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developed large populations that appeared to outnumber and outcompete the indigenous kinds. Among these problems, hybridization presents the greatest danger to species integrity and survival. By the 1970s, genetic contamination had reduced Apache trout to less than 5.0 % of its former range. The Gila trout similarly survives in natural populations in only a few headwater streams. Only inaccessibility to stocking and natural barriers to subsequent upstream movement stopped invasions of non-native trouts and prevented complete loss of these two special natives. Both are federally listed, as threatened and endangered, respectively. Figure 11. Juvenile trout (no common name), 62 mm in total length, also an undescribed species from the Rio Mayo drainage (Arroyo de la Casita), Chihuahua. All of the native Mexican trouts are considered of "special concern" by an international committee of the American Fisheries Society. All these trouts have similar ecologies. Large individuals live in pools, while smaller ones remain near obstructions or other cover such as overhanging trees or brush in runs and riffles. All spawn shortly after snowmelt, and young hatch and grow rapidly in the warming waters of early summer. Variations in color patterns are pronounced, even within single populations, some of which may reflect hybridization with rainbow and cutthroat trouts in both the Colorado and Yaqui systems. In addition to the dangers of introduced relatives that hybridize with them, and the invasions (or stocking) of predatory and competitive species, Figure 12. Apache trout, 21.0 cm in total length, East Fork of the White River, Arizona. This threatened species generally remains small (less than 20 cm) in smaller streams, but attains 45 cm in artificial lakes. a ~~. ~.;--~~~~ ,~ ......: t-...........~.. .... - ~...,,,.. .. ~~ ...... - -.. .. .-lll'" - ~ ,. ~ "- ......... . ... .., ---... :... - iIlI III a III 8