In order to decide on quotas, the government needs to know the total number of fish that live there. They rely on scientists to find out. They couldn't possibly count every fish, especially when they can't see them all. The truth is, scientists don't count every animal. Instead, they count some of the animals, then make an estimate using basic math.
Scientists are especially interested in the number of fish that live within limited areas.
They can find out whether the population density of this kind of fish is growing or shrinking by keeping track of the population density within a habitat over a number of years. One method researchers use is fairly simple. One week, for example, they will tag twenty fish. The next week, they will return to the same area of the ocean and count every fish they see. Maybe they will see and count two hundred.