A battle between humans and mice is raging in the Dongting Lake area of China’s Hunan Province. According to the province’s Department of Agriculture, the number of mice in the area has exploded to up to 2 billion in recent months. In the Datong Lake section of the area, authorities captured more than 90 tons of mice within three days.

Since June 20, when the gates of the Three Gorges dam were opened to release flood waters caused by upstream rainfall, the water level of Dongting Lake has risen by nearly half a meter a day on average. The higher water has forced many mice that inhabited the lake’s low-lying shoal to migrate to the higher ground of the dike.

From a timing perspective, the massive migration of the mice appears to have some connection to the Three Gorges flood discharge. But how close is the link?

In May 2002, Professor Cong Guo and his students at what is now the Institute of Subtropical Agriculture under the Chinese Academy of Sciences published a paper evaluating the impacts of the Three Gorges project on the Dongting Lake mouse population. They analyzed historical data on the lake’s evolution, changes in the water level, and the population dynamics of the mice.

The paper predicted that completion of the dam project would reduce the flood discharge flow in October and December, causing the water level of Dongting Lake to drop and increasing the period during which the lake’s shoal would remain above the surface. Prolonged exposure of the shoal would subsequently lengthen the reproduction period of the mice that inhabited it. According to the paper, the earlier the water ebbed during the previous year, the later it would rise in the current year, and the greater the mouse population would be.

The researchers concluded that, “in Dongting Lake area, in the short term after the Three Gorges dam is completed, the change in the mouse population will be sudden, while over the middle and long term, such change will become a gradual and slow process. The common characteristic is that the number of mice will increase, and the harm to the surrounded farmland will worsen.”

Guo, who is now teaching at the School of Life Sciences at Sichuan University, confirms that reduced flow from the Three Gorges dam could expose the Dongting Lake shoal early and lead to an extended mouse reproduction period. He cautions, however, that his 2002 paper was qualitative and should not be used to predict the exact impact of the dam on this year’s mass mice explosion. “Compared with the economic benefits brought by the project, the mice disaster is only partial and temporary, and we can build barriers around the lake to prevent mice from migrating,” he says.

Guo believes that this year’s mouse explosion is ”a special case” related mainly to continuous drought in 2006 and to the lake’s low water level—and that it has little connection to the dam project. “Normally, the shoal of the lake would only be above the water surface in September and October, but due to the drought last year, it was exposed early, thus extending the reproduction period of the mice.”

Mice can reproduce at an astonishing rate, and having an additional month for reproduction can have huge effects on the population, Guo said. “It is possible for the mouse species in Dongting Lake to reproduce two or three times a year, and we have observed in the lab that it can even go into estrous within two days after giving birth. While the average gestation period is 20 days, lactation usually will not be finished before the next litter comes out.”

Mouse population explosions in Dongting Lake were first observed in the 1970s, but the scale of the phenomenon has never been as large as it was this year. This year’s migration represented the 11th such mass incidence, according to Wangyong Guo, an agronomist with National Agricultural Technology Promotion and Service Center.

The onslaught of mice is currently under control, though the danger still remains. On July 19, a survey by the Institute of Subtropical Agriculture indicated that the capture rate of mice is still around 30 percent, and the task of prevention remains challenging. “The number of mice living on the shoal is still large, and they will come back again once the water rises,” said the Institute’s Yong Wang.