This video, we're going to talk about sampling mobile organisms using the mark and recapture method. For highly mobile organisms that move around a lot, those transects and quadrats that we talked about in our previous lesson videos are not going to work at all. Instead, ecologists use the mark and recapture method to sample these mobile organisms. This mark and recapture method can be broken up into 3 steps that we have numbered in the top half of our image right here. Let's imagine a scenario where we're trying to determine the total population size of these highly mobile fish in this lake.
It would be really hard for us to go underwater and count up all of these moving fish, and good luck not counting Bill multiple times as he looks almost identical to Sally over here. In situations like this, we use the mark and recapture method. In step number 1, all we have to do is go out and capture a small subset of the population, and in this situation, we've gone and captured 5 fish. Then all we need to do is mark or tag these fish in some way that does not interfere with their normal activity, and all of the marked fish have this reddish background behind them.
In step number 1, all we gotta do is capture and mark. Now, in step number 2, all we do is release all of those fish that we captured and marked in step number 1, and then we just wait a long enough period of time, maybe a few days or weeks, for those marked individuals that we released to mix back into the population evenly, as you can see here in this image. Then, in step number 3, we go back and we capture again, and hopefully, we recapture some of the fish that we captured the first time around. Notice that in this image we captured a total of 5 fish, but only one of the fish is marked, so that is the only fish that we captured the first time around. Once we've done this, we can go on and calculate the unknown total population size that we were trying to determine all along, and we can do that by using this formula that you can see down below.
This formula or equation says that the ratio of mn is equal to the ratio of rc. Here, m refers to the number of marked individuals in the first capture, which in this situation was 5 total fish. n is the unknown total population size that we were trying to determine all along. r is the number of marked individuals in the second capture, and c is the total size of the second capture. There were 5 fish caught in the second capture, but only one of them was marked, and so that one would be the r.
If we have any 3 of these 4 variables, we can calculate the unknown missing variable using our algebra skills, and we're going to do that in our next video. So, I'll see you all there.