THOOTHUKUDI: A large number of fish and sea organisms were found dead along a 2 km stretch of the seashore between Tharuvaikulam Beach and Pattinamaruthur on Tuesday. Researchers say the deaths could have been triggered by the algal bloom phenomenon in the sea.
Fishermen said the seashore of Tharuvaikulam turned bright green and calm all of a sudden this week. “We are not aware why this is happening, but nature is indicating something serious,” they said.
Speaking to TNIE, K Diraviya Raj, Associate Professor, Suganthi Devadason Marine Research Institute (SDMRI), said the characteristics of algal bloom is the rapid multiplication of the microalga called ‘Noctiluca scintillans’ triggered by high temperatures and lack of sea current. “The surge in Noctiluca scintillans population, a single-celled phytoplankton, depletes the dissolved oxygen in the water causing temporary hypoxic conditions, which in turn leads to death of both mobile and sessile organisms in the sea and also fish species in the sea. These microalgae also clog the gills of fish,” he further said.
Diraviya Raj has been monitoring the characteristics of algal bloom since 2019, and he is the lead author of a 2020 study about N. scintillans blooms occurring in the Gulf of Mannar. Referring to the unusual rise of temperature to 30 degree Celsius during the month of October in the past five years, he attributed the algal bloom to the impact of climate change.
“Usually, the sea temperature gradually declines to 25 degree Celsius after May every year. However, in the past few years, the Gulf of Mannar region has been experiencing a peak in October triggering the rapid multiplication of Noctiluca scintillans,” he pointed out. The unusual development of algae reduces dissolved oxygen levels in the seawater to less than 2 mg per litre, as against a usual average of above 5 mg per litre, which is required for the survival of fish and other organisms, Diraviya Raj said.
The associate professor further said that the algal bloom phenomenon has been occurring during October and November in the past three years, leading to huge fish production loss in addition to triggering mortality of sea grass, sea anemone, sea snakes, sea cucumbers, sea turtles, crabs, starfish and other organisms that live in the sea bottom. It is pertinent to note that the algal bloom also causes substantial damage to the corals and soft corals, for which the Gulf of Mannar is famed, he said.
Previously, the algal bloom occurred in 2008 and 2019 at Keelakarai and Mandapam respectively. “Though the researchers first thought the phenomenon is restricted to Keelakarai and Mandapam, the algal bloom was observed widely from Mandapam to Kanniyakumari in 2022. Apart from fish mortality, several islands in the Gulf of Mannar region experienced coral loss also,” he added.
Marine researchers from SDMRI said the fast-growing coral species namely Acropora, Montipora, and Pocillopora are vulnerable to N. scintillans blooms because they require the ambient oxygen to thrive. It is a hypothesis that needs more focussed studies to find the reasons, they said.