Biological Survey led explorations into the Mexican mainland (1898) and Baja California peninsula (1905) to document flora and fauna.
[10] Between 1997 and 2005, members of the group explored the rivers and streams of the Sierra Madre Occidental collecting specimens for study and documenting the diverse populations of Mexican native trout.
He described a number of local forms of the Mexican rainbow trout primarily based on the river systems they occurred in.
The authors, in discussing their findings, state:[12] Here, we again confirm the significant genetic diversity present in trout populations inhabiting northwestern Mexico.
Genetic similarity of the southernmost SMO populations with O. mykiss could be the result of a third, more recent colonization event by steelhead from California or further north, or could be due to introgression by or naturalization of imported hatchery rainbow trout.
[2] The endemic range of Mexican native trout extends from near the U.S.–Mexican border in western Chihuahua and eastern Sonora south through the Sierra Madre Occidental mountains of Durango.
During Edward W. Nelson's 1898 expedition, he observed trout in the Rio del Presidio basin near the town of El Salto, but did not collect any specimens.
In 1946, Ralph G. Miller an American researching Mexican ichthyofauna near El Salto collected the earliest surviving specimens of the Rio del Presidio trout, which now reside in the Smithsonian Institution.
[3] As of 2002, according to ichthyologist Robert J. Behnke, the Rio del Presidio trout were the southernmost natural distribution of any member of the family Salmonidae.
[29] Truchas Mexicanas reported in 2006 that all the populations of Mexican trout face threats from habitat loss due to logging, mining and aquaculture impacts.