ESTIMATING POPULATION SIZE USING "MARK-RECAPTURE"
The way we will estimate how many turtles are living in our neighborhood is by using a simple "mark-recapture" technique. Here's how it works. Say I give you a large sack of marbles, and you want to estimate how many marbles are in the sack, without having to count them all. So you scoop out a big handful of marbles and count how many are in your hand. Let's say you got 100 marlbes in one big scoop. Then you paint those 100 marbles gold, throw them back in the sack, and shake the sack real good so the gold marbles get mixed in with the other marbles. Next you take another scoop of marbles. In your second handful, you count 80 marbles, 20 of which are painted gold. It seems that 1/4 of the marbles in the sack have been painted gold, and thus the original scoop of 100 marbles represented 1/4 of the total, and so we would guess that there are 400 marbles total in the bag. The same technique can be applied to turtles. Instead of painting the turtles gold, taking a picture of the unique patterns on the turtles' shells serves as our "marking" technique because it allows us to recognize individuals on successive encounters.