Cuttbow

What a Cuttbow Is

A cuttbow is a hybrid between rainbow trout (Oncorhynchus mykiss) and cutthroat trout (Oncorhynchus clarkii). The two are congeners with compatible chromosome numbers, and unlike most fish hybrids their offspring are fertile. That single fact is what makes the cuttbow biologically consequential rather than a curiosity: a fertile hybrid can backcross into either parent population, and repeated backcrossing produces introgression — the movement of genes from one species into the genome of the other — and eventually a hybrid swarm, a population in which no genetically pure individuals remain.

The visible fish anglers call a cuttbow is usually not a first-generation cross. F1 hybrids are a small fraction of what is caught in most introgressed populations; the majority are later-generation backcrosses with varying proportions of each parent’s ancestry. This matters because it means “cuttbow” describes a genetic continuum, not a category with edges.

No Native Range

The cuttbow has no natural native range. Neither parent is exotic to western North America, but the hybrid as a widespread entity is a consequence of fish stocking, not of natural biogeography. Rainbow trout were native to the Pacific slope and to interior basins west of the Continental Divide; the interior cutthroat subspecies — Yellowstone, westslope, Colorado River, greenback, Lahontan, Rio Grande, Bonneville — occupied basins that rainbow trout largely did not reach. The two lineages were separated by drainage divides for most of their evolutionary history.

Rainbow trout have been stocked across nearly every state since the late 1800s, and the U.S. Geological Survey’s nonindigenous species account for the species records directly that they “hybridize with other, more rare trout species, thereby affecting their genetic integrity,” naming Lahontan cutthroat, golden trout, Gila trout and Apache trout among the natives affected (USGS Nonindigenous Aquatic Species). The contact that produces cuttbows was created by hatchery trucks.

Where the parents do genuinely co-occur naturally — coastal cutthroat trout and coastal rainbow trout and steelhead share hundreds of Pacific coastal streams — hybridisation happens but does not typically dissolve the two species into one. Genetic work on naturally sympatric anadromous rainbow trout and coastal cutthroat found F1s, backcrosses to both parents and later-generation hybrids present, yet the parental forms persist as distinguishable entities (Young et al. 2001). WDFW describes coastal cutthroat and rainbow trout as sharing habitat across western Washington while remaining separately identifiable. Reproductive timing and spawning-habitat separation appear to do much of that work: where cutthroat and rainbow spawn at different times or in different reaches, hybridisation stays rare, and where stocking collapses that separation it does not (DeRito et al. 2010).

Where They Are Common, and Why

Cuttbows are most abundant where three conditions coincide: a native interior cutthroat population, a long history of rainbow trout stocking, and a stream network warm enough at its lower end to favour rainbow ancestry. That describes much of the interior West — the upper Colorado and its tributaries, the Yellowstone and its Idaho and Montana headwaters, the Snake River system, the upper Missouri, and many Great Basin drainages. In practice, mainstem and lower-tributary reaches carry the highest rainbow ancestry, and the proportion of cutthroat ancestry rises with elevation and distance upstream.

Both parents are spring spawners, which is the precondition for the whole problem. Cutthroat and rainbow trout build redds in the same size of gravel, in the same kind of riffle tailout, on rising and then falling spring flows. Where they historically overlapped, differences in timing and in the tributaries chosen kept the two largely separate. Introduced rainbow trout, and particularly hatchery strains selected for early maturation, frequently spawn in the same window and the same water as the native fish, and the isolating mechanism disappears. Once F1 hybrids exist and are fertile, their spawning timing is intermediate, which bridges any remaining gap and accelerates the collapse of separation in later generations.

Identification Is Unreliable

Field identification of cuttbows does not work well, and this is the single most important practical fact about them. The classic characters — the red or orange slash under the lower jaw, basibranchial teeth at the back of the tongue, spotting pattern, jaw length relative to the eye — are all inherited independently and reassort in hybrids. A backcross that is 90 percent cutthroat may show a clean rainbow-like spotting pattern; a fish with a vivid slash may be substantially rainbow.

The best direct test of this compared visual identification against genotyping at seven nuclear DNA loci for 323 fish from an Idaho stream where Yellowstone cutthroat trout were introgressed with rainbow trout. Phenotype correctly identified 94 percent of pure Yellowstone cutthroat trout, 79 percent of hybrids, and only 71 percent of rainbow trout, with every misidentification occurring between pure and hybrid categories (Meyer et al. 2017). In other words, roughly one hybrid in five is misread by trained field crews using the standard characters — and hybrids are the category that matters most for management.

The implication runs both directions. Angler reports of “cutthroat” in introgressed drainages overstate purity, and removal programmes that rely on appearance leave hybrids in the population while killing pure fish. Where the answer matters, it requires genetic screening.

What Hybridisation Does

Hybridisation is not neutral for the native parent. Introgression has been shown to reduce reproductive success in westslope cutthroat trout, and controlled comparison of fitness-related traits found that rainbow trout admixture correlated with increased embryonic survival and ova energy but decreased juvenile weight and burst swimming endurance (Drinan et al. 2015). Hybrids also differ morphologically and in swimming stamina from both parents (Seiler and Keeley 2007), and chromosome rearrangements between the parent species suppress recombination in hybrids, which shapes how ancestry blocks move through a population (Ostberg et al. 2013).

Spread is not slow. In one well-documented invasion, rainbow trout admixture declined with upstream distance from a hybrid swarm site that was 92 percent rainbow in genetic contribution, tracing an invasion front (Boyer et al. 2008), and repeat sampling found new rainbow trout introgression in seven of eleven populations that had been non-hybridised in 1984 (Hitt et al. 2003). Long-term monitoring in the Stehekin River has tracked the same process in Washington (Ostberg et al. 2012). In Colorado, hybridisation dynamics between native cutthroat and introduced rainbow trout have complicated even the basic question of which native lineages survive where (Metcalf et al. 2008).

Temperature Is the Control Variable

The clearest environmental predictor of where hybridisation spreads is water temperature. Analysis of local habitat, watershed and biotic factors found hybridisation between native westslope cutthroat trout and introduced rainbow trout to be positively associated with mean summer water temperature and with the number of upstream road crossings, and negatively associated with distance from the source population (Muhlfeld et al. 2009).

The mechanism is a thermal mismatch between the parents. Rainbow trout have an ultimate upper incipient lethal temperature of 24.3 °C — about 76 °F — against 19.6 °C, about 67 °F, for westslope cutthroat trout, a gap of roughly 4.7 °C, and rainbow trout hold a distinct survival advantage above 20 °C. Optimum growth temperatures are nearly identical at 13.1 °C for rainbow and 13.6 °C for westslope cutthroat (Bear et al. 2007). Cold headwater reaches therefore act as refuges where cutthroat hold their ground; as streams warm, the rainbow-derived genome gains a survival edge and the hybrid zone moves upstream. Hybrids themselves are generally intermediate, which places their thermal tolerance somewhere between the two parents and makes ancestry proportion a rough predictor of where an individual can persist.

This makes live stream temperature an unusually direct readout of hybridisation risk. A drainage whose summer maxima are climbing through the mid-60s °F is a drainage whose upper limit for pure cutthroat is retreating.

Cuttbows as an Angling Fish

Cuttbows are common enough in the interior West that in many drainages they are the default fish rather than the exception, and in heavily introgressed systems most of what is caught and called cutthroat is not genetically pure. They are frequently described as combining the cutthroat’s willingness to take a surface fly with the rainbow’s speed and jumping, which is a fair generalisation about intermediate phenotypes even if it is not a rule.

Their status in management is genuinely divided, and the site should not pretend otherwise. In waters where a native cutthroat subspecies is the conservation target, hybrids are a problem to be removed or contained. In waters where the native population is already a hybrid swarm and restoration is not feasible, the same fish are managed as a fishery. The relevant distinction is not what the fish looks like but which drainage it came from and what remains genetically intact there — and, given that phenotype misidentifies roughly a fifth of hybrids, appearance is a poor guide to which situation applies.