'Very harsh winter on calf survival'
Out of 33 calves in 2015, ENR says only four survived to yearlings
April Hudson
Northern News Services
Monday, October 31, 2016
DEH CHO
A higher-than-normal winter mortality rate for the Nahanni population of bison in the Deh Cho has officers with the Department of Environment and Natural Resources pointing to a warm spell last winter as the likely culprit.
Bison from the Mackenzie herd walk near Fort Providence. In the Nahanni population, only 13 per cent of calves from 2015 survived to be yearlings in 2016. - photo courtesy of Thorsten Gohl |
Nic Larter, the department's regional manager of wildlife, research and monitoring, said only 13 per cent of calves from 2015 survived to be yearlings in 2016.
Larter said the average winter survival rate for calves is a little more than 50 per cent.
Since 2002, the department has gathered data for its classification survey of bison.
Calf survival rates vary year to year, sometimes dropping as low as 10 and other times rising as high as 37.
But out of 33 calves in 2015, only four survived the winter.
"This past year seems to have been a very harsh winter on calf survival - the harshest that we've recorded," said Larter.
"They were very noticeably absent (during our survey), and as well the number of young males was noticeably lower than in other years."
Larter says the winter mortality rate could be due to a warm spell that struck the area toward the end of March.
Four days of warm weather were followed by 10 days of freezing weather, causing the snow pack to melt, sink and then freeze.
"That dropped the snowpack down to half the height it was at and made it like concrete," Larter said. "Even the bison can't break through this."
For at least a week, bison would have found it difficult to forage for food as a layer of ice had formed over vegetation.
On top of that, the hard snowpack made it easier for predators to go after bison.
"It's a double whammy," Larter said.
And although a high calf crop this year could help to offset the mortality rate, Larter said as the bison age there will be a noticeable hole in their life table.
On top of calf mortality, young bulls had a high mortality rate as well this year.
Whereas the Department of Environment and Natural Resources usually counts around 20 to 25 young bulls for every 18 to 20 yearlings - that is to say, animals in their first or second year of life - this year they only counted two or three.
"They just weren't there," Larter said.
The department has no way of telling if young females were equally impacted, since female bison mature quicker than males.
