Extreme heatwave triggers mass fish, shrimp deaths across ponds

“Floating fish” whiten ponds, “drown shrimps” redden the water surface

In the final days of May, Duong Cao Dai (born in 1976, Xuan Bai hamlet, Thai Hoa commune, Phu Tho province) visits his fish pond as usual, only to freeze in shock. Across the 5,000-square-meter pond he has leased for the past 10 years, fish are floating belly-up everywhere. Grass carp, mud carp, and black carp lay dead in large numbers. A few dead fish from time to time are normal, but he has never before seen an entire pond covered in dead fish. “At first, I panicked, then felt devastated, and only afterward thought about how to deal with it,” he says.

The cause is not hard to identify. A heavy rain earlier had washed grass and plankton from the hillside through the culvert into the pond. The fish gorged themselves. Right afterward, the scorching heatwave of recent days caused thermal shock, leading to mass fish deaths. “One day it was 32 degrees Celsius, the next it shot up to 39 - 40 degrees. The fish floated belly-up, and even the pigs we raise stopped eating because of the heat,” Dai says with frustration.

Dai raises fish on a two-to-three-year cycle before harvesting, allowing them to reach market weight. This year is supposed to be harvest season, with an estimated four to five tons of fish in the pond. He has planned to wait a few more months for the fish to grow larger before selling them, expecting revenue of around VND 150 million.

Yet after just three to four days of peak heat, around one ton of fish had died, causing losses of VND 50 - 60 million. “Never before have fish died in such massive numbers. The whole pond turned white with dead fish. In more than 10 years of fish farming, I have never witnessed anything like this,” he says.

Not far away, the shrimp and fish ponds in Song Lo hamlet (Song Lo commune, Phu Tho province), owned by Nguyen Thi Quynh Hoa (born in 1977), are facing a similar situation. On peak heat days, dead shrimp turn the pond surface red. Silver carp and common carp float up before sinking again. “Everything is wiped out. Fish and shrimp die every year around this time, but this year the heat is far more extreme,” she says bitterly.

Hoa can not bring herself to harvest shrimp for the past month because she already knows the result. Every time she pulls up the nets, the shrimp are dead red before they can be sold. Live shrimp can fetch VND 300,000 per kilogram, but dead shrimp are worthless.

On days when the pond surface turns red with dead shrimp, she lets a neighbor collect them to feed ducks, but after only a few days, the ducks suffer severe diarrhea. This season, she feels completely powerless against the heat.

Praying for heavy rain

Dai runs a water pump to spray water and create oxygen from 3 a.m. to 6 a.m. - the exact hours when fish suffer the greatest oxygen shortage and rise to the surface gasping for air. He also cuts banana trees from his garden and throws them into the pond, hoping to reduce the sunlight reflecting onto the water. That is all fish farmers can do at the moment.

He tosses chlorine tablets into one corner of the pond to reduce the foul smell, clear the water, and lessen pollution. As for Hoa, she spreads lime into her pond, first 100 kilograms, then 200 kilograms, following the water flow in hopes of keeping the water cleaner and preventing fungal infections in the shrimp and fish gills.

But they both know these efforts are only temporary measures. Dai says, “There is no medicine for this. It is not a disease that can be treated with drugs. The weather is simply too hot, and there is no way to cool the pond.”

Twice a day, he collects dead fish and buries them beneath bamboo groves. Large pits once intended for planting trees have now become burial sites for fish. After each collection, another wave of dead fish appears because many sink to the bottom and only float up six to seven hours later, once their bellies swell.

The dead fish decomposing at the bottom have made the pond water so foul that every time Dai wades in to collect carcasses, his skin becomes itchy and irritated, forcing him to scrub himself immediately after returning home. The stench spreads throughout the area, lingering even after most dead fish have been removed. “At noon, standing on the bank, I can still smell the overwhelming stench even after I had already cleared away the dead fish,” he says.

What aquaculture farmers hope for most right now is a heavy rainstorm. Not to add more water to the ponds, but to completely replace the polluted water layer. “The heavier the rain, the better. Once the rain comes, no more fish will die,” Dai said.

According to the Thai Hoa Commune Economic Office, during the heatwave from May 24 to 28, 2026, at least four aquaculture households across the commune were affected. The total damaged water surface area exceeded 11,000 square meters. Fish deaths reached several tons, with estimated losses approaching VND 100 million.

In response, the Thai Hoa Commune People’s Committee issued technical guidance on protecting livestock and aquaculture from heat stress. For aquaculture ponds, farmers are advised to maintain water depth at 1.5 meters or more, operate water paddle wheels or aerators from 10 p.m. to 5 a.m., and during early afternoon hours to increase dissolved oxygen. On peak heat days, feed amounts should be reduced by 30 - 50% to avoid bottom pollution, while black mesh coverings can be used to shade one-third to one-half of the pond surface to reduce heat radiation.

*$1 = VND 26,393. Source: Vietcombank.