Yeah, I didn't understand how hard it was to get wildlife population surveys right until I read about some of the work of MacArthur Grant winner Ted Ames (https://en.wikipedia.org/wiki/Ted_Ames). Some years, the random sampling wouldn't find any fish. Some years, it would end up sampling a school of fish that happened to be moving through the area (as far as I understand it). That data is used to determine fishing limits, so it's important to figure out how to survey (local limits might be different in different areas, etc.).