FAIRBANKS (AP) — When you bring an animal back into the Alaska wild for the first time in a century, you have to be prepared for some mishaps.

Although at least nine of the 100 newly-released wood bison drowned after walking on rotting ice, biologists said the herd is healthy and the deaths fall within the expected range, reports the Fairbanks Daily News-Miner.

Biologists are trying to reintroduce the 2,000 pound wood bison — North America’s largest land animal — in the Lower Innoko River area near Shageluk. The bison were once widespread in Alaska and Canada, but populations began falling in the 1800s.

They were thought extinct until a herd was discovered in Canada in 1957.

Re-introduced wildlife populations can lose as many as half their members before stabilizing, according to a Tuesday news release from the Alaska Department of Fish and Game.

The release said drowning is a leading cause of death for wild wood bison in Canada and a significant cause of death of moose in Alaska.

In addition to the nine bison that drowned, five have died from causes that are still under investigation and at least six calves have born.

The animals were flown to Innoko River by Fish and Game personnel in March. They were released on April 3, each wearing radio collar that broadcasts its location, after being given time to recover from the plane ride.

Before that, the bison were living in captivity at the Alaska Wildlife Conservation Center near Girdwood.

The new herd has been separated into group sizes typical of wild bison and quickly became skittish around humans.

Only one 11-month-old calf returned to the pen area where the bison were kept before being released. That calf rejoined the herd after about two weeks.

The next step is to add 29 bulls to the heard in time for the late July breeding season. Those animals will be barged down the Tanana and Yukon rivers from Nenana.