Mikomoto's Visibility Crash: From 14.3m (2019) to 9.8m (2025) — Data Investigation
2026-03-16
Mikomoto Island off the Izu Peninsula is world-renowned for its hammerhead shark schools. However, visibility has crashed dramatically in recent years — from a peak of 14.3m in 2019 to just 9.8m in 2025, a 4.5m drop in only 6 years. The 2026 provisional figure of 8.6m suggests further deterioration.
Yearly Visibility Trend
| Year | Avg Vis | YoY | Note |
|---|---|---|---|
| 2015 | 12.6m | - | |
| 2017 | 13.5m | +0.9m | |
| 2018 | 14.2m | +0.7m | |
| 2019 | 14.3m | +0.1m | Peak |
| 2020 | 12.6m | -1.7m | |
| 2021 | 12.5m | -0.1m | |
| 2022 | 12.7m | +0.2m | |
| 2023 | 11.6m | -1.1m | |
| 2024 | 10.9m | -0.7m | |
| 2025 | 9.8m | -1.1m | |
| 2026 | 8.6m | -1.2m | Provisional |
4.5m Crash in 6 Years, 2026 Even Worse
Since the 2019 peak (14.3m), visibility has declined almost every year. 2025 was the first time it dropped below 10m. The 2026 provisional 8.6m is the lowest since data collection began.
Kuroshio Retreat Hypothesis
Mikomoto Island's visibility heavily depends on Kuroshio proximity. When the Kuroshio approaches, clear oceanic water flows in; when it retreats, turbid coastal water dominates.
Prolonged Kuroshio Large Meander
The Kuroshio Large Meander has persisted since August 2017 (as of March 2026). The meander pushes the Kuroshio's main stream far south of the Izu Peninsula, reducing the supply of clear oceanic water to Mikomoto. The acceleration of decline after 2019 is consistent with changes in the meander pattern.
Ref: JAMSTEC Kuroshio Large Meander Monitoring
Coastal Water Temperature Rise
In addition to the Kuroshio retreat, general coastal water temperature increases may be promoting phytoplankton growth, further accelerating visibility decline.
Ref: JMA 'Long-term Sea Surface Temperature Trends'
Impact on Hammerhead Diving
In sub-10m visibility, the viewing distance for hammerhead shark schools is severely limited. Mikomoto diving is a 'big animal' experience heavily dependent on visibility, and this declining trend may affect diver satisfaction and return rates. Unless the Kuroshio Large Meander ends, a short-term recovery seems unlikely.
About the Data
Mikomoto visibility data collected from Kaiyusha (290.jp) blog posts. Annual averages used. 2016 excluded due to data gaps. 2026 is provisional (through March).
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