>Dolly Varden (salvelinus malma) are not a true trout, but a member of the char family like Arctic char and bull trout. They look almost exactly like bull trout, but their life history is much closer to that of the Arctic char. In fact, these three species are so closely related, that anglers and scientists have confused the species for decades!<

(click here for more, and pic)
http://fishing-guides.gordonsguide.com/dolly-varden-fishing.cfm

Now, as for whether the fly or the fish captured the nomenclature first. I would surmise that flies tied tto catch Dolly Varden trout assumed the name. BUT, there is a chance that certain colorful flies, in general, were named after the Dickens character without specifically being targeted for the fish of the same name, just because the flies were colorful patterns.