In looking at larger scale trends in environmental quality, we have pulled together population data over time across the Cook Islands. This is the overall trend (summing across all 15 of the islands that comprise the Cook Islands):
Here is a bit more complicated graph wherein an island representing each of the temporal patterns we have seen is plotted. We have seen some islands that have a declining population since record-keeping began (1901; Palmerston), islands that peaked in the 1950s (e.g. Mauke), the 1960s (e.g. Nassau), 1970s (e.g. Aitutaki & Atiu), and now (e.g. Rarotonga).
You can find the absolute population numbers below.
These data were all sourced from the 2011 Cook Island Census.