A maritime expert says reports of a so-called 'red blood sea' in Tonga's Vava'u are the result of overgrown algae releasing toxic gases.
Ocean and fisheries specialist, Vailala Matoto, told TBC the toxins kill sea organisms and create an unpleasant smell.
Dr Matoto says the process usually occurs when the temperature of the sea rises and that is exacerbated by factors such as climate change.
However, he says the algae can also be useful as it releases oxygen that is of benefit to the environment.
Similar sightings of red seas have been reported in China and other countries.