Another two Minnesota wolves have been moved to Isle Royale in recent days as the National Park Service moves to establish a viable pack of predators on the largest island in Lake Superior.

The wolves, both females trapped on the Grand Portage Chippewa Reservation at the tip of Minnesota's Arrowhead, joined two other Minnesota wolves brought to the island last week.

This week's transfers took different transportation modes: The first took a boat ride, the second was flown to the island.

The wolf released on Tuesday weighs approximately 70 pounds and shows signs of having had pups in the past. Veterinarians estimated she is five years old. Once on the island, the wolf waited in the crate for less than an hour before walking away to her new home. Two days later she was seen on one of the park's remote wildlife cameras.

On Thursday the other female wolf was flown on a U.S. Fish and Wildlife Service airplane to the island and carried by park staff over a hiking trail to her release site.

So far, all four new wolves appear to be doing all right. A fifth wolf trapped in Minnesota died in the process before it could be released on the island.

The Park Service hopes to add at least two more wolves, from Minnesota or Michigan, in coming days, with a goal of at least 20 and up to 30 new wolves moved to the island over the next three years. Half the wolves will be male and half female. They will be spaced out on their new island home to avoid inter-pack fighting. All will be fitted with GPS collars to keep track of their progress or problems.

Fall is considered the best time to move wolves because there are few if any people on the island over the winter, with the park closed to the public after Oct. 31 each year.

Wolf numbers on the island crashed from 24 as recently as 2009 to just a pair earlier this year, a 7-year-old female and 9-year-old male, as inbreeding spurred genetic defects that have crippled the wolves' ability to survive and reproduce.

Scientists with Michigan Technological University, who have been studying the island's wolf and moose populations for 59 years, have said for years that the wolves need new blood or they face extinction. The last two wolves, a father-daughter pair which have failed to produce any viable offspring, are almost certainly the end of the line for the island's natural wolfpack. No new wolves have crossed ice to the island from Ontario or Minnesota, and stayed, in decades. And the chances of that natural movement fades as ice bridges form less often due to climate change.

The 45-mile-long, 143,000-acre island, about 15 miles off Minnesota's North Shore, is mostly dedicated as federal wilderness.

There is plenty of food for the new wolves. In their annual survey last winter, scientists estimated the moose herd had grown to about 1,600 on the island with no other major predators.