Alaska's biggest bison herd is shrinking, and state wildlife biologists don't know why.

The Delta Bison Herd has dropped from about 480 bison two years ago to an estimated 400, according to this year's count.

"I am definitely not finding the number of animals I expected to find this year," said state wildlife biologist Steve DuBois with the Alaska Department of Fish and Game in Delta Junction.

There are any number of possible reasons for the herd's decline.

"They range from there's a group of them that I can't find them to a disease issue to natural mortality to unnatural mortality," said DuBois, adding that poaching would fall into the latter category. "We had a hot, dry year last year; it could be a forage issue."

Or it could be a combination of factors that have conspired to hit the herd at the same time, DuBois said.

"At this point I don't have the answer," he said.

The decline prompted Fish and Game officials to issue only 65 Delta bison permits to hunters this year rather than the 120 it was planning to distribute. That's the lowest number of permits issued since 1994, DuBois said.

Delta bison permits are the most coveted hunting permit in the state, in part because the herd is so accessible and the animals are so big. This year, the state received 13,952 applications for the 65 permits issued. That translates to one permit for every 215 applications.

DuBois first detected a decline in the herd last year, when the estimate came in at 421. He issued only 75 permits as a result, down from 130 the year before, and expected the herd to rebound this year as a result of the reduced hunting pressure.

But that didn't happen, according to what DuBois saw when he surveyed the herd last month and came up with a high count of 400. Both the number of adults and calves were down this year, he said.

The fact that both adults and calves were lacking adds perplexity to the situation, according to DuBois.

"If it were a pregnancy issue or a calf-mortality issue or something related to calf production and survival, you would expect the number of adults to be at normal levels and the number of calves down," he said. "If it was something like poaching you would expect that would be occurring to adults and not calves.

"Because it's occurring across the board makes it more confusing."

And as hard as it may be to believe given the size of the animals, there is a possibility that a large group of bison may have escaped DuBois' eye, too, the biologist said.

"It would be real easy for a group of bison to go some place and not be found if none of them had radiocollars," DuBois said. "You can put 20 to 50 bison in the trees and never find them."

All 12 of the bison in the herd that are fitted with radiocollars are accounted for, which opens the door for a possible undetected satellite herd.

Though he hasn't ruled it out, predation is not on the list of potential reasons for the herd's drop, DuBois said. There has been "little if no" predation on the herd in the past.

"We don't really have any evidence predation has increased," he said.

The Delta herd was started in 1928 when 20 bison from Montana were planted in Delta. The herd grew to the point where a hunt was established in 1950 to keep the herd in check. Natural dispersal and transplants have resulted in two smaller herds on the Copper River near Chitina and one in Farewell.

Tim Mowry can be reached at 459-7587 or tmowry@newsminer.com .