First sighting in 45 years of a basking shark off Mie, Japan, and occurrence records of the species in Japanese coastal waters
摘要
Understanding species occurrence is crucial for elucidating distribution, habitat use, migration, and conservation. The basking shark Cetorhinus maximus (Gunnerus, 1765) is a large planktivorous shark distributed from subarctic to tropical waters. In Japan, a harpoon fishery off Mie Prefecture landed about 100 individuals annually between the 1940s and 1970s, but records became extremely rare after the 1980s. On 2 April 2025, a 6.0–7.0-m female basking shark was recorded in a set net off Owase, Mie Prefecture. The sea surface temperature was 17.1 °C, and chlorophyll-a concentration was 0.415 mg/m3, consistent with known favorable ranges. The shark showed no mating scars, parasites were observed on its dorsal surface, and appeared alone, suggesting an immature or recently mature individual likely migrating for feeding. A review of Japanese records from the 1940s to 2025 revealed 1056 individuals from 24 reports, including this study. Sightings peaked in spring during large phytoplankton blooms and occurred most frequently on the Pacific side. Reported lengths ranged from 2 to 10 m, most around 7 m. Where sex was recorded, males outnumbered females by about 2.8:1. No mature females were reported, whereas many males were mature. Only three juveniles have been recorded, suggesting that coastal occurrence in Japan is primarily linked to feeding. This study updates baseline data on basking shark occurrence in Japanese waters and contributes to future research on their conservation and habitat use.