Experts said hunger and water pollution might be causing the old turtle at Hanoi iconic lake to emerge above water frequently in the past month.
The turtle has risen at least 12 times since last month, after its leg injuries were treated and it was returned to the natural environment at the Sword Lake in July, news website VnExpress said Tuesday.
The turtle emerged at least four times this month.
Dr. Nguyen Viet Vinh, an aquaculture expert, said the turtle was hungry.
"There's not a lot of fish cast into the lake, and they were all alive. It's not easy for the turtle to catch the small number of fish in such large water area," Vinh told VnExpress.
He also said that during the three-month treatment, the turtle built up a habit of being fed.
Officials from Hanoi Aquaculture Department said they cast around 60,000 fish into the lake in July when the turtle was returned to the lake.
When people took the injured turtle away for treatment earlier this year, there were signs indicating that there might be other turtles living in the lake. But no clear traces of a second turtle has been found.
Aquaculture expert Bui Quang Te agreed that food is the reason for the turtle's frequent appearances.
But Te suggested that the turtle also may be affected by the dirty environment in the lake and the hotter weather.
Ha Dinh Duc, the professor who has been campaigning to protect the precious Sword Lake turtle since 1991, said the turtle is rising to breathe because it has been hotter and there has been more sunshine, causing weeds in the lake grow faster and reducing the oxygen in the lake.