Canadian filmmakers Jack Weisman and Gabriela Osio Vanden knew they were onto something when their short film “Nuisance Bear,” an unconventional nature documentary centered on the annual polar bear migration in Manitoba, was picked up by the New Yorker Studios following its Toronto Film Festival premiere in 2021 and shortlisted for the Academy Awards.  But while the 14-minute short was conceived as “proof of concept” for a feature-length documentary, neither filmmaker could predict the success