What happened to this trout?, Missing piece

[sniperlee]

So I caught a trout and noticed a part of his gill plate was missing. At first I thought it got cut off from the fish wrapping itself around the line. The wound didnt look fresh so I was perplexed. It seemed healthy so i released it. Later I was helping a guy land a fish and his fish was missing a piece of his gill plate also. He said it was from a lack of oxygen at some point of its life.
If anybody can shed some light on this please do.
I have a pic, its not very clear but shows what im talkin about.

[Disco]

It’s a bacterial disease that causes that. Lots of info on it available for all salmonids.

I see this commonly also on Chinook.

[sniperlee]

Wow really!? That sucks. Fortunately for me and the fish It's the first time I've seen it in my 3 years fishing for them. Odd that I see 2 in one day. Well I hope I don't come across anymore. Thanks for the info.

[Disco]

I see them much more common from fisheries that are heavily stocked. I am sure the bacterial disease if found in hatcheries would result in this being more common in heavily stocked areas.