Mayflies typically appear on the surface of the river during the third month of the lunar calendar, when there are changes in the weather, especially when periods of heat and moisture are followed by rain.
In the past, people usually captured mayflies using net rackets. But in the past three years, mayfly prices have been rising and people switched to using motor boats and large nets to catch them in much greater quantities.
Those who capture mayflies usually do so at Red River sections near Ba Dinh and Hoang Mai districts.
Mayflies are aquatic insects that go through several life cycles. They begin life as larvae, become nymphs, then subimago. After emerging from the water surface, they molt to gain functional wings before molting a second time, a unique trait that separate them from other insects. The molting process lasts about 60 minutes.
Late in the afternoon, motor boats patrol the river, with people using light to detect locations with lots of mayflies.
People retract nets every 500 meters to relieve weight on the boat and prevent mayflies from being crushed.
"Mayflies usually appear in small numbers, but will turn into huge swarms the next morning. We've stayed up the entire night to prepare tools and vehicles to capture them, even dividing into shifts," said 60-year-old Nguyen Van Minh in Hoang Mai District.
Minh said mayflies may show up at around 4:30 a.m., and start to emerge in large numbers at around 5:15 a.m. It would only take 10-15 minutes before the entire river is filled with them, so boats need to be quick.
"There was a time, about 10 years ago, when there were no mayflies to be seen. Areas where wastewater is released into the river usually had no mayfly," said Nguyen Tho Sinh, who has spent 50 years living and working on the river.
In peak periods, each family could capture up to 40 kilograms of mayflies a day. They typically fetch between VND300,000 ($13.05) and VND500,000 per kilogram.
Tran Minh Chien carries a basket of freshly cleaned mayflies. She uses them to cook meals, saying her regular customers eat so much of them that mayflies are sold out as quickly as they are harvested.
