Native Trout in North America: Impacts of Multiple Stressors

Cutthroat Trout (Oncorhynchus clarkii spp.) were once widespread across western North America and consisted of 14 subspecies.1 However, populations have since been declining due to a variety of threats; leaving only 9 subspecies left in the wild, all of which are under state or Federal protection.2

Cutthroat Trout (Oncorhynchus clarkii)

One of the primary threats to Cutthroat Trout populations is the introduction of nonnative trout. Beginning in the 1800s, nonnative trout introductions became common practice for improving recreational sport fishing in the western United States.3 The ecological ramifications caused by these introductions were not recognized until much later. Brown Trout (Salmo trutta), a native of Europe, have been introduced extensively throughout the Cutthroat Trout’s historical range. Negative effects have been observed in streams with sympatric (i.e. occurring within the same geographical area) populations of Brown and Cutthroat Trout. Due to an overlap in diet and a more aggressive predatory nature, nonnative Brown Trout outcompete native Cutthroat Trout.4 This disadvantage in foraging for food has resulted in a reduction of growth rates and reproductive function.5 Inability to compete in the presence of nonnative fish has also contributed to an upstream shift in the distribution of native trout. In many mountain streams today, the headwaters (i.e. the upstream portion of a river nearest its original source) are occupied by native Cutthroat Trout and the lower portions are dominated by nonnative Brown Trout. This zoning pattern has become useful in conservation practices that implement an “isolation management” strategy, whereby physical barriers are installed to prevent the movement of nonnative trout upstream into the last remaining habitat of native populations.6 Isolating populations in headwater streams away from the negative effects of nonnative Brown Trout is vital in preserving native trout populations. Likewise, preventing contact with nonnative Rainbow Trout (Oncorhynchus mykiss) is also important because of the potential for hybridization and loss of genetic diversity in Cutthroat Trout populations.7

Another growing concern for native Cutthroat Trout populations in western North America is climate change. The amount of snow in the Rocky Mountains is expected to decrease in coming years and with warmer temperatures also forecasted, snowmelt will occur earlier in the springtime thereby altering hydrological patterns in streams and rivers.8 This is problematic for fish that depend on a continuous flow of water throughout the year. Drought has been shown to cause a decrease in the overall abundance and size of Cutthroat Trout populations.9 It is likely that these conditions are a result of a scarcity of food. Trout often feed on aquatic insects that are swept downstream in the moving waters. The number of insects drifting in the water is greatest when flow rates are high, thus under conditions of drought when less water is flowing through a stream the abundance of aquatic insects is rather low.10 Also, when the water level is lower, fish become more concentrated in narrow streams and the effects of competition increases which can have negative impacts on abundance and size.11 Additionally, in response to drought and increased temperatures, Cutthroat Trout have also been shown to move upstream in search of deeper pools and colder water where they can best endure the challenging conditions created by climate change.9 However, this is near impossible when native trout are already living in the upper limits of their range.

Combined effects of nonnative trout and climate change are predicted to extirpate (i.e. the extinction of a population at a local level) roughly 40% of the Colorado River Cutthroat Trout subspecies and leave another 40% vulnerable to extirpation.12 Loss at this scale would be devastating to the species as a whole. Appropriate management is needed to prevent this catastrophe from happening. New introductions of nonnative trout need to be prevented across all regions of North America. Fish barriers need to be repaired or improved to prevent any further migration of nonnative trout upstream. Where possible, nonnatives need to be removed and habitat restored for expanding the current range of native trout. Furthermore, protection of the genetic diversity of native trout is important for allowing populations to have the capacity for adapting to the growing threat of climate change.


  1. Behnke RJ (1988) Phylogeny and classification of Cutthroat Trout. In: Gresswell RE, eds. Status and management of interior stocks of Cutthroat Trout. American Fisheries Society, Symposium 4, Bethesda, Maryland, pp 1-7.
  2. Wilson WD, Turner TF (2009) Phylogenetic analysis of the Pacific Cutthroat Trout (Oncorhynchus clarkii : Salmonidae) based on partial mtDNA ND4 sequences: a closer look at the highly fragmented inland species. Molecular Phylogenetics and Evolution 52: 406-415.
  3. Pister EP (2001) Wilderness Fish Stocking: History and Perspective. Ecosystems 4: 279-286. doi: 10.1007/s10021-001-0010-7
  4. Meredith CS, Budy P, Thiede GP (2015) Predation on native sculpin by exotic brown trout exceeds that by native cutthroat trout within a mountain watershed (Logan, UT, USA). Ecology of Freshwater Fish 24: 133-147. doi: 10.1111/eff.12134
  5. Al-Chokhachy R, Sepulveda AJ (2019) Impacts of Nonnative Brown Trout on Yellowstone Cutthroat Trout in a Tributary Stream. North American Journal of Fisheries Management 39: 17-28. doi: 10.1002/nafm.10244
  6. Kirk MA, Rosswog AN, Ressel KN, Wissinger SA (2018) Evaluating the Trade-Offs between Invasion and Isolation for Native Brook Trout in Pennsylvania Streams. Transactions of the American Fisheries Society 147: 806-817. doi: 10.1002/tafs.10078
  7. McKelvey KS, Young MK, Wilcox TM, Bingham DM, Pilgrim KL, Schwartz MK (2016) Patterns of hybridization among Cutthroat Trout and Rainbow Trout in northern Rocky Mountain streams. Ecology and Evolution 6:688–706.
  8. Stewart IT, Cayan DR, Dettinger MD (2005) Changes toward earlier streamflow timing across western North America. Journal of Climate 18: 1136–1155.
  9. VerWey BJ, Kaylor MJ, Garcia TS, Warren DR (2018) Effects of Severe Drought on Summer Abundance, Growth, and Movement of Cutthroat Trout in a Western Oregon Headwater Stream. Northwestern Naturalist 99(3): 209-221.
  10. Harvey BC, Nakamoto RJ, White JL (2006) Reduced streamflow lowers dry-season growth of Rainbow Trout in a small stream. Transactions of the American Fisheries Society 135: 998–1005.
  11. Uthe P, Al-Chokhachy R, Shepard BB, Zale AV, Kersher JL (2019) Effects of Climate-Related Stream Factors on Patterns of Individual Summer Growth of Cutthroat Trout. Transactions of the American Fisheries Society 148: 21-34. doi: 10.1002/tafs.10106
  12. Roberts JJ, Fausch KD, Hooten MB, Peterson DP (2017) Nonnative Trout Invasions Combined with Climate Change Threaten Persistence of Isolated Cutthroat Trout Populations in the Southern Rocky Mountains. North American Journal of Fisheries Management 37: 314-325. doi: 10.1080/02755947.2016.1264507

*All images are the property of J.Evans

Save the Turtles – Salvar a las Tortugas


Sea Turtle Conservation and Temperature-dependent Sex Determination

In 2018, a group of marine biologists reported on an alarming trend they discovered in one of the largest populations of sea turtles in the world. They found that the northern Great Barrier Reef population of Green sea turtles (Chelonia mydas) has produced primarily females (99%) over the past two decades. Whereas the southern Great Barrier Reef population is experiencing a moderate female bias (67%) in their population. When overlapped with temperature data from their respective nesting beaches, warmer temperatures due to global climate change was ascribed as the leading cause of this extreme feminization in the northern population.1

Figure 1 Temperature-dependent Sex Determination (TSD) curve

Why does temperature matter in determining the sex of sea turtles? The sex of most vertebrates is determined by the genetic information of the gametes that join at fertilization. This strategy is referred to as Genetic Sex Determination (GSD), however sea turtles and several other groups of reptiles utilize an alternative strategy known as Temperature-dependent Sex Determination (TSD).2 In this case, the differentiation of gonads into male or female parts occurs during the thermosensitive period which is the middle third of embryonic development. The transitional range of temperature (TRT) is a narrow range of temperatures that produces both male and female offspring and the pivotal temperature (PT) is the temperature at which the sex ratio is 50/50.3 Therefore, if the average nesting temperatures during the thermosensitive period is above the PT, then there will be a bias towards the development of more females. It is also to be expected that average temperatures above the TRT will produce 100% females (see Figure 1).

Around the world, sea turtle conservation projects have been using hatcheries as a management tool for protecting eggs against predation and producing a higher number of hatchlings.4,5,6 When nests are laid below the high tide line or in areas of high predation risk, they are relocated to the hatchery. Simulated nests are dug by hand at the appropriate depth, depending on the species of sea turtle, in order to re-create the egg chamber environment. Doing so allows for researchers to insert temperature probes into each nest monitor temperature during embryonic development and estimate sex ratios. Also, natural predation and egg poaching are eliminated when the nests are in an enclosed and monitored area. In some hatcheries, shade-cloth coverings are used to reduce the amount of direct sunlight on nests and have been shown to improve hatching success along with cooling nesting temperatures in favor of males.6

There is a possibility that turtles will be able to respond naturally to climate change by altering nesting behaviors. Females opting to lay their eggs in cooler/shaded areas of the beach or shifting their nesting season to cooler/wetter periods are several ways turtles would be able to compensate for the rise in global temperatures. Likewise, differences in PT and TRT across populations may provide enough variation for turtles to adjust to a new climate. However, due to their long generation times it seems unlikely that turtles will be able to adapt rapidly to these changes.7 Feminization of entire populations is likely to occur in the near future.1 The use of turtle hatcheries as a management tool is vital for the conservation of sea turtle populations. The protection afforded to sea turtle eggs in a hatchery during crucial developmental periods not only produces a higher number of hatchlings entering the population but also helps to alleviate the gender bias between males and females.

Olive Ridley Hatchling (Lepidochelys olivacea)

Conservación de tortugas marinas y determinación del sexo dependiente de la temperature

En 2018, un grupo de biólogos marinos informaron sobre una tendencia alarmante que descubrieron en una de las poblaciones más grandes de tortugas marinas en el mundo. Encontraron que la población del norte de la gran barrera de coral de las tortugas marinas verdes (Chelonia mydas) ha producido principalmente hembras (99%) durante las últimas dos décadas. Mientras que la población del sur de la gran barrera de coral está experimentando un sesgo femenino moderado (67%) en su población. Cuando se superponen con los datos de temperatura de sus respectivas playas de anidación, las temperaturas más cálidas debido al cambio climático global se atribuía como la principal causa de esta feminización extrema en la población del norte.1

¿Por qué la temperatura es importante para determinar el sexo de las tortugas marinas? El sexo de la mayoría de los vertebrados está determinado por la información genética de los gametos que se unen a la fertilización. Esta estrategia se conoce como determinación genética del sexo (GSD), sin embargo, las tortugas marinas y varios otros grupos de reptiles utilizan una estrategia alternativa conocida como determinación del sexo dependiente de la temperatura (TSD).2 en este caso, la diferenciación de las gónadas en partes masculinas o femeninas se produce durante el período termosensible que es el tercio medio de desarrollo embrionario. El rango de temperatura de transición (TRT) es un rango estrecho de temperaturas que produce la descendencia masculina y femenina y la temperatura pivotante (PT) es la temperatura a la que la proporción de sexo es de 50/50.3 Por lo tanto, si las temperaturas medias de anidación durante el período termosensible está por encima del PT, entonces habrá un sesgo hacia el desarrollo de más hembras. También es de esperar que las temperaturas medias por encima de la TRT producirá 100% hembras (ver figura 1).

En todo el mundo, los proyectos de conservación de tortugas marinas han estado utilizando criaderos como una herramienta de gestión para proteger los huevos contra la depredación y producir un mayor número de crías.4, 5, 6 Cuando los nidos se colocan por debajo de la línea de marea alta o en áreas de alto riesgo de depredación, se trasladan al criadero. Los nidos simulados se cavan a mano a la profundidad apropiada, dependiendo de las especies de tortugas marinas, con el fin de recrear el ambiente de la cámara de huevo. Esto permite a los investigadores insertar sondas de temperatura en cada nido temperatura del monitor durante el desarrollo embrionario y estimar relaciones sexuales. Además, la depredación natural y la caza furtiva de huevo se eliminan cuando los nidos están en una zona cerrada y vigiladas. En algunos criaderos, los revestimientos de telas de sombra se utilizan para reducir la cantidad de luz solar directa en los nidos y se ha demostrado que mejoran el éxito de la eclosión junto con la refrigeración de las temperaturas de anidación en favor de los machos.6

Existe la posibilidad de que las tortugas puedan responder de forma natural al cambio climático alterando los comportamientos de anidación. Las hembras optan por poner sus huevos en las zonas más frías/sombreadas de la playa o cambiar su temporada de anidación a períodos más fríos/húmedos son varias formas en que las tortugas podrían compensar el aumento de las temperaturas globales. Del mismo modo, las diferencias en PT y TRT a través de las poblaciones pueden proporcionar suficiente variación para que las tortugas se adapten a un nuevo clima. Sin embargo, debido a sus largos tiempos de generación, parece improbable que las tortugas puedan adaptarse rápidamente a estos cambios.7 es probable que la feminización de poblaciones enteras ocurra en un futuro próximo.1 El uso de criaderos de tortugas como herramienta de gestión es vital para el conservación de las poblaciones de tortugas marinas. La protección otorgada a los huevos de tortuga de mar en una incubadora durante periodos cruciales de desarrollo no sólo produce un mayor número de crías que entran en la población, sino que también ayuda a aliviar el sesgo de género entre machos y hembras.

Leatherback Hatchlings (Dermochelys coriacea)


  1. Jensen MP, Allen CD, Eguchi T, Bell IP, LaCasella EL, Hilton WA, Hof CAM, Dutton PH (2018) Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World. Current Biology 28: 154-159.
  2. Janzen FJ (1994) Climate change and temperature-dependent sex determination in reptiles. Natl. Acad. Sci. 91: 7487-7490.
  3. Mrosovsky N, Pieau C (1991) Transitional range of temperature, pivotal temperatures and thermosensitive stages for sex determination in reptiles. Amphibia-Reptilia 12: 169-179.
  4. Vannini F, Sánchez AR, Martínez GE, López CS, Cruz E, Franco P, García HP (2011) Sea turtle protection by communities in the Coast of Oaxaca, Mexico. Cuadernos de Investigación UNED 3(2): 187-194.
  5. Mutalib AHA, Fadzly N (2015) Assessing hatchery management as a conservation tool for sea turtles: A case study in Setiu, Terengganu. Ocean & Coastal Management 113: 47-53.
  6. García–Grajales J, Hernando JFM, García JLA, Fuentes ER (2019) Incubation temperatures, sex ratio and hatching success of leatherback turtles (Dermochelys coriacea) in two protected hatcheries on the central Mexican coast of the Eastern Tropical Pacific Ocean. Animal Biodiversity and Conservation 42(1): 143-152.
  7. Binckley CA, Spotila JR (2015) Sex Determination and Hatchling Sex Ratios of the Leatherback Turtle. In: Spotila JR, Santidrian Tomillo P, eds. The Leatherback Turtle: Biology and Conservation. Johns Hopkins University Press, Baltimore, pp 84-93.

*All images and figures are the property of J. Evans