The sea off this island town and in the Palk Straits has turned an unusually dark green colour due to the presence of ''Natalulaka'' algae.
Scientists at the Central Marine Acquarium Research Centre said the phenomenon was visible in the 10 km stretch from Pamban to rameswaram and from Singli to Kurusada islands in Palk Straits.
They said the algae spreads in the sea from July to September, during which period the colour turns green.
The change is seen when the current is low, they said, adding that the sea would return to its orginal colour in the next few days.
The scientists said a large number of fish would have died due to the low current and suffocation caused by the algae.
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