Neural Network (or something else) for fish photos

[Hiero]

Hello guys,
I was wondering if anyone had  this trouble, I have several images like this above (pic attached), I was thinking how to quickly get all the standard lenght (fish mouth to base of caudal fin) and total lenght (fish mouth to end of caudal fin) of my fishes. I saw some papers about training neural network that could do this, but I´m new in this field. Anyone could help me?!
I just need a way to do the lenght measures. The scale convertion I can do manualy.
Thanks in Advance