Unfortunately, it is not that straightforward. To give you an idea have a look here:When does a creature cease to be on the endangered list? is there a formula for working out how many animals per acre of land or per mile of river bank are needed in order that the animal is no longer considered endangered?
Lutra lutra (Common Otter, Eurasian Otter, European Otter, European River Otter, Old World Otter)
Have a sit down first though as it might make you quite cross! ;-)
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