Izu Nudibranch Season (Feb–May) Visibility: How Clear Is the Water Really?

March 11, 2026 · 8 Izu sites analyzed from real observation data

Spring Izu: 'Nudibranchs Are Visible, but the Water Turns Murky'

The Izu Peninsula's February–May is nudibranch season. As water temperatures drop to 10–18°C and begin warming, nudibranch species and individual counts explode. Sites like Futo, Osezaki, and Kumomi record over 100 nudibranch species during this period — but spring plankton bloom also progresses simultaneously. For divers wondering about clarity during nudibranch season, we provide detailed real visibility data for 8 Izu sites throughout spring.

Monthly Visibility by Site: January–June

Site	Jan	Feb	Mar	Apr	May	Jun
		← Nudibranch Season →
IOP	18.6m	17.7m	13.7m	10.1m	10.6m	11.3m
Koganezaki	16.2m	15.7m	15.1m	12.8m	10.7m	13.2m
Futo	15.6m	14.5m	11.6m	9.5m	9.2m	9.8m
Futone	14.8m	14m	11.9m	11.4m	10.4m	12.1m
Mikomoto	13.6m	12.6m	11.3m	9.9m	9.7m	12.3m
Kumomi	14.2m	14.2m	10.8m	10m	8.8m	9.2m
Osezaki Outer	12.8m	12.6m	11.1m	9m	10.4m	11.5m
Osezaki Bay	11.8m	10.7m	8.6m	6.6m	7.8m	7.5m

Purple columns = nudibranch season (Feb–May). Darker color = higher visibility.

Best Windows Balancing Visibility and Nudibranchs

Site & Month	Visibility	Water Temp	Why
Koganezaki (Feb)	15.7m	15.4°C	15.7m clarity with abundant nudibranchs. Drysuit at 15°C but best balance
IOP (Feb)	17.7m	16°C	Izu's best winter–spring clarity at 17.7m. Many nudibranch species. Water temp 16°C
IOP (Mar)	13.7m	16°C	Still good at 13.7m. Prime window before spring bloom starts in late March
Futone (Mar)	11.9m	16.3°C	11.9m clarity; start of nudibranch density peak season

Site-by-Site Analysis

IOP: Best Clarity in Izu Even During Nudibranch Season

IOP maintains relatively high clarity in February (17.7m) and March (13.7m) before spring bloom begins. It drops to the 10m range in April–May, but consistently stays 1–2m higher than other Izu sites. For nudibranch seekers who don't want to sacrifice visibility, February–March at IOP is the best choice.

Osezaki Bay: Japan's Top Nudibranch Density but Visibility Drops to 6.6m in April

Osezaki Bay is famous as Japan's prime nudibranch diving location, but the visibility data is sobering. April's average of 6.6m is the lowest of all 8 sites, severely limiting what you can see at distance. Paradoxically, this low-visibility spring season is exactly when nudibranch species and individual counts peak. For 'close-range macro photography of nudibranchs right in front of you,' low visibility is no barrier — and many advanced divers deliberately choose this period.

Koganezaki: Hidden Gem with Good Visibility-Marine Life Balance

Koganezaki maintains relatively high transparency in February (15.7m) and March (15.1m) while still offering nudibranch encounters. Even in April–May when spring bloom begins, it stays above 10m — making it the top recommendation for those who want both good visibility and nudibranch encounters. Located in western Izu, good water exchange from Suruga Bay currents likely contributes to this balance.

Water Temp Guide: What Suit Do You Need for Spring Izu?

Jan–Feb (15–16°C): Drysuit or 5–7mm wetsuit (experienced divers)
Mar–Apr (16–17°C): Drysuit recommended; 5mm wetsuit possible for short dives
May (17–20°C): 5mm wetsuit manageable; drysuit also comfortable

Summary

Izu nudibranch season visibility worsens in order: February > March > April = May. The highest-clarity entry into nudibranch season is February–March: IOP at 17.7m, Koganezaki at 15.7m. By April–May, nearly all sites drop below 10m — but nudibranch species and individual counts are at their peak during this period. Matching your goal (visibility-first vs. marine life-first) with the right timing and site is the key to successful spring Izu diving.

Data source: scraped from public dive logs. Nudibranch species counts based on general local dive service information.