Moose don't gather in large groups. Their population estimate surveys are performed in the fall and winter when the right combination of weather, snow depth, and day length allow biologists to fly and see the moose. The area to be censused is broken into smaller sample units based on terrain or habitat types. Some sample units in each stratum are randomly chosen to be intensively surveyed. The actual densities of moose found within the surveyed units are extrapolated to estimate densities in the other sample units within the same stratum.
Different methods work for estimating different species.
Then they count as many bears as they can find and note the proportion of marked bears to unmarked bears. Since they know how many bears in the total population are marked, they arrive at an estimate of total numbers of grizzly bears. Of course, the different methods of counting wildlife are not foolproof. All of them depend on getting humans out to a place where they can see wildlife to count them, often in a plane. Wind, clouds, and snow have prevented many a photocensus and moose census.