The cutbow (Oncorhynchus sp. × mykiss) is an interspecific fertile hybrid between rainbow trout (Oncorhynchus mykiss) and cutthroat trout (Oncorhynchus sp.). Based on currently accepted taxonomy, four species-specific hybrid names are recognized for cutbow:Trotter, P., P. Bisson, B. Roper, L. Schultz, C. Ferraris, G.R. Smith and R.F. Stearley. 2018. A special workshop on the taxonomy and evolutionary biology of cutthroat trout. Pages 1–31 in Trotter P, Bisson P, Schultz L, Roper B (editors). Cutthroat Trout: Evolutionary Biology and Taxonomy. Special Publication 36, American Fisheries Society, Bethesda, Maryland. Coastal cutthroat trout × rainbow trout (Oncorhynchus clarkii × mykiss), or coastal cutbow Westslope cutthroat trout × rainbow trout (Oncorhynchus lewisi × mykiss), or westlope cutbow Lahontan cutthroat trout × rainbow trout (Oncorhynchus henshawii × mykiss), or Lahontan cutbow Rocky Mountain cutthroat trout × rainbow trout (Oncorhynchus virginalis × mykiss), or Rocky Mountain cutbow Of these four species-specific hybrids, only coastal and westslope cutbow have natural range overlap; Lahontan and Rocky Mountain cutbow are the result of rainbow trout stocking and invasion. Due to these introductions, many populations of cutthroat trout are at risk of genetic pollution. Significant management intervention at state and federal levels has occurred to preserve native populations of cutthroat trout. History Cutbow can occur naturally where the native ranges of both parent species overlap, such as between coastal rainbow trout (O. mykiss irideus) and coastal cutthroat trout (O. clarkii) and between Columbia River redband trout (O. mykiss gardineri) and westslope cutthroat trout (O. lewisi).Kozfkay, C. C., Campbell, M. R., Yundt, S. P., Peterson, M. P., & Powell, M. S. (2007). Incidence of Hybridization between Naturally Sympatric Westslope Cutthroat Trout and Rainbow Trout in the Middle Fork Salmon River Drainage, Idaho. Transactions of the American Fisheries Society, 136(3), 624–638. https://doi.org/10.1577/T06-085.1Donald E. Campton and Fred M. Utter. 1985. Natural Hybridization between Steelhead Trout (Salmo gairdneri) and Coastal Cutthroat Trout (Salmo clarki clarki) in Two Puget Sound Streams. Canadian Journal of Fisheries and Aquatic Sciences. 42(1): 110-119. https://doi.org/10.1139/f85-014 However, stocking of nonnative rainbow trout in watersheds that contained cutthroat trout throughout the 19th and 20th centuries increased the occurrence of cutbow in North America. Increased hybridization imperiled or extirpated many populations of cutthroat trout, and hybridization was recognized as problematic by early North American ichthyologists and fishery scientists. Description Like most fish hybrids, cutbow are difficult to identify based on external characteristics alone.Seiler, S. M., Gunnell, K., Ptacek, M. B., & Keeley, E. R. (2009). Morphological Patterns of Hybridization between Yellowstone Cutthroat Trout and Introduced Rainbow Trout in the South Fork of the Snake River Watershed, Idaho and Wyoming. North American Journal of Fisheries Management, 29(6), 1529–1539. https://doi.org/10.1577/M08-128.1 This is further complicated by phenotypic variation of cutthroat trout across their range.Kevin A. Meyer, Patrick Kennedy, Brett High & Matthew R. Campbell (2017) Distinguishing Yellowstone Cutthroat Trout, Rainbow Trout, and Hybrids by Use of Field-Based Phenotypic Characteristics, North American Journal of Fisheries Management, 37:2, 456-466Meyer, K. A., Hillyard, R. W., & Campbell, M. R. (2022). Detecting rainbow trout introgression in Bonneville cutthroat trout of the Bear River basin using field-based phenotypic characteristics. Western North American Naturalist, 82(1), 117-127. Subsequently, many anglers confuse cutbow with rainbow or cutthroat trout. Cutbow generally have a reddish or orange slash under the jaw like cutthroat trout, but the slash is usually fainter than would be expected in a pure cutthroat trout. In some populations such as Yellowstone cutthroat trout (Oncorhynchus virginalis bouvierii), a white leading edge on the anal or dorsal fin suggests hybridization with rainbow trout. In crosses between coastal cutthroat trout and rainbow trout, the presence of hyoid teeth and jaw slash intensity can be useful external traits to identify potential hybridization. Benjamen M. Kennedy, Jason Baumsteiger, William L. Gale, William R. Ardren & Kenneth G. Ostrand (2009) Morphological, Physiological, and Genetic Techniques for Improving Field Identification of Steelhead, Coastal Cutthroat Trout, and Hybrid Smolts, Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science, 1:1, 45-56, DOI: 10.1577/C08-006.1 Reproduction In a hatchery setting, cutbow are created when the female cutthroat trout's eggs are fertilized by a male rainbow trout.Rohrer, R.L. and Thorgaard, G.H. (1986), Evaluation of Two Hybrid Trout Strains in Henry's Lake, Idaho, and Comments on the Potential Use of Sterile Triploid Hybrids. North American Journal of Fisheries Management, 6: 367-371. https://doi.org/10.1577/1548-8659(1986)6<367:EOTHTS>2.0.CO;2 Spawning cutthroat trout may experience prolonged spatial and temporal overlap with spawning rainbow trout, thus increasing the likelihood for hybridization. Henderson, R., Kershner, J.L. and Toline, C.A. (2000), Timing and Location of Spawning by Nonnative Wild Rainbow Trout and Native Cutthroat Trout in the South Fork Snake River, Idaho, with Implications for Hybridization. North American Journal of Fisheries Management, 20: 584-596. https://doi.org/10.1577/1548-8675(2000)020<0584:TALOSB>2.3.CO;2 Management In many instances, wild (non-stocked) cutbow trout are considered a threat to the persistence of wild cutthroat trout and are managed similarly to invasive fishes. For example, some programs focus on targeted removal of cutbow.Gresswell, R. E. (2011). Biology, Status, and Management of the Yellowstone Cutthroat Trout. North American Journal of Fisheries Management, 31(5), 782–812. https://doi.org/10.1080/02755947.2011.608980 Lampton, J., Kovach, R., & Dunnigan, J. L. (2023). Efficacy of one-time rotenone application for Westslope Cutthroat Trout restoration in Boulder Creek, Montana. North American Journal of Fisheries Management, 43, 1081–1. https://doi.org/10.1002/nafm.10918 In other locations where introgression with wild cutthroat trout not an immediate threat, cutbow are stocked to establish a fishery.Parker, D., Avers, T. & Courtney, M. Weight, Length, and Growth in Cutbow Trout (Oncorhynchus mykiss x clarkii). Nat Prec (2011). https://doi.org/10.1038/npre.2011.6432.1 External links The following external links contain visual comparisons of rainbow trout, cutthroat trout, and their hybrids. Figure 2 showing comparison of Yellowstone cutthroat trout, rainbow trout, and their hybrid. Table 2 showing a comparison of multiple traits between Bonneville cutthroat trout, rainbow trout, and their hybrids. Utah Division of Wildlife Resources agency report containing images of Bonneville cutthroat trout and suspected hybrids from the Clear Creek Drainage. Manuscript figure showing a comparison of westlope cutthroat trout and their hybrids. Ardren, W.R., Bernall, S.R. Dams impact westslope cutthroat trout metapopulation structure and hybridization dynamics. Conserv Genet 18, 297–312 (2017). https://doi.org/10.1007/s10592-016-0906-6
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