Ito's Declining Visibility: From 18.1m (2017) to 15.1m (2025) — Investigation
2026-03-16
Ito in Tateyama, Chiba Prefecture, is a popular dive site famous for shark-feeding dives. However, data analysis reveals a steady decline from 18.1m average visibility in 2017 to 15.1m in 2025 — roughly 0.3m per year. This article examines possible causes behind the trend.
Yearly Visibility Trend
| Year | Avg Vis | YoY |
|---|---|---|
| 2017 | 18.1m | - |
| 2018 | 16.8m | -1.3m |
| 2019 | 16.5m | -0.3m |
| 2020 | 16.7m | +0.2m |
| 2021 | 16.3m | -0.4m |
| 2022 | 14.8m | -1.5m |
| 2023 | 16m | +1.2m |
| 2024 | 14.8m | -1.2m |
| 2025 | 15.1m | +0.3m |
3.0m Decline Over 9 Years (~0.3m/year)
A consistent decline from 18.1m (2017) to 15.1m (2025). There was a brief recovery to 16.0m in 2023, but it dropped back to 14.8m in 2024.
Possible Causes
1. Kuroshio Current Path Shifts
The Kuroshio Large Meander has persisted since 2017. Ito sits at the southern tip of the Boso Peninsula, directly influenced by the Kuroshio. When the meander pushes the current offshore, less clear oceanic water reaches the coast, allowing turbid coastal water to dominate. The meander's onset (August 2017) closely matches the visibility decline, suggesting a strong correlation.
Ref: JAMSTEC Kuroshio Large Meander Monitoring
2. Water Temperature Changes and Phytoplankton
Rising water temperatures can promote phytoplankton growth, reducing visibility. Sea surface temperatures off Boso have been trending upward in recent years (JMA data), potentially affecting nutrient cycling and biological productivity.
Ref: JMA 'Long-term Sea Surface Temperature Trends'
3. Impact of Diving Activity
Ito's shark-feeding dives have grown in popularity, increasing annual diver numbers. Sediment disturbance from many divers and ecological effects of feeding could affect visibility. However, this impact is likely local and short-term, with limited contribution to annual averages.
Conclusion
The Kuroshio Large Meander's persistence appears to be the most likely cause of Ito's visibility decline. While visibility may recover if the meander ends, other factors like rising water temperatures complicate the picture. We'll continue collecting data to track changes.
About the Data
Ito visibility data collected from Bommie dive shop blog posts. Annual averages used for each year. Pre-2017 excluded due to limited data.
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