Native Trout South of the Border?

July 2022

What if I told you there are multiple species of native trout found in North America as far south as the tip of the Baja California peninsula? That's nearly to the Tropic of Cancer! Are these the rarest trout? Let's take a look.

While it's a common assumption that trout native to North America span from northern New Mexico up into Canada and Alaska, there are isolated pockets of native trout found in Mexico's Sierra Madre Occidental (Penaluna et al., 2016). Though not quite rainbow trout, these native species are closely related. Within the family of rainbow trout, the coastal rainbow lineage of California now branches out into Mexico where recent genetic studies have identified at least 10 new, previously undescribed species that call the Sierra Madre Occidental home (Hendrickson, 2020-1). Due to remote and rugged terrain, progress has been slow in uncovering their secrets. We all know trout require cool, clean streams to survive, so available habitat is limited for these fish within the Sierra Madres to elevations over 6,300ft (Hendrickson, 2020-2). According to the International Union for the Conservation of Nature (IUCN) Red List, what populations remain of these Mexican trout are either threatened or endangered.

Unique Geography

How is it possible for native cold-water fish to survive so far south in North America? The Sierra Madre Occidental mountains that rise above the warmer lands below (some peaks reach 10,000ft), create cooler suitable islands of potential habitat. To the west of these mountains, easterly winds deliver warm, moist air from the Gulf of California up these high elevation mountains, forcing the wet air upwards which leads to condensation, clouds and precipitation. This effect, termed orographic lift and orographic precipitation, provides the required cool and clean water that support trout and their habitat.

On the rainy, windward, side of the mountains, yearly rainfall averages up to 80" in some places compared to only 40" of rain on the drier, leeward side (Bruman, 2022). However, this mountain range is still set in an arid landscape, subject to flooding and drought (Behnke, 2002).

Isolation and Evolution

How is it that so many different species exist in the Sierra Madre Occidental? The likely cause for so many different native fish within this small geographic range stems from the gradual isolation between populations over time. One possibility of isolating events may be volcanic activity like lava flows, or landslides, which reshape river morphology over millennia. Another hypothesis suggests that stream habitats warmed, and forced trout closer to headwater streams to find cooler water during the period of receding glaciers following the Pleistocene Ice Age (Behnke, 2002). Once connected habitats become separated and fish that may have once been able to swim between drainages were isolated. Either way, given enough time, genetic differences between drainages started to arise. Without the mixing of genetic material between the different drainages to maintain one large population, each isolated population evolved with distinct genes and characteristics over time. As warming has continued, populations have been forced to even higher elevations, further disconnecting populations from each other, and fueling genetic variation. However, all of these trout have one thing in common—brilliant coloration.

Current and Future Threats

As we discussed in our Western Trout Rivers: An Angler’s Prospectus article, trends for trout in western North America (including the Sierra Madres) are trending toward warmer and drier conditions. Though this special geography allows native trout to survive in the Sierra Madre Occidental may continuously provide precipitation, disturbances in hydrological timing and warming stream temperatures could continue to force fish upstream towards cooler water. But how long can these native fish keep chasing cool water before they run out of upstream real estate? Barring some serious and unforeseen plate tectonic movement, these mountains have a limited elevation. Living on these isolated geographic islands of high elevation, they are prone to running out of livable habitat.

Another concern regarding isolated populations is the risk of genetic drift through bottlenecking. This occurs when a population dwindles to a small population and it can become inbred, potentially concentrating negative mutations that affect their ability to survive. This is usually not a problem in larger populations or where dispersal and immigration between populations are possible, but can hinder the conservation of species with a small number of remaining individuals. Isolated populations can also become overly specialized in a specific ecological niche that is susceptible to environmental changes, including climate change, making the population vulnerable to even small changes in their habitat.

Luckily, conservation efforts have turned a focus on these trout and are working to save them. Though these trout face unique obstacles, they also face a host of other threats familiar to native trout across North America: logging, mining, industrial agriculture, human-derived erosion, ground and surface water depletion, invasive species (like whirling disease), and rainbow trout hybridization (De Lourdes Lozano-Vilano & Contrere-Baldera, 2007). It's an uphill battle, but we hope it's not too large of a lift for those working to conserve these beautiful and remarkable Mexican trout.

Sources:

1. Penaluna, B.E. et al. 2016. Conservation of Native Pacific Trout Diversity in Western North America, Fisheries, 41:6, 286-300. https://www.fs.fed.us/pnw/lwm/aem/docs/penaluna/2016_penaluna_etal_fisheries_pacifictrout.pdf
2. Hendrickson, D. 2020-1. Treasures of the Sierra Madre - Mexico's little known native trout diversity. University of Texas at Austin. https://repositories.lib.utexas.edu/bitstream/handle/2152/81747/Hendrickson_2020_Treasures_Sierra_Madre_native_trout.pdf?sequence=3&isAllowed=y
3. Perez, J. A. 2012. Las Truchas Mexicanas.https://www.dondepescar.com/dondepescar/1515-truchas_endemicas_de_mexico
4. Hendrickson, D. 2020-2. Trout of Mexico's Sierra Madre. University of Texas at Austin: Biodiversity Center. https://biodiversity.utexas.edu/news/entry/trout-of-mexico
5. Bruman, H. 2022. Sierra Madre mountain system, Mexico. Britannica. https://www.britannica.com/place/Mexico
6. Behnke, R. 2002. Trout and Salmon of North America. The Free Press. New York.
7. Tomelleri, J. 2020. Illustrations of Mexican trout. Treasures of the Sierra Madre - Mexico's little known native trout diversity.
8. De Lourdes Lozano-Vilano, M. & Contrera-Baldera, A. J. 2007. Studies of North American Desert Fishes in Honor of E. P. (Phil) Pister, Conservationist. Universidad Autonoma de Nuevo Leon. http://w3.biosci.utexas.edu/tnhc/fish/research/truchas_mexicanas/pub/reports/Hendrickson_etal_2007_conservation_Mexican_trout.pdf
9. Hendrickson, D. 2022. Trout Conservation. Hendrickson Lab. https://sites.cns.utexas.edu/hendricksonlab/conservation