When asked to define the difference between a Rainbow Trout and a Cutthroat Trout, a biologist would tell you that although both species have different origins and ranges, they are both members of the same family and genus Salmonidae oncorhynchus but, that they are also two distinct and different species. However, if you asked the same question of an avid fly fisherman and he would tell you that the two species are not only found in different areas, they each have very distinct appearances. But, while both descriptions are technically correct, the fact is that both species have numerous different recognized subspecies; each with different markings. Nonetheless, all subspecies of both Rainbow and Cutthroat Trout do display certain common markings that gave rise to their common names.
For instance, Rainbow Trout are classified by taxonomists as Oncorhynchus mykiss and their native range extends from the Kamchatka Peninsula in Russia, east along the Aleutian Islands into most of Alaska, and then south along the Cascade, Sierra-Nevada, and the Rocky Mountains all of the way into northern Mexico. However, they are also commonly found inhabiting eastern streams throughout the Appalachian Mountains as well as some northern lakes where it is believed that they were transported and stocked by unknown individuals. Also, Rainbow Trout differ in appearance from Cutthroat Trout in that their caudal fin is somewhat square and only mildly forked and, they generally display a blue-green or olive-green overall coloration with black spots over the length of their sides and back. However, their main distinguishing feature is the broad, reddish, stripe that extends along their lateral line from their gills to their tail and which is the origin of their popular name “Rainbow” Trout. But, it should be noted that some coastal Rainbow Trout (O. m. irideus), as well as some Columbia River Redband Trout (O. m. gairdneri) populations also sometimes display red or pink throat markings similar to that of Cutthroat Trout.
On the other hand, Cutthroat Trout are classified as Oncorhynchus clarki (in honor of explorer William Clark who was the co-leader of the famous Lewis and Clark Expedition) and, while they are also native to the western United States, their range only extends east to the Rocky Mountains where they have evolved through geographic isolation into 14 distinct subspecies; each of which is native to a different major drainage basin. Therefore, unlike Rainbow Trout, their coloration can range from golden to gray to green on the back and, depending on subspecies, strain, and habitat, most have distinctive red, pink, or orange linear marks along the underside of their lower jaw and in the lower folds of the gill plates which are responsible for the common name “Cutthroat” which first used by outdoor writer Charles Hallock in an article published in The American Angler in 1884. However, it should be noted that these markings are not unique to the species since some coastal rainbow trout (O. m. irideus) and Columbia River Redband Trout (O. m. gairdneri) populations also display reddish or pink throat markings. Therefore, the real distinguishing difference between a Cutthroat Trout and a Rainbow Trout is that Cutthroat Trout have a set of basibranchial teeth at the base of their tongue and a maxillary that extends beyond the posterior edge of the eye.
Last, it should be noted that even though animals of different species generally cannot interbreed, such is not the case with Rainbow and Cutthroat Trout. Therefore, because the two species commonly inhabit the same range and habitats, some populations have interbred to produce hybrid offspring populations which are commonly known as “Cutbow” Trout. Thus, members of such populations often display both the characteristic pink or red stripe along their lateral lines as well as the distinctive pink or red markings on their lower jaws and gill plates and thus, identifying a “cut-bow” can be somewhat confusing to an angler who is not familiar with the species.
So, although both Rainbow Trout and Cutthroat Trout are members of the same Family and Genus which commonly inhabit the same range and even the same streams, they are actually two distinctly different fish species with distinct anatomical differences as well as distinct differences in coloration which are the origins of their common names. But, due to their unique ability to interbreed, they are also capable of producing a hybrid species which often displays both the anatomical and coloration differences of both species. Thus, each species’ common name is a product of the unique differences in their markings.
Photos Courtesy of Andrew Engel from @theflydudes on Instagram. Be sure to check their page out for more epic Colorado fishing content!
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