Thanks to all and especially to the pond guru Mr Lusk for your valuable comments and advice. Very appreciated.

@ ewest very interesting your graph. Which software did you use to create it?

@ Theo Gallus unfortunately we have here herons and cormorants in abundance and absolutely protected. I'm not sure but I think those old scars have been made by cormorants.
http://www.leccotoday.it/video/video-cormorano.html
In the video you can see how cormorant use its beak to keep and turn the fish, probably causing that kind of scars.

@ Bob Lusk I am really happy that you have intervened with your great experience.
All is clear, I'd just like to ask you some clarifications, if possible:
- is it correct to record the measurements throughout the year or is it better to do them only in certain periods?
- can I start culling right away or it's better to wait the end of the spawn?
- do I keep the limit of 88% valid for the whole year or is it only valid now that they are in the middle of the spawn?

Filippo