The edible brown alga Sargassum fusiforme (Fucales) at the southern distributional limit in mainland Japan has declined on coasts facing the open ocean, showing reduced size and density

SUMMARY The status of the edible brown alga Sargassum fusiforme (Fucales) in Kagoshima in 2025, located at the southern distributional limit in Kyushu, mainland Japan, was investigated to assess whether its populations were stable or declining, based on comparisons with herbarium specimens and records from previous studies conducted between the 1960s and 2010s. Population coverage and biomass (population volume index), individual weight and height, and the degree of herbivory were measured in both field and laboratory settings. As a result, stable populations with high coverage and large biomass were observed in sheltered or semi‐enclosed areas such as the inner part of Kagoshima Bay and the Yatsushiro Sea. In contrast, this alga had largely declined or disappeared in areas facing the open ocean, including many former collection sites in previous studies. Modeling the relationship between site topographic openness and the thallus height at the study sites, in comparison with past collection sites, revealed that the overall morphology of S. fusiforme exhibited significant elongation at moderate openness levels in both the 2025 and past datasets. Conversely, limited algal heights were recorded at highly open sites facing the open ocean, suggesting that poor growth or herbivory impacts directly or indirectly associated with climate change may be influencing these populations. Furthermore, given the strong positive correlation observed between reduced population density and algal height, suppression of thallus elongation by these factors likely results in a decline in population density, ultimately leading to population decline or local extirpation.