Has Diving Visibility in Japan Changed Over 10 Years? Trend Analysis 2015–2026
2026-03-11
Analyzing a decade of visibility data across Japan. Yonaguni dropped from 28m to 24.7m; Tajiri and Futo improved by +2m. Site-by-site trends and oceanographic explanations.
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)
| Year | IOP (Izu) | Yonaguni | Echizen | Kushimoto | Mikomoto | Observations |
|---|---|---|---|---|---|---|
| 2015 | — | 28 | 7.7 | — | 15.1 | 1,324 |
| 2016 | — | 26.7 | 10.4 | 14.6 | 15.4 | 1,853 |
| 2017 | — | 26.2 | 10.9 | 13.5 | 16.4 | 2,415 |
| 2018 | 16.6 | 25.2 | 7.4 | 11.9 | 17.1 | 3,163 |
| 2019 | 16.1 | 25.2 | 9.2 | 12.8 | 17.4 | 3,885 |
| 2020 | 17.2 | 24.5 | 11 | 13.1 | 15.9 | 3,684 |
| 2021 | 14.6 | 23.9 | 10.2 | 12.7 | 15.5 | 4,755 |
| 2022 | 15.4 | 24.1 | 8.3 | 14.1 | 16.4 | 5,040 |
| 2023 | 15.3 | 24.5 | 8.3 | 13.8 | 15 | 4,843 |
| 2024 | 16.2 | 24.7 | 8.3 | 12.4 | 14.5 | 4,458 |
| 2025 | 14.9 | 24.7 | 9 | 13.9 | 12.9 | 5,057 |
10-Year Change by Site (2015–17 vs 2023–25)
| Site | Early (2015–17) | Recent (2023–25) | Change | Trend |
|---|---|---|---|---|
| Tajiri | 10.7m | 13m | +2.2m | ↑ Improving |
| Futo | 12.1m | 13.1m | +1.1m | ↑ Improving |
| Hirasawa | 9.8m | 10.9m | +1.1m | ↑ Improving |
| Kumomi | 11.7m | 12.7m | +1m | ↑ Improving |
| Ishigaki | 20.3m | 20.6m | +0.3m | → Stable |
| Omijima | 10.6m | 10.3m | -0.3m | → Stable |
| Kushimoto | 14.1m | 13.4m | -0.7m | ↓ Declining |
| Kashiwajima | 14.5m | 13.8m | -0.7m | ↓ Declining |
| Echizen | 9.6m | 8.4m | -1.2m | ↓ Declining |
| Mikomoto | 15.6m | 14.1m | -1.4m | ↓ Declining |
| Miyakejima | 12.6m | 10.9m | -1.7m | ↓ Declining |
| Yonaguni | 26.9m | 24.6m | -2.3m | ↓ Declining |
| Shirahama | 17m | 13.2m | -3.7m | ↓ Declining |
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)
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