Jellyfish Arrival Calendar

Hawaii Jellyfish Arrival Calendar - Jellyfish Forecast for Hawaii Beaches

Planning to hit the beach in Hawaii? This jellyfish calendar forecasts the likely arrival of box jellyfish on Oahu's south and leeward shores, typically 8 to 10 days after a full moon. Learn how to stay safe, what to do if stung, and which beaches are most affected - plus first aid tips based on University of Hawaii research.

This calendar is for informational purposes only. Ocean conditions change quickly, and jellyfish may appear on any day of the year. Therefore, always obey posted warning signs, swim at lifeguarded beaches when possible and ask a lifeguard about jellyfish, Portuguese man-of-war, currents, surf and other hazards. For broader ocean planning, review Hawaii beach safety tips.

Full Moon

Jellyfish Invasion Likely

Note: This calendar is for informational purposes only. All jellyfish arrival dates are estimates. Jellyfish may be present on any day of the year. Always obey warning sings posted on beaches. Swim on guarded beaches only and ask a lifeguard about the presence of jellyfish and other ocean hazards.

When Do Box Jellyfish Arrive in Hawaii?

Box jellyfish in Hawaii often arrive near shore about 8 to 10 days after a full moon. This pattern appears most often on Oahu's south-facing and leeward beaches, including areas such as Waikiki Beach, Ala Moana Beach and Hanauma Bay. Windward beaches, including Kailua Beach, may see Portuguese man-of-war floating in nearshore waters or wash up on the beach.

However, wind, currents, tides and other ocean conditions can affect where jellyfish appear. Also, warning signs usually get placed only at lifeguarded beaches, so other beaches may not have posted alerts.

Where Are Jellyfish Most Common in Hawaii?

Box jellyfish most often affect south and leeward shores. On Oahu, this includes many popular beaches that face south or west, so visitors should pay extra attention to jellyfish forecasts when planning beach days in these areas.

Portuguese man-of-war are commonly seen on windward beaches, such as Kailua Beach when tradewinds or onshore winds push them toward land. They have a blue or purple float and long tentacles that can still sting after washing onto the beach.

Jellyfish First Aid

In case you get stung by a jellyfish or Portuguese man-of-war:

1. Carefully remove (pluck, not scrape) visible tentacles from the skin. Use a towel, napkin or other item. Do not use bare fingers if you can avoid it.

2. Rinse the affected area with vinegar (if available) to help prevent additional venom discharge.

3. Get medical help in case you suffer from a severe reaction. Immediate medical care may be necessary as jellyfish stings may cause an anaphylactic shock. Get the help of a lifeguard or call 911.

A team of University of Hawaii researchers published a report in the journal Toxins about the best treatment for box jellyfish stings. According to the researchers, vinegar is effective in preventing more venom from being released into the skin, as is a product called Sting No More, which has a venom-inhibiting effect. A hot water compress can be used to treat the sting's effects. The report comes to the conclusion that seawater, soda, urine, alcohol and ethanol had no effect, and that freshwater poured over a sting or icing the area could make things worse.

How to Reduce Jellyfish Sting Risk

You cannot remove all sting risk from ocean swimming, but you can lower the chances of getting stung. First, check the jellyfish calendar before visiting south or leeward beaches. Then, look for posted warning signs when you arrive.

• Ask lifeguards about current jellyfish and man-of-war conditions.
• Avoid swimming when jellyfish warning signs are posted.
• Stay out of the water if you see jellyfish or Portuguese man-of-war on the beach.
• Keep children away from beached jellyfish and tentacles.
• Swim at lifeguarded beaches whenever possible.
• Do not swim alone at remote or unguarded beaches.
• Leave the water if you feel a sting or see others getting stung.

Jellyfish Info

The most common type of jellyfish in Hawaii is the Carybdea alata and Carybdea rastonii, also known as box jellyfish because of its squared body. The box jellyfish is most commonly found on Hawaii's leeward (west) and south shores. Their arrival on Hawaii's beaches is quite predictable. They usually arrive around 8-10 days after a full moon (but occasionally they have been observed on off-cycle days as well). This is when warning signs are usually posted on many Oahu beaches (warning signs are generally only posted on popular beaches).

Don't swim at remote and unguarded beaches for your own safety. There are many other ocean hazards besides jellyfish, including strong currents, rip tides, sharp coral, etc. Similar to a jellyfish, the Portuguese man-of-war also causes a painful sting. It has a purplish body and is commonly found on windward (east) shores as the prevalent northeastern tradewinds blow these creatures close to shore.

If you see a jellyfish on the beach, don't touch it as its tentacles can cause a painful sting even if the animal is dead. Keep children away from beached jellyfish. If you get stung, see a lifeguard for first aid. For more serious cases or if you feel unwell, call 911 immediately.