According to experts, this marks the third consecutive annual decline for the butterflies.
A decrease in the monarch population has been observed in six of the past seven years. This decline now fits into a long-term statistical trend that can not be attributed to "one-off" seasonal events, like extreme drought.
Illegal logging, climate change and the wide-spread use of pesticides that kill plants the butterflies feed on have contributed to the trend.
Omar Vidal, director of WWF's Global Fund in Mexico warned that Canada, the United States and Mexico must share responsibility in protecting the monarchs, adding that Mexico has taken great strides towards combating illegal logging.
The Mexican mountains where monarchs spend the winter was designated a nature reserve in 2000.
This year, the butterflies covered 1.19 hectares of forest, compared to 2.89 hectares last year.
With files from the Canadian Press