Has Diving Visibility in Japan Changed Over 10 Years? Trend Analysis 2015–2026

A question every diver asks: "Is the visibility getting worse over the years?" Rather than relying on gut feelings, we analyzed 46,000+ real observations from 2015 to 2026. The verdict: there is no uniform national trend — some sites are improving significantly while others have declined.

Year-by-Year Visibility at Key Sites (2015–2025)

10-Year Change by Site (2015–17 vs 2023–25)

Sites Improving: Tajiri, Futo, Hirasawa, Kumomi

Tajiri (Tottori) improved by +2.2m (10.7m→13.0m) — a substantial gain likely linked to changes in Sea of Japan coastal current patterns. Three Izu sites (Futo +1.1m, Hirasawa +1.1m, Kumomi +1.0m) all improved simultaneously after a trough around 2021, coinciding with the Kuroshio returning from its large meander phase.

Sites Declining: Yonaguni, Shirahama, Miyakejima, Mikomoto

Yonaguni's -2.3m decline (26.9m→24.6m) is statistically robust with 2,000+ observations from each period. The Pacific Decadal Oscillation and El Niño-related changes in western Pacific oceanography may be contributing. Mikomoto peaked at 17.4m in 2019 and fell to 12.9m by 2025 — a sharp decline closely tracking Kuroshio route variability.

Is the Ocean Getting Murkier Overall?

The all-site average dropped from 15.6m (2015) to 13.6m (2021) before recovering to 15.5m (2025). However, this is largely a composition effect — early years covered mainly offshore clear-water sites (13–17 sites), while later years added many more coastal and bay sites. Individual site analysis tells a more honest story: no uniform national trend of worsening visibility.

Key Takeaways

• No nationwide decline trend — improving and declining sites coexist
• Izu sites have recovered since 2021 — likely Kuroshio route stabilization
• Yonaguni and Mikomoto are declining — possible ocean climate signal
• "The sea was clearer back then" is true at some sites (Yonaguni) but false at others (Tajiri, Futo)