Increasing global warming leading to fish mortalities in Arabian Sea: Indo-US study

Delhi: In what is set to aggravate concerns over climate change, a new joint Indo-US study has found that global warming is leading to large-scale fish kills in the Arabian Sea by fuelling the growth of harmful bioluminescent algae.

The striking green algae, Noctiluca scintillans, which is behind the scintillating phenomenon observed in oceans when they glow fluorescent blue at night, are indicators of fish mortality. Scientists highlight that their growing abundance is a cause of concern, because they adversely affect fish, which do not thrive in such waters.

“Less dense water comes to the surface because of the warming of oceans, encouraging these intense blooms, which has an adverse impact on fisheries. Currently, the western coast, Persian Gulf, and Oman are largely affected, but if it keeps on increasing, it will have drastic effects on fisheries along the Indian coast. That could be alarming,” said Dr S.C. Shenoi, director, Indian National Centre for Ocean Information Services (INCOIS).

The study jointly conducted by scientists of INCOIS, Hyderabad, and those of the National Oceanic and Atmospheric Administration (NOAA) under the US Department of Commerce, published its findings in ScienceDirect.

The new research contradicts another study, which had indicated that the massive outbreak of Noctiluca was occurring because of an increase in oxygen minimum zones.

Dr Shenoi pointed out that oxygen minimum zones are in the deeper layers of the sea and said the research team has found that there was no oxygen depleted water at the surface, where the bloom is prevalent. The dissolved oxygen in the surface water was always >70% saturated, research showed.

“A recent study had laid the blame for fish mortality on the coastal pollution caused by dumping of fertilisers and waste along the Indian coast. But this research has shown that rather than effluents or chemical pollution, it is actually global warming that is contributing to the observed abundance of Noctiluca,” said Dr Shenoi.

The researchers used sensors on board satellites to observe the distribution of these harmful algae from 2009 to 2016.

The researchers said that warming causes sea-water layers to stratify more intensely and slows upward movement of nutrients like silicate from the ocean bottom. “This discourages growth of diatoms, which need silicate and sunlight. It instead leads to flourishing of Noctiluca which preys on diatoms,” said the study.

“The present findings show no evidence that cultural eutrophication has contributed to the decadal scale shifts in plankton (algae) composition in the north- eastern Arabian Sea. Instead, the lessons that are learned here may help to forecast the effects that climate change may have in other productive oceanic ecosystems,” stated the research paper published in ScienceDirect.

Following the research, INCOIS has announced the setting up of a Marine Observation System along Indian Coast (MOSAIC) to monitor the water quality of the Indian coastal waters.