Low oxygen levels leading to high mortality of fish in Gulf of Mannar

troubled waters: Blooming of a harmful marine and the ‘red tide’ phenomenon have led to discolouration of the ocean surface in the Mandapam area.

troubled waters: Blooming of a harmful marine and the ‘red tide’ phenomenon have led to discolouration of the ocean surface in the Mandapam area.   | Photo Credit: — PHOTO: SPECIAL ARRANGEMENT

Shastry V. Mallady and C. Jaisankar

Study blames it on blooming of a harmful marine, dinoflagellate

MADURAI/RAMESWARAM: Studies conducted in the waters of the Gulf of Mannar point to low levels of oxygen leading to high mortality of fish.

The Regional Centre of Central Marine Fisheries Research Institute (CMFRI), Mandapam, has found out that this is due to the blooming of a harmful marine, dinoflagellate (noctiluca scintillans), a minute marine species.

Another study undertaken by Madurai Kamaraj University’s Centre for Marine Studies has revealed that oxygen levels have depleted due to outbreak of ‘red tide’ phenomenon caused by blooming of organisms, which discolour the ocean surface.

Breathing problems

The massive blooming of the organism has reduced the oxygen level in many of the landing centres between 1 and 1.2 ml per litres as against the normal level of 4.5 to 5 ml, causing breathing problems for fishes, the CMFRI study says. It has caused the death of a large number of fish in the past few days. The size of the organisms is between 200 and 2000 microns.

Speaking to The Hindu on Saturday G. Gopakumar, scientist-in-charge, CMFRI, said fish species such as rabbit, parrot, goat, serrandis, silverbellies, surgeon fishes and other benthic fishes, mostly associated with coral reefs, were the ones mainly affected due to the blooming of dinoflagellate. The origin of the bloom was from Periapattinam and it had spread to several areas from Keelakarai to Thonithurai.

Although the species was colourless, the presence of photosynthetic green endosymbiont made the water green. The colour of the sea might change to pinkish red or orange depending upon the endosymbiont of the species.

The cause

Although the blooming of noctiluca had occurred a few times in Gulf of Mannar in the past, large-scale blooming happened only in the recent past. Drastic changes in environment of the sea such as very high temperature, extreme low wind velocity and no water current were the reasons.

The study found the noctiluca to be non-toxic, Mr. Gopakumar said. However, he cautioned people against eating dead fish found along the landing centres of the Gulf of Mannar. He said the problem might come down shortly, when the organisms find it difficult to get food. However, they might spread to other areas, as they multiplied into millions in no time.

The MKU study concluded that extensive patches of green-coloured water were observed along the 50-km coast from Mandapam to Keelakarai. Subsequent laboratory analysis found that the organism responsible for this phenomenon was ‘Noctiluca.’

Red tide threat

“A variety of marine organisms were killed and washed ashore along the coast, including edible/ornamental fishes, crabs, lobsters, sea cucumbers, eels etc. The red tide phenomenon is a threat to marine biodiversity ,” A. K. Kumaraguru, Director, Centre for Marine and Coastal Studies, Madurai Kamaraj University, told The Hindu on Saturday.

The centre, located at Pudumadam near Mandapam, has come out with a report that the dissolved oxygen in coastal waters has been depleted by red tide organisms. Living organisms in sea depend on the dissolved oxygen for their survival.

Dr. Kumaraguru cited the laboratory report that showed the increasing density of algal cells in sea water. “It is usually a few hundred cells per litre but it has gone up to 12.7 lakh cells per litre. The dissolved oxygen level. which used to be 5 to 6 mg per litre has gone down to as low as 0.64 mg per litre,” he said.

The team of scientists attributed the outbreak of ‘algal bloom’ to coastal marine pollution along the Gulf of Mannar due to indiscriminate dumping of municipal and domestic sewage. Absence of monsoon winds might have caused rapid multiplication of algal cells.

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