How do scientists actually know, how many animals of an endangered species are actually left?

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Saw a post about ‘only 366 North Atlantic Whales left. Whales are big and easy to spot, but are these numbers really exact? Especially with animals who live in the oceans. Some time ago there was headline about a white rhino being the last of its kind. This makes sense for me, since it is a rhino, it is white , hard to miss if you look for it and this combination most likely only appears in a certain spot on Earth. Also in forests with many hiding spots you can probably use wildlife cameras for a rough number.

In: Biology

Usually there is a group of scientists that specialize and will tag larger animals with gps or rf tags that they can track movement. For smaller individuals they probably use some sort of algorithm based on population density over a set amount of area.

First of all you are probably talking about the North Atlantic Right Whale. There are certainly tens of thousands of large whales in the North Atlantic. And the number of whales of other species like the Humpback whale is posing a danger to Right whales as they compete for the same food resources.

But for your question the most common way to count animals is to collect reported sightings and then estimate how many are not sighted. There are also expeditions with the aim of counting various animals with more accurate predictions for how many they missed. A big part of this is to look for distinguishing markings on each animal and in some cases collect DNA evidence. This allows you to determine if you are seeing the same animals several times or if it is unique animals. With DNA tracking we are also able to see how related they are to each other and can therefore determine if the whales we do observe is the child of an unobserved whale. We have made mistakes before where animals have been recorded as extinct as we could not find evidence of any of them but then decades later they turned up. So the methods we have are not foolproof.