Swimmers and surfers in Hawaii are being warned to stay out of the water near Honolulu Harbour after 1,400 tonnes of molasses accidentally leaked from a pipe into the ocean.

The sticky caramel-like substance was being transferred to a tanker from a sugar plantation.

Hundreds of fish have been killed in the spill, attracting a number of sharks to the area to feed on the dead marine life.

Professor of Environmental Biotechnology at Bangor University in Wales, Barry Johnson, says the deaths were caused by a lack of oxygen in the water.

"Molasses itself is not a toxic material."

"The problem here is what happens when something like that goes into any water body."

"Molasses is a very sugary substance; it’s made from the processing of sugar cane."

"When it gets into the ocean...what happens is the sugars are broken down by the bacteria in the seawater or freshwater, and when they do that they consume the oxygen that’s dissolved in the water."

"So the water in the immediate vicinity of the molasses becomes oxygen depleted and so all the life forms that require oxygen, and of course here we’re thinking mostly of the fish, they just suffocate."

Professor Johnson told the spill isn’t harmful to humans, but the lack of oxygen was the reason many of the fish were seen gasping for air at the ocean’s surface.

He says molasses is not as severe as an oil spill, but it will affect marine life.

"Oil spillages, they tend to float on the surface."

"Molasses is just like putting a bit spoon full of sugar in your cup of cold water, most of it sinks to the bottom, and of course that's where a lot of the important sea life is to be found, and so you will get these zones of oxygen depletion at the bottom of the sea and that could kill off coral reefs in the long term."

But the spill shouldn’t take too long to clear.

"It’s going to last weeks, possibly a couple months down in the ocean depths; it depends on how quickly the oxygen is replenished," Professor Johnson said.

It’s not the first time molasses has spilled into a waterway.

According to Professor Johnson there was a deliberate dumping of molasses into a reservoir in Mexico earlier this year.

"That basically killed all the fish, sea bream and carp, and there were 180 families apparently depending for their livelihood on fishing in that lake."

"That is an act of environmental vandalism."

"Accidents like the present one, they do happen, it’s not like an oil spill, but in the immediate term it’s not good for the environment."