Box Jellyfish in Hawaii: Calendar and Safety Guide

Regional safety guide · Last reviewed 22 August 2026

Hawaii’s most predictable box jellyfish events occur on some south- and west-facing shores about nine to ten days after a full moon. That pattern is useful enough to support public calendars, but the dates are estimates and jellyfish may appear on any day.

What the lunar forecast means

Research on Alatina alata links near-shore migrations to a specific interval of darkness between twilight and moonrise, alongside reproductive maturation. It is not simply that a full moon “creates jellyfish,” and it is not a rule for every Hawaiian shore.

Before swimming

• Check Hawaii Ocean Safety warnings and the current jellyfish calendar.
• Choose a lifeguarded beach and ask the lifeguard.
• Stay out when jellyfish signs are posted or stranded animals are present.
• Remember that surf, currents and shorebreak may be the larger hazard that day.

Box jellyfish or Portuguese man o’ war?

Both produce painful stings but are different animals with different arrival patterns. Hawaii Ocean Safety notes that man o’ war often arrive with strong onshore winds. Do not use one name or one treatment assumption for both.

Official and research links

• Hawaii Ocean Safety: beach warning signs
• Hawaii Ocean Safety: lifeguarded beaches
• Research on Alatina alata lunar migrations

Independent clinical review is pending. Follow Hawaii Ocean Safety rather than imported first-aid instructions.