Rainbow Trout/Steelhead vs Coastal Cutthroat Trout

April 27, 2017

Question: I have a question regarding regulations on non-adipose fin-clipped (“wild”) rainbow trout/steelhead and coastal cutthroat trout in tributaries on the North Coast (e.g. the lagoons in northern Humboldt County). Anglers are not permitted to keep wild rainbow trout/steelhead but are permitted to keep wild coastal cutthroat trout. However, these two species are well known to hybridize and hybrid offspring are reproductively viable.

Hybrids also exhibit a continuous spectrum of phenotypic expression that runs from the rainbow phenotype (few spots below the lateral line, small head, maxillary terminating before the rear of the eye and no throat slashes) to the cutthroat phenotype (heavily spotted including below the lateral line, large head, maxillary extending past the rear of the eye and throat slashes present). These phenotypes are what the California Department of Fish and Wildlife (CDFW) website recommends for identification of the two species, but there is no reference to the basibranchial teeth that are specific to cutthroat.

Therefore, if an angler catches a non-adipose fin-clipped trout that has no throat slashes, no spots below the lateral line, a small head and a maxillary that does not extend beyond the rear of the eye, but has basibranchial teeth, is the angler allowed to keep the trout? The fish described is likely a hybrid “cuttbow.” Alternatively, if an angler catches a trout that outwardly looks like a coastal cutthroat but does not have basibranchial teeth, is the angler allowed to keep the trout? Again, this fish is likely a cuttbow. (Brian P., Sacramento)

Answer: According to CDFW Environmental Program Manager Roger Bloom, it is true that rainbow trout/coastal cutthroat hybrids exist at some low level in sympatric populations. However, based on a recent scientific study, the practice of using phenotypic traits to distinguish hybrids is not very effective. Although the presence of basibranchial teeth are a strong indication of a cutthroat trout lineage, it should not be used exclusively as a definitive sign to retain/harvest a fish.

From a regulatory/enforcement perspective, field identification of coastal cutthroats should be based on commonly agreed upon morphology of red/orange slashes found under the jaw. If there is a question about a fish being a hybrid coastal cutthroat crossed with a rainbow trout, anglers should err on the side of caution. It must have observable red/orange slashes if the trout is to be considered a coastal cutthroat for harvest.

Interestingly, some Central Valley hatchery steelhead may exhibit orange/yellow slashes which could stem from genetic influences via ancestral redband trout. Hence, if an angler encounters an adipose-clipped fish that looks like a rainbow trout but has these characteristics, it can be retained/harvested as there are currently no hatchery coastal cutthroats with clipped adipose fins.