I'm seeing the same thing in my pond here in northern Indiana although the spawn is almost over. The redness on the bottom of the caudal fin is normal from the stress of spawning, and quickly warming waters cause bacteria to flourish on certain parts of stressed fish. In fact, in some cases fish can be so stressed so much they die after spawning. However I usaully see this with some of my bluegills.

It's usually the males as they make and fan the bed, but contrary to popular believe it's not limited to males.

As a fish taxidermist I see this every year at this time when bass are brought in. In fact, I like to paint in a reddened area on the bottom of the caudal fin to add realism.

So you've had problems with the herons too eh? It's seems to have been especially bad this year with a late spring leaving the herons in short supply of food. However, I think I've got them whipped now or they are finding feed somewhere else. All spring I have been sicking the dog on them (she's 1 1/2 the times the size of a normal German Shepard and is part collie), and I installed a heat sensing "scarecrow" sprayer on the pier. Anything that sets foot in front of this "scarecrow" within 35 feet gets blasted. On top of that I have driven two foot stakes around the pond in shallow water and connected 10 lb. mono a foot off the water at least two times all the way around the pond. The idea is they don't like walking into the line and won't step over it if it is a foot off the water. Additionally the outside of the mono is too deep to wade. I haven't seen any wading herons at all.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.