Determine object size (fish) in image

To develop a program able to determine 1) fish length 2) fish surface and 3) fish width on an image, based on preset lythographic (100% black and white, no greyscale) treatment of the image and pre-defined zone of interest.
The fish will always be placed oriented with head on the right/ tail on the left, so we do not need any rotation feature to keep the programm simple.
All details in file attached.