Snake River Fine-Spotted Cutthroat Trout

The Snake River fine-spotted cutthroat trout is the most taxonomically contested fish in the cutthroat group. It occupies a native range of roughly 100 river miles, it is visually distinctive to the point of being unmistakable, it shares that range with a large-spotted cutthroat it does not appear to blend into, and the genetic evidence gathered so far has not shown it to be distinct from that neighbor. All four of those statements are true at once, which is why the question remains open.

Range

The USGS Nonindigenous Aquatic Species database defines the native range precisely: the upper Snake River, and tributary streams downstream of and including the Gros Ventre River, between Palisades Reservoir and Jackson Lake, in Wyoming and Idaho. Wyoming Game and Fish maps the fish it calls the Snake River cutthroat in the Hoback, Salt, Greys, Gros Ventre and Snake drainages. That is a small native range by any measure — a single river reach in Jackson Hole and the canyon below it, plus its tributaries.

The stocked range is far larger. USGS records nonindigenous occurrences in Colorado (15 hydrologic units, 1976-2009), Idaho (19 units, 1982-2011), Maryland (2 units, 1981-1998), New Mexico (5 units, 1982-1990) and Wyoming outside the native reach. The fine-spotted fish became a hatchery workhorse because it grows well, tolerates hatchery conditions and produces a visually striking trout, and “Snake River cutthroat” in a stocking record from Colorado or New Mexico usually means this strain.

Identification

The spotting is the whole identification. Wyoming Game and Fish describes “numerous small, relatively evenly-sized dark, black spots covering the entire body except the belly”, over a body that runs dark-olive to brown dorsally with yellowish-brown to silver or brassy bronze flanks. The contrast with the Yellowstone cutthroat sharing the same drainage is immediate: Yellowstone fish carry larger, sparser spots concentrated toward the tail. Placed side by side, the two are easy to tell apart. That obvious visual difference, holding across a shared range where the two ought to blend, is the core of the argument that the fine-spotted form is a real biological entity.

Is It a Valid Taxon?

This is the question that matters, and it is genuinely unresolved. The positions are as follows.

• Formally described and currently valid in ITIS. ITIS carries Oncorhynchus clarkii behnkei Montgomery, 1995 (TSN 914090) as a valid subspecies with a credibility rating of “verified — standards met”, parent taxon Oncorhynchus clarkii, and cites Behnke’s Trout and Salmon of North America (2002) as a key reference. The name honors Robert Behnke, who studied the form extensively without ever formally naming it himself.
• USGS treats it as a distinct subspecies for distribution accounting, maintaining a separate fact sheet with its own native and introduced range.
• USFWS does not. The Service’s cutthroat trout profile lists “Snake River fine-spotted cutthroat trout” as an alternate common name for Oncorhynchus clarkii itself, and there is no separate FWS species profile for behnkei. In effect the Service recognizes the name without recognizing the taxon.
• Wyoming Game and Fish treats it as a management unit but not a named subspecies. The Cutt-Slam program requires anglers to catch four native Wyoming cutthroat including the Snake River fish, yet the department’s Fish of Wyoming list carries a single entry for cutthroat trout, Oncorhynchus clarkii, with no subspecies breakdown — and the state’s record book likewise keeps one undifferentiated cutthroat record.
• The genetic evidence has not supported distinctness. Loxterman and Keeley (2012), BMC Evolutionary Biology 12:38, sampled across nearly the entire cutthroat range and reported that “fish representing fine-spotted Snake River cutthroat trout had the same haplotype of other Yellowstone cutthroat trout.” At the mitochondrial markers examined, the fine-spotted fish was not separable from the Yellowstone cutthroat; both fell within a single intermixed clade that also contained Bonneville cutthroat.

Mitochondrial DNA is maternally inherited and represents one locus, so a shared haplotype does not by itself prove the two forms are one population — recent divergence, or divergence driven by nuclear loci under selection, could produce exactly this pattern. But it is the published evidence, and it points against subspecies status. The honest summary is that the Snake River fine-spotted cutthroat is a recognized, named and managed entity whose distinctness from the Yellowstone cutthroat has not been demonstrated genetically, and readers should not treat its subspecies status as settled.

The American Fisheries Society’s 8th edition names list (2023) reorganized cutthroat trout into multiple species. Whether that list retains the fine-spotted form at any rank could not be verified for this article and is not stated here.

Conservation Status

The fine-spotted form is not listed under the Endangered Species Act. The ITIS record notes it was identified as a “potential candidate without sufficient data on vulnerability” in the Federal Register of 15 November 1994 (volume 59, number 219) — a designation that captures the position exactly, since a taxon whose validity is disputed cannot easily be assessed for listing.

The practical conservation concerns are the group’s standard set. Introduced rainbow trout hybridize with cutthroat throughout the Snake system, and regional management plans treat rainbow introgression into native cutthroat as a standing priority. Widespread hatchery stocking of the fine-spotted strain into other cutthroat waters, documented by USGS in Colorado, New Mexico and Idaho, creates the reverse problem: a native cutthroat lineage being introgressed by a different cutthroat lineage. And within its own range, the fine-spotted form interbreeds with Yellowstone cutthroat, an interaction whose significance is entirely contingent on whether the two are separate taxa in the first place.

Water Temperature

No thermal tolerance study specific to the fine-spotted form was verified for this article. The available benchmark is Bear, McMahon and Zale (2007), who measured an ultimate upper incipient lethal temperature of 19.6 C (67.3 F) for westslope cutthroat against 24.3 C (73.7 F) for rainbow trout, with growth optima near 13-14 C for both species and a recommendation to hold maximum daily temperatures near 13-15 C (55-59 F).

If the mitochondrial evidence is right and the fine-spotted fish is the Yellowstone cutthroat, no separate thermal profile should be expected. The upper Snake between Jackson Lake and Palisades is a cold, high-elevation, snowmelt-dominated river that spends most of the season well inside the cutthroat comfort band; the constraint is late-summer warming in the lower tributaries and in the canyon reaches below the dam, where releases and air temperature interact.

Life History

Fine-spotted cutthroat are spring spawners, moving into tributaries on the rising limb of snowmelt. Both fluvial fish — rearing in tributaries and maturing in the mainstem Snake — and resident tributary fish occur, and the mainstem fluvial population produces the largest individuals. Feeding is opportunistic and surface-oriented, dominated by drifting aquatic insects and by terrestrial insects blown or washed in from the sagebrush and cottonwood corridor along the river. The willingness to feed at the surface through a long season is the behavioral trait the Snake is known for, and it is a straightforward consequence of a cold, clear, insect-rich river with a short productive window.