Hallo Ben, like u mentioned, I noticed it also recently then on gutting the one I found fish roe's inside, I felt so sh!t afterwards to think that the fish would have spawned in the near future. I will from now on make an effort to check if the redness got to do with the spawning.
Hi Envor, I don't thing its sex related. I caught a male kob 58cm last week and it also had the red tinge. Notable between the pecs was a nipple protruding for spraying of the eggs when the roe is released by the female.