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Before the first reintroduction, in 2000 on a 100-square-kilometer island off Estonia called Hiiumaa, hunters and a trapper killed the island’s entire American mink population, the legacy of a defunct fur farm.

Hiiumaa now has a thriving population of at least 100 animals.

A Baltic ringed seal rests on a rock in Estonia's Hiiumaa Islets Landscape Protection Area.

Estonia said on Monday that it was boosting surveillance around the islands of Hiiumaa and Saaremaa islands -- a potential resupply route from the West in the event of conflict with Russia to the east.

It stretches across the frozen surface of the Baltic Sea, connecting the Estonian coastline to the island of Hiiumaa.