Dead in the water: How heatwaves are killing fish

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nearly 50 degrees Celsius. While the official human toll as of 1 June stood at 87, the tragedy did not end there. Life underwater, too, perished before fishermen had a chance to extinguish it.

Inland water bodies have been hit hard by the rising temperatures. An alarming 2-5% of fish in lakes and ponds may have been wiped out, according to Trivesh Mahekar, a fisheries scientist at the Indian Council of Agricultural Research-Central Coastal Agricultural Research Institute (ICAR-CCAR), Goa. “Warmer water temperatures can decrease dissolved oxygen levels and increase the incidence of harmful algal blooms, both of which can affect fish health and survival rates in natural as well as farmed environments," says Mahekar.

In general, for farmed, freshwater or brackish water fish to thrive, the ideal water level should be 1-1.5 metres, says Mahekar. But this year water levels in the ponds that he is studying have shrunk below 1 metre. “I have observed that the water level shrank 67-75 centimetres due to intolerable temperatures, resulting in mortality.

This year, fish mortality in my ponds could be around 2-5%. If the temperature keeps rising like this every year, the mortality burden is bound to go up." It isn’t just ponds and lakes. Since October 2023, marine heatwaves—unusually high ocean temperatures—have led to extensive bleaching of corals in the Lakshadweep Sea, depriving them of essential nutrients and compromising their survival.