Rainbow Trout are a popular sportfish in the United States. The species has been stocked in lakes and streams outside of its native range since the late 1800s. Interestingly, in many areas where stocking has stopped, Rainbow Trout populations continue to thrive. This is because stocked Rainbow Trout have been able to survive and reproduce in their introduced ranges (Muhfeld et al., 2017). Over time, these stocked individuals and their offspring have immigrated out from the stocked locations and colonized new habitats. Rainbow Trout populations are now established across much of the United States, and the species is considered invasive in many areas.
Rainbow Trout populations are negatively impacting native trout species in these systems. This is because Rainbow Trout can hybridize with other closely related trout species. Hybridization occurs when two separate species reproduce. Hybridization between a native and invasive species can be dangerous. Native species are usually adapted to their habitat. However, invasive species often do not have the same set of adaptations. Thus, when a native and non-native species hybridize, some of the beneficial traits of the native species are lost or diluted by traits from the invasive species.
A team of researchers and USGS scientists have been studying this phenomenon in trout populations in the Northern Rocky Mountains since the 1980s (Muhfeld et al., 2017). Specifically, the team is interested in “Cutbow” Trout, which are produced when native Cutthroat Trout and invasive Rainbow Trout reproduce (Figure 1). Cutbow Trout are generally fertile, which means they can produce offspring of their own. This is an issue because as hybrids reproduce, the gene pool continues to get polluted.
Figure 1. Hybridization between native Cutthroat Trout and invasive Rainbow Trout produces “Cutbow” Trout.
Scientists expect climate change to favor Rainbow Trout, which will result in more hybridization between the invasive and native trout species (Bear et al., 2007; Muhfeld et al., 2014; Muhfeld et al., 2017). All species have a range of temperatures that they can live comfortably in (Randall et al., 2001). This range varies within and between species. At temperatures above or below this range, individuals experience decreased performance. Climate change results in warmer temperatures, which Rainbow Trout can tolerate better than Cutthroat Trout (Bear et al., 2007). In addition, Rainbow Trout have a higher capacity for growth at higher temperatures than native Cutthroat Trout (Bear et al., 2007). As waters continue to warm, Rainbow Trout are expected to continue to spread into new areas. Rainbow Trout will encounter more native trout populations in these areas, which will result in more hybrid trout populations (and continued decline of native trout populations).
Fortunately, invasive Rainbow Trout populations are being managed in several ways (Muhfeld et al., 2017). For example, barriers are often installed so that Rainbow Trout cannot reach uninvaded areas. Additional management strategies include habitat restoration and protection and fishing efforts targeting the invasive Rainbow Trout. These efforts will be critical for biodiversity conservation, especially in the face of climate change (Muhfeld et al., 2017).
Check out the following links for more information:
- USGS News Release: https://www.usgs.gov/news/hybridization-between-native-and-invasive-trout-increasing-west
- NPR Podcast: http://www.npr.org/2014/05/27/316316952/hybrid-trout-threaten-montanas-native-cutthroats
Figure created with images attributed to the National Park Service (http://www.yellowstone.co/fish.htm#wscut)
Bear EA, McMahon TE (2007) Comparative thermal requirements of westslope cutthroat trout and rainbow trout: implications for species interactions and development of thermal protection standards. Trans Am Fish Soc 136: 1113 – 1121.
Muhfeld CC, Kovach RP, Al-Chokhachy R, Amish SJ, Kershner JL, Leary RF, Lowe WH, Luikart G, Matson P, Schmetterling DA, Shepard BB, Westley PAH, Whited D, Whiteley A, Allendorf FW (2017) Legacy introductions and climatic variation explain spatiotemporal patterns of invasive hybridization in a native trout. Glob Change Biol.
Muhfeld CC, Kovach RP, Jones LA, Al-Chokhachy R, Boyer MC, Leary RF, Lowe WH, Luikart G, Allendorf FW (2014) Invasive hybridization in a threatened species is accelerated by climate change. Nat Clim Change 4: 620 – 624.
Randall D, Burggren W, French K (2001) Eckert Animal Physiology: Mechanisms and Adaptations. W. H. Freeman and Company, New York.