Sometimes, for certain ecological investigations, there is no need to know the absolute population densities. Relative densities serve the purpose equally well. In this case, population size is indirectly estimated without actually counting them. For example, the number of fishes caught per trap is good enough measure of its total population density in the lake. The tiger census in our National parks and tiger reserves is often based on pug marks (animal's foot print) and faecal pellets.