The hounds! They love the herring! (Right?)
Literary historians initially misunderstood its mention in Gentleman's Rec to mean it was used to train hunting dogs to follow scents -- totally understandable given the pungency of the actual fish. This, itself, was a misdirection; it later turned out that kippers were sometimes used to smell-train the horses to follow the dogs, who were usually smell-trained with dead cats or dogs. Super!
Another red herring in the search for the definition of red herring: colonial fugitives supposedly dropped kippers to throw dogs off their trails when fleeing from capture. This may have been true, but it probably didn't work -- Mythbusters found that bloodhounds would eat the fish, then resume tracking.
But back to those dogs! In 1807, notes Quinion, a journalist published a story in the Weekly Political Register about a boy leading hounds away from a rabbit by dragging red herring across the trail. It was a parable meant to chastise the contemporary British media for being easily distracted by rumors, the fish being a figurative stand-in for a false clue. Now we're talking.