Sophon Thongdee, deputy director general of the Marine and Coastal Resources Department, said a letter had been sent to the Surat Thani governor to put warning signs at the Chalok Lar Bay. 
The governor was also asked to have officials inform tourists of the danger of jellyfish at the spot. 
Jatuporn Burutphat, director general of the department, said the jellyfish have been sighted since September 8 and their numbers appeared to have increased with 25 members of the Chironex indrasaksajiae species seen in the shallow bay. 
Tourists were advised against swimming until further notice.

Pha-ngan issues jellyfish alert

Tourists were warned on Friday against swimming in a bay off Koh Pha-ngan after many dangerous box jellyfishes have been sighted.