It seems that you did not provide possible answers, but the mark recapture method is one method that is not very invasive. The population size is estimated by marking a certain number of individuals and releasing them into the wild, followed by a second capture later on. The members captured the second time with the markets from the original capture are used to estimate population size in the formula s*n/x where s and n are the first and second captured population sizes and x is the number of marked inidividuals from the second capture.