- Science Name: Oncorhynchus gilae gilae
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- Environment: stream
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The Gila trout is a member of the salmon and trout family (Salmonidae). Gila trout was not formally described until 1950, using fish collected in Main Diamond Creek in 1939 (Miller 1950). It is most closely related to Apache trout (Oncorhynchus apache), which is endemic to the upper Salt and Little Colorado River drainages in east-central Arizona. Gila trout and Apache trout are more closely related to rainbow trout (O. mykiss) than to cutthroat trout (O. clarki), suggesting that Gila and Apache trouts were derived from an ancestral form that also gave rise to rainbow trout
The Gila trout is readily identified by its iridescent gold sides that blend to a darker shade of copper on the opercles (gill covers). Spots on the body are small and profuse, generally occurring above the lateral line and extending onto the head, dorsal (back, top) fin, and caudal (tail) fin. Spots are irregularly shaped on the sides and increase in size on the back. On the dorsal surface of the body, spots may be as large as the pupil of the fish eye and are rounded. A few scattered spots are sometimes present on the anal fin, and the adipose fin (fleshy fin located behind dorsal fin) is typically large and well- spotted. Dorsal, pelvic, and anal fins have a white to yellowish tip that may extend along the leading edge of the pelvic fins. A faint, salmon-pink band is present on adults, particularly during spawning season when the normally white belly may be streaked yellow or reddish orange. A yellow cutthroat mark is present on most mature specimens. Parr marks (diffuse splotches on the sides of body, usually seen on young trout) are commonly retained by adults, although they may be faint or absent (Miller 1950; David 1976).
Historic declines in Gila trout populations were associated with habitat degradation and competition and hybridization with exotic fish species, most notably rainbow and brown trout. Mining, logging, and cattle-grazing activities altered habitat through increased erosion and sedimentation as well as by causing changes in water levels, increased water temperatures, and reduced bank cover.
Reasons for listing the Gila trout as endangered included hybridization, competition, and/or predation by non-native rainbow, cutthroat, and brown trout, and habitat degradation.
- Overfishing was also problematic.
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- Forest fires (that now burn more severely that they did historically) and associated fire suppression activities also pose a threat to Gila trout populations.
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