How to Repeatedly capture fixed screen region to sequence of images
You can use screencapture commandline utility. Create an Automator Service (takes no input) which will do a shell command and assign keyboard shortcut to it. 

The shell script will look like this:

/usr/sbin/screencapture -R0,0,1000,400 /path/to/save/FileName$(date +"%m_%d_%Y_%H_%M_%S_%s").png


screencapture has -R modifier not listed in man but it's listed in help as:

-R&lt;x,y,w,h&gt; capture screen rect


The values I used above captured this image which also shows how the rect coordinates worked.

Change x,y,w,h values for those You need.

The date +"%m_%d_%Y_%H_%M_%S_%s" adds a timestamp to the file name





You can actually use Screencapture to find the rect you want by using cmd +shift+4  which will give you some cross hairs with the numbers on for click drag release capture rect.





In Terminal.app  typing: /usr/sbin/screencapture -h  will give you the help documentation

usage: screencapture [-icMPmwsWxSCUtoa] [files]
  -c         force screen capture to go to the clipboard
  -C         capture the cursor as well as the screen. only in non-interactive modes
  -d         display errors to the user graphically
  -i         capture screen interactively, by selection or window
               control key - causes screen shot to go to clipboard
               space key   - toggle between mouse selection and
                             window selection modes
               escape key  - cancels interactive screen shot
  -m         only capture the main monitor, undefined if -i is set
  -M         screen capture output will go to a new Mail message
  -o         in window capture mode, do not capture the shadow of the window
  -P         screen capture output will open in Preview
  -s         only allow mouse selection mode
  -S         in window capture mode, capture the screen not the window
  -t&lt;format&gt; image format to create, default is png (other options include pdf, jpg, tiff and other formats)
  -T&lt;seconds&gt; Take the picture after a delay of &lt;seconds&gt;, default is 5
  -w         only allow window selection mode
  -W         start interaction in window selection mode
  -x         do not play sounds
  -a         do not include windows attached to selected windows
  -r         do not add dpi meta data to image
  -l&lt;windowid&gt; capture this windowsid
  -R&lt;x,y,w,h&gt; capture screen rect
  files   where to save the screen capture, 1 file per screen