Was 2020 a Good Year for Diving Visibility? Did COVID Help?
2026-03-16
In 2020, COVID-19 dramatically reduced human activity worldwide. Did decreased boat traffic and tourism affect diving visibility? Analysis of Izu Peninsula dive logs reveals that some sites did show improved visibility in 2020. But was this a 'COVID effect' or natural oceanic variation? Let's examine the data.
14.3m
IOP 2020
8.8m
Hirasawa 2020
12.2m
Futo 2020
IOP Yearly Visibility Trend
| Year | Avg Vis |
|---|---|
| 2017 | 13.5m |
| 2018 | 13.8m |
| 2019 | 14.2m |
| 2020 | 14.3m |
| 2021 | 12.8m |
| 2022 | 13.1m |
| 2023 | 13.6m |
IOP's 2020 at 14.3m was nearly identical to 2019 (14.2m). However, 2021 dropped to 12.8m, confirming 2020 was relatively a good year.
Site Comparison: 2019–2021
| Site | 2019 | 2020 | 2021 | 2019→2020 |
|---|---|---|---|---|
| IOP | 14.2m | 14.3m | 12.8m | +0.1m |
| Hirasawa | 7.9m | 8.8m | 7.5m | +0.9m |
| Futo | 11.5m | 12.2m | 10.8m | +0.7m |
Hirasawa (+0.9m) and Futo (+0.7m) showed relatively large improvements. IOP was +0.1m — essentially unchanged.
Did COVID Improve Visibility? Analysis
Arguments for COVID Effect
During Japan's 2020 state of emergency, boat traffic decreased significantly — including leisure and dive boats. Reduced propeller wash and discharge may have improved coastal water quality. The largest improvement at Hirasawa (+0.9m), which is near a port, aligns with this hypothesis.
Arguments for Natural Variation
However, IOP's improvement was only +0.1m — hardly attributable to COVID. Annual average visibility naturally fluctuates by 1–2m, and the 2020 improvement falls within normal variation. Kuroshio Large Meander changes since 2017 may also be a factor.
Possible Reporting Bias
With far fewer dives in 2020, shops may have only operated and logged on good-visibility days. This means 'bad day data' may be missing, artificially inflating the average.
Conclusion: COVID Effect Uncertain, Natural Variation More Likely
While 2020 visibility improvements are real, attributing them to COVID is premature. The small magnitude (IOP +0.1m), normal year-to-year variation range, and possible reporting bias prevent us from concluding that 'COVID cleaned the ocean.' The 2019→2020 slight increase versus the 2020→2021 large decrease (-1.5m) suggests natural oceanic variation has far greater influence.
Lessons from the Data
- Diving visibility naturally fluctuates 1–2m year over year. Drawing conclusions from single-year comparisons is unreliable.
- Kuroshio path changes, typhoon frequency/paths, and rainfall patterns have far greater impact than human activity.
- Identifying long-term trends requires at least 5–10 years of data. This underscores the value of continuous data collection in this project.
About the Data
Annual average visibility calculated from dive shop daily logs at IOP, Hirasawa, and Futo. IOP has ~300–500 observations per year, Hirasawa ~200–400, Futo ~300–500. Observation counts decreased in 2020 due to COVID, introducing possible sampling bias.
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