There are always a more noticeable mortality in freshwater fish at this time of year after or during spawning. Some fish develop cysts or growths on the sides of their bodies. Sometimes these growths will heal but sometimes become infected.

Herons do pierce them as you say, then can,t take off or are disturbed.
I use childrens windmills as deterrents......

That was a surprise to go with my Chinese meal! Still, just shows what this site can throw up! Heron, I'd have said. Small children also make great heron deterrents, without their windmills, especially when bird watching, I've found!

Thanks for all the help. I bought a large windmill today---I'll see what that does!