Distance between points

[I.M.]

Hi everyone,
I am completely new to ImageJ (or any other similar software), and unfortunately, so far I don’t really understand any of it :(  

Here is the “problem”: I am going to observe the behavior of spawning fish (salmon) when they are being disturbed by a predator (in my case a human). I will set up a video camera (sony handycam  HDR-CX190, on a tripod) to capture the behavior, and I want to analyze these video’s later on (or stills). One of the things I want to measure is the flight distance (how far do they swim away from their starting location, distance between points).

Somebody advised me to use imageJ to analyze my video material (use stills from the camera). I figured out that I will need an object for the scale reference (I know how to do that),  and I will also measure the width of the river after the observations,  but I don’t know how to deal with the perspectives. So far I can only find information of aerial pictures or 3D. Is it even possible to use ImageJ for what I want to measure?

Any help would be appreciated!!