I suppose I could have started this with something more poetic, like what's in a name? Nah...quit with the bullshit! Brook Trout are trout too. It is the "too cute-ism" of the internet, they're not trout, they're char. I'm SOOO much smarter than you...I know the difference between a trout and a char. Just quit it! Yes, they are char but they are also trout.
It is unnecessary sophistry. Brook Trout are trout too - unless you want to make one hell of a mess of the rest of the "trout" world. Brook Trout are in the family Salmonidae - as are ALL the other trout and salmon along the graylings and whitefishes. The Salmonidae family includes three subfamilies. The grayling (subfamily Thymallinae), the whitefishes (subfamily Coregoninae), and the trout and salmon (subfamily Salmoninae). These are all thing things we think of as trout and salmon - including the Brook Trout.
Salmoninae - the subfamily that are referred to as trout - include the genera, Salmo (Brown Trout and Atlantic Salmon and others), Oncorhynchus (the Pacific Salmon along with Rainbow, Cutthroat, and a host of other Western trout species), and Salvelinus, the Char (Brook Trout, Lake Trout, Arctic Char, and other things). It also includes a number of species we do not see in North America - things like the Lenok, Taimen, and other species. They're all trout or none are trout.
If Brook Trout are not trout because they are char, how do you rectify Atlantic Salmon (Salmo salar) being salmon but Brown Trout (Salmo trutta) are trout? Similarly, the Pacific Salmon like Coho, Chinook, Pink, and Chum - Oncorhynchus spp. and Rainbow Trout (Oncorhynchus mykiss) are salmon and trout in the same genus.
You see, common names are sort of useless. Scientific names on the other hand inform us about evolutionary relationships. Maybe you remember when Rainbow Trout were placed in the genus, Salmo as Salmo gairdneri. Salmo is the group that now includes the Brown Trout and Atlantic Salmon and depending upon if you are a "lumper" or a "spliter", a TON of other species. Genetics - and some pretty simple logic - changed the Rainbow Trout scientific name. Salmo are native to the Atlantic drainages of Europe, northern Africa, and western Asia. Oncorhynchus, where Rainbow Trout now reside, are the Pacific salmonids - the anadromous salmon and the resident trout of the west. It never made much sense that "trout" had to be in one genus and "salmon" in the another. That or we should have found better names that align better with evolutionary history.
Evolutionary relationships are built upon monophyletic groups. A monophyletic group includes the common ancestor and all of the taxa that share that common ancestor. It is why birds are dinosaurs (no really! It's pretty cool...) and biologists often refer to dinosaurs as "non-avian dinosaurs" because there is not a dinosaur monophyletic group that does not include the birds (Class Aves). Within the subfamily Salmoninae, there are two rather distinct groups - a monophyletic group that includes the Hucho and Lenoks and another monophyletic group that included the three taxa we have discusses - Oncorhynchus, Salmo, and Salvelinus - along with the genus Parahucho which includes a single Asian species. Monophyletic groups are hierarchical - each genus, each subfamily, each family, each order, etc. are all monophyletic groups at different levels of resolution.
This is a long-about way to say that, yes, Brook Trout are char (genus Salvelinus) but they're also trout. They belong to a monophyletic group that includes the things we call trout and salmon (Oncorhynchus and Salmo). According to evolutionary history, if you're going to call some Salmo species and some Oncorhychus species "trout" and others "salmon", you really need to call the char (Salvelinus) "trout" as well. Or we're back to birds and dinosaurs (actually that is a much more coarse resolution that trout vs. char).
Looking at the figure above, Salvelinus (the char) are more closely related to Oncorhynchus (Pacific salmon and Rainbow Trout relatives) than they are Salmo - the Atlantic Salmon and Brown Trout. Which is always interesting as I've never heard of a Brook Trout - Rainbow Trout hybrid (they generally spawn at very different times) but "tiger trout", a Brook x Brown trout hybrid does naturally exist despite the fact they that have different numbers of chromosomes. To put it kindly, chromosome numbers in the family Salmonidae is one huge jumbled up mess. If you're not already convinced of the mess that are Salmonidae genetics, lets muddy things up a bit further.
To confuse things a bit more - because that is what we needed - the scientific name for the Brook Trout, Salvenlinus fontinalis, is roughly translated to "little salmon of the springs". And there are anadromous populations of Brook Trout - "salters" of the East, "coasters" of the Great Lakes. The desire to wander - the evolutionary drive to grow larger, faster by moving to forage-rich oceans and lakes - has not left the char. There appears to be no genetic determination for the "wandering" gene. Why some wander and others do not is partially a mystery but I'd suggest it happens in most Brook Trout populations. They can get larger by accessing better areas to forage - but that is generally offset by greater competition (now, generally by non-native species like Brown Trout) and greater risk of predation, particularly in the ocean but also in the Great Lakes.
Quite simply, if we could do the whole common names thing over, it would have made a lot more sense. there would be char (Salvelinus), some name for the Pacific salmonids (Oncorhynchus), and another for the (mostly) Atlantic salmonids (Salmo). Instead, we have trout which are generally the non-migratory, the Salmo and Oncorhynchus species, salmon - the migratory Salmo and Oncorhynchus species, and our char. And none of that is simple because like the other trout species (see, I called them trout!), there are homebodies and there are wanderers.
The quick and dirty, if you quit reading and skipped to the end (I don't blame you...), QUIT IT, Brook Trout are trout and they are char. Common names are a mess and tell us nothing about evolutionary relationships which are what really matter. At least if you are a biology geek like myself.