A regular part of the weather forecast in Hawaii is the prediction of when the monthly box jellyfish swarm will hit. About ten days after a full moon there is a warning for swimmers to stay out of the water along affected beaches.
There are two important parts of that warning. First, the jellyfish invasion is very regular: the City and County of Honolulu has a calendar predicting future swarms that allows you to plan Hawaii activities. When my sister and her husband visited, we wanted to spend one day snorkeling at Hanauma Bay. The week of their vacation was close to the time that boxjellies are expected. I sent a message by Twitter to a local TV weather person, who assured me that the waters should be clear by the day we selected.
The second part of the warning is that it does not affect all beaches. Hanauma Bay is affected and occasionally closes if there is a h3 swarm of jellyfish. However, the beaches on the windward side of Oahu are not affected — I never see them in Kailua.
The Hanauma Bay website warns swimmers to avoid contact with jellyfish, either in the water or dead ones on the beach. They have powerful venom. If you are stung, you may feel a stinging, burning sensation, redness and swelling and even long red welt lines where the tentacles touched your skin.
A final caution. The monthly jellyfish swarm usually lasts three days. If those are days you planned to spend in the water, either move to a different beach or re-arrange your schedule. On the day I took this picture in Waikiki, I saw people walking past the warning sign into the water — including children. Dealing with a venomous sting is not a memory you want to take home from Hawaii. Heed the warning and give the little jellies their space.