"It's clear that moose are continuing to go down, down, down," Department of Natural Resources researcher Mark Lenarz told the Minneapolis Star Tribune. "I'm very pessimistic."
"I don't think it's a single thing," Lenarz said, noting wolves and hunters don't appear to be behind the decline. "The majority of mortality appears to be related to disease and parasites."
A recent aerial survey showed a population of 4,230 moose, down 14 percent from 2011, the newspaper reported Friday.
The population was 8,840 as recently as 2006, officials said.
Lenarz said he believes warmer temperatures in northeastern Minnesota in recent decades also may be having an impact on moose populations
"I still believe there is a link between climate change and the mortality we're seeing," he said, as even slightly warmer temperatures may be making moose more vulnerable to disease and parasites.
"It's not that moose are dying because of heat stress on a hot day; it's likely a cumulative process," he said. "Perhaps their immune system is compromised."