Wow, that's a beauty! Just to ask, is that from the States?
What part of "western South Dakota" didn't you get?....WK
That is really a nice fish. That said, I have seen hundreds of brook trout and that one just didn't look right to me. I shared the photo on another wedsite with professionals who have the credentials to give a dependable identification. While there was no concensus, none of them said it was a brook trout. Most who saw it believe it is a splake...a hybrid of a brook trout and a lake trout. Congrats!!