Which island nation had the highest population decline rate of all nations in 2023?

The Pacific Islands nation, The Cook Islands.

In 2023, the Cook Islands suffered a population decrease of about 2.31%, compared to 2022. This was the highest country's population decline rate for 2023. The second and third countries were American Samoa (1.74%) and Puerto Rico (1.29%).

Many of the countries with high population decline are island nations with high emigration rates to large neighbours like Australia, New Zealand and the United States.

https://www.statista.com/statistics/264689/countries-with-the-highest-population-decline-rate/