Fort Smith (May 02/05) - The number of bison in Wood Buffalo National Park has taken a sharp turn upwards.

An aerial population count in March spotted more than 4,500 animals.

"It's somewhere between 4,500 and 4,700," says Cam Zimmer, the supervising senior park warden.

"That's the minimum count."

Based on that number, the park also calculates an estimated total number of bison in the park.

In 2003, there were 3,743 animals spotted and an estimated population of 4,947.

Zimmer says the estimated population for this year has not yet been calculated, but will be in a week or two.

"It's definitely going to be up," he says. "It's a large increase."

Zimmer says there seems to be an upward trend, after the population was stable at about 2,500-3,000 animals in the 1990s.

Zimmer says the increase cannot be easily explained without further study. "The 'why' is always the hard part. There are so many factors involved."

In particular, he is reluctant to say the herd is getting healthier.

Many of the animals have tuberculosis and brucellosis, and there is an occasional outbreak of anthrax.

The bison population in the park is believed to have peaked at about 12,000 in the 1930s and 1940s.