Marine Heatwaves and Herbivorous Fish Drive Kelp Forest Collapse off Japan

29 October 2026
Edited By: Haley McQueen

Kelp forests, ecosystems formed by large brown seaweeds in coastal waters, provide habitats for numerous fish and invertebrates and are vital to coastal productivity and carbon cycling. However, in recent years, kelp forests have declined throughout Japan, alongside the spread of isoyake, a phenomenon in which rocky reefs become largely devoid of large seaweeds. This ecological shift has been primarily attributed to rising sea temperatures and intensified grazing by herbivorous fish.

Kelp forests along Japan’s Izu Peninsula support rich marine biodiversity and play an important role in coastal ecosystems. However, rising ocean temperatures and prolonged grazing by herbivorous fish are driving widespread kelp forest decline, threatening these important underwater habitats. (Image credit: Shigeki Wada)

To identify the drivers behind this collapse, researchers investigated the causes of kelp forest decline by integrating more than 40 years of data on the biomass of Ecklonia cava (kajime kelp) and long-term surveys of kelp cover across approximately 200 km of the Izu Peninsula coastline. These datasets were combined with satellite-derived sea surface temperature and marine heatwave records, field surveys of herbivorous fish, and laboratory experiments examining the thermal tolerance of E. cava.

The study found that the Izu coast experienced a dramatic surge in warming and marine heatwave activity from 2017 to 2025, during the prolonged Kuroshio Large Meander event. Laboratory experiments demonstrated that the photosynthetic performance of E. cava declined rapidly at temperatures above approximately 29.5°C. At the same time, warming extended the annual window of temperatures suitable for herbivorous fish grazing by up to 86 days. These findings suggest that the combination of heat-induced physiological stress and a longer grazing season has hindered E. cava recovery, ultimately contributing to widespread loss of kelp forests and the transition to isoyake conditions along the Izu Peninsula.

To read the full article published in the Journal: Global Change Biology, visit: 10.1111/gcb.71106