I've had the same thing happen to me-just last weekend on the Stehekin in fact. I think that they either are trying to slap the fly around a bit with their tail (kinda like an orca flinging a seal around) or they decide at the last second that they don't want to eat the fly. I ended up breaking off the fish I hooked, without a net there was no way I was going to land him. Caught the same fish the next morning, the fly was gone-fish barbless!
LDR
LDR
