Question 1

When do box jellyfish arrive in Hawaii?

Accepted Answer

Box jellyfish typically arrive on Oahu's south-facing shores 8 to 10 days after each full moon. The pattern is remarkably predictable — they show up in pulses, peak for a day or two, then disperse. The most heavily affected beaches are Waikiki, Ala Moana, Hanauma Bay, and other south shore spots. The Ocean Safety Division posts warnings when box jellyfish are expected.

Question 2

What does a Portuguese man-o-war look like in Hawaii?

Accepted Answer

Portuguese man-o-war (locally called 'bluebottles') have a translucent blue or purple gas-filled float, usually 3-5 inches long, with long trailing tentacles that can extend many feet. When washed up on the beach they look like a deflated blue balloon. Beware — the tentacles remain venomous even when the animal is dead and dry on sand. They typically wash ashore when trade winds die and winds blow onshore from the north.

Question 3

What should I do if stung by a jellyfish in Hawaii?

Accepted Answer

Sting guidance varies by species and source, especially for Portuguese man-o-war. Safe to Swim Hawaii does not provide treatment instructions. Alert a lifeguard or local first-aid provider, follow posted beach guidance, and seek emergency medical care for severe symptoms such as difficulty breathing, chest pain, faintness, severe swelling, or symptoms involving the face or throat.

Question 4

Which Hawaii beaches are most affected by jellyfish?

Accepted Answer

Box jellyfish most frequently affect Oahu's south-facing beaches: Waikiki Beach, Ala Moana Beach Park, Hanauma Bay, Sandy Beach, and Makapuu Beach. Portuguese man-o-war can appear on any beach island-wide when northeast trade winds shift. Kailua Beach and windward Oahu beaches sometimes see man-o-war. North Shore beaches see them less often for box jellyfish but man-o-war can still appear.

Question 5

Is there a jellyfish calendar for Hawaii?

Accepted Answer

Yes — box jellyfish on Oahu follow a lunar cycle. Count 8-10 days after each full moon and south shore beaches are likely to see an influx. The City and County of Honolulu Ocean Safety Division posts forecasts on their website and social media. You can also ask lifeguards at staffed beaches for current conditions. This lunar pattern applies primarily to Oahu south shore; other islands and other jellyfish species don't follow the same schedule.