained about 35 days. The authors’ interpretation is that summer heat weakened and killed kelp while longer grazing stopped surviving or new plants from recovering, and that combination drove the collapse.
The authors are careful about the limits of their evidence. They state that their datasets can’t separate exactly how much each driver contributed. They had only one reef of each type to compare, satellite temperatures ran almost a degree warmer than measurements taken in the water, and their lab thresholds come from a single kelp population, so they aren’t a universal limit for the species. For restoration, they suggest pairing kelp seeding with managing herbivorous fish, timing work for cooler periods, and keeping up long-term monitoring. The meander ended in spring 2025, and whether Izu’s kelp can come back now that it has is a question this study can’t yet answer.
Further reading: Harvey, B.P. et al., “Unprecedented Warming and Increased Herbivory Drive Collapse of Japan’s Kelp Forests,” Global Change Biology (2026); University of Tsukuba, “Ocean Warming and Herbivorous Fish Grazing Drive Kelp Forest Collapse Along Japan’s Izu Peninsula” (2026).