Hanoi's Hoan Kiem Lake and Temple of Literature are seeing a boom in red-eared slider population.
Every morning, scores of sliders climb up on rocks and branches in the lake to sunbathe, according to VnExpress.
Vendors in the area say pagoda visitors released the sliders into the lake as a symbol of mercy.
In the meantime, slider peddlers catch them by the score using fishing hooks.
They range in size from a kilogram to about the size of a big toe.
Ha Dinh Duc, a scientist who has authored research about the lake for the past decade, said he first captured a photo of the sliders in the lake in 2004.
Before that, he said, someone had produced an image of the invasive turtles in the lake dating back to 1997.
"I can't tell how many red-eared sliders there are in the lake right now, but there has been a sudden increase from the past," Duc said. "This species reproduces very fast, just like the yellow snail (oc buou vang) which is destructive to paddy fields."
Oc buou vang served as Vietnam's first big lesson on the dangers of foreign fauna.
Several pairs were imported in 1975 from South America and bred in ponds as a potential food source.
But the creatures went on to become the nemesis of the region's rice and water spinach crops.
Duc said the sliders won't endanger the famed Hoan Kiem Turtle but they will disrupt the food sources and habitat of smaller creatures.
"I have, on many occasions, brought up the dangers posed by the red-eared sliders but no agencies have taken measures to mitigate them."
The Thien Quang Well inside the Temple of Literature also contains many sliders.
They've even come to entertain visitors to the UNESCO heritage site by swimming on the water surface or climbing up the stairs of the well.
Dang Kim Ngoc, a manager at the temple, said the sliders have come from two sources: from waterways when the city was flooded in heavy rains and from visitors who deliberately released them.
Nguyen Minh Tuan, head manager of the Sword Lake area, said if the government doesn't want to keep the sliders, it should issue a policy to get rid of them -- such as using them for food.
Duc added that the government should enforce a strict ban on the import and sale of the sliders. It should also seek to educate residents about the ecological harm they cause.
According to the Ministry of Agriculture and Rural Development, red-eared sliders came to Vietnam from North America and can live from 50-70 years.
The creature invades the habitat of local turtles, disrupting the ecological balance by gobbling up food supplies and potentially spreading typhoid.
The turtle (Trachemys scripta elegans) was included on a list of the world's 100 worst invasive species by the Invasive Species Specialist Group (ISSG) a global network of scientific and policy experts on invasive species, organized under the auspices of the Species Survival Commission (SSC) of the International Union for Conservation of Nature (IUCN).