WELLINGTON -- An international study showed the global population of emperor penguins was declining around 1.3 percent per year, with scientists failing to know "the cause of this apparent decrease."

Across Antarctica, there were fewer emperor penguins in 2018 than in 2009, the study said on Wednesday.

Researchers from countries including New Zealand and Australia found that in 2018 there were around 24,000 fewer adult emperor penguins at breeding colonies in spring compared to 2009 numbers, according to the study published in the Proceedings of the Royal Society B: Biological Sciences journal.

Changes across the continent varied by region and observed trends were not linked with sea ice conditions, said scientists from New Zealand's University of Canterbury and Australian Antarctic Division, a government agency.

Like many polar animals, it is challenging to monitor emperor penguin populations because of the species' life history and remoteness. Consequently, it has been difficult to establish its global status, a subject important to resolve as polar environments change, said the published research article.

In order to gain empirical evidence of emperor penguins' global status to understand the population declines, scientists have used counts at colonies, satellite images, and Bayesian modeling, and established a framework for global monitoring of emperor penguins and other ice-obligates in Antarctica.