WEST GLACIER, Mont. — When “A River Runs Through It” was released 21 years ago, moviegoers who had never seen a fly rod descended upon tackle shops. Once outfitted, they made a beeline to Montana for a chance to cast to oversize brown and rainbows like those Brad Pitt seemed to land on every other cast. Those anglers probably did not realize that like themselves, the browns and rainbows were relatively recent arrivals, shouldering into the historical domain of Montana’s state fish.

Westslope cutthroat trout, or cutts, are among western Montana’s original salmonid inhabitants; more celebrated species like rainbow, brown and brook trout were introduced to the state in the late 1800s, at least in part because settlers from the East were accustomed to fishing for them. Westslopes were first described by Captain Meriwether Lewis after a member of Lewis and Clark’s expedition, Silas Goodrich, caught several specimens in the Missouri River below the site of the present-day town of Great Falls in June 1805.

In the vernacular of the times, Lewis wrote: “These trout are from 16 to 23 inches in length, precisely resemble our mountain or speckled trout in form and the position of their fins, but the specks on these are of a deep black instead of the red or goald colour of those common in the U.’ States. These are furnished long sharp teeth on the pallet and tongue and have generally a small dash of red on each side behind the front ventral fins; the flesh is of a pale yellowish red or, when in good order, of a rose red.”

Lewis vividly captured the cutthroat’s beauty. Although genetic markers are the only true distinguishing feature for trout species, westslopes (as well as the other 13 subspecies of cutthroat) can generally be identified by the “small dash of red” beneath their jaws. (In a nod to the expedition’s discovery, westslopes are formally recognized as Oncorhynchus clarkii lewisi.)