Illustration by Steven Schalla, FlyFishingtheSierra.com, used with permission
Check first for the distinct orange-red cutthroat slash below the jaw. If you find one, and you’re in the right range, it’s almost certainly a Coastal Cutthroat, as no other cutt inhabits this same area. However, the cutthroat slash sometimes disappears in sea run fish. Distinguishing a Coastal Cutt from a Steelhead (a sea run coastal rainbow) is difficult in such cases, even for experts, as other distinguishing marks also fade.
In stream-resident and fluvial forms, coastal cutts can show a brassy or golden yellow color. They are the most intensely spotted of all cutthroat; spots are almost always present on top of the head and can often be found on the ventral surface and the anal fin. Cutthroats are typically more slender and finer scaled than rainbows. They also have a longer mouth, often extending well beyond the eye. If you look down its throat, you may see basibranchial teeth, small protrusions between the gill arches and behind the tongue. 90% of cutthroats will have these teeth, which are usually absent in rainbows/steelhead.