system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the population of deep sea fish?

People bring up that the bottom of the ocean is less known than the surface of the moon, and yet, when I go to the Wikipedia page for Frilled Sharks, I see that it's classified as 'near threatened'. How do we know the population of such a deep sea species well enough to term it 'near threatened'?
gpt
In biology, where a species is either widespread, or for some other reason, counting individuals exhaustively is too difficult, numbers are calculated from exhaustively surveying a set area of habitat then extrapolating based on area of known territory. 

E.g. A bird species has confirmed sightings in 10 different areas that correspond to its known habitat. The combined area of these 10 sites is 1000km^2. Biologists sample 10km^2 and find 20 individuals and so the population is estimated at 2000 individuals.