Nine of the original 100 wood bison released near the Lower Innoko/Yukon Rivers area on April 3 have drowned while five have died of unknown causes, according to a release from the Alaska Dept. of Fish & Game.

However, biologists say the animals are “on course to form a strong, successful herd.”

The herd, consisting of mostly cows and young, was the culmination of a project almost 20 years in the making, according to Fish and Game spokeswoman Cathie Harms.

Wood Bison Project Leader Tom Seaton led the animals to their new home from their release pen via snowmachine.

“The wood bison are showing behaviors of a wild population, including feeding, bedding and avoiding humans. The herd is doing well,” Seaton said in a prepared statement, adding, “This is an experimental population. They’re not inside a fence anymore, their fate is up to them and nature. We continue to have very high hopes.”

Already, six calves have been born to the herd, with more on the way, Harms wrote. Seaton noted that calving season runs between April and June.

The deaths of the nine bison that drowned after falling through ice are considered “normal,” according to Seaton.

“Bad ice is a problem for many wildlife species, and we knew it would be a factor in the Lower Innoko area. Bison released from captivity need time to learn about natural hazards,” Seaton said in the release. “Natural selection is a difficult and challenging process for wild animals, but it ensures that only the strongest and best adapted animals survive to establish the next generation. Through the process, the herd will get stronger and stronger as the years go by.”

Fish and Game needed to release at least 30 animals in order to establish a successful herd, but generous donations allowed the Alaska Wildlife Conservation Center and Fish and Game transport and release the 100-head herd.

An additional 28 adult males are expected to be transported and released in the same area in late May or early June, just in time for breeding season. The males — some weighing nearly a ton — will be transported by truck and boat, rather than plane, as was done previously.