76 UNIVERSITY OF COLORADO STUDIES 



Depth 3.7 to 4. 5, head 4 to 5 in the length; dorsal rays 10 to 12; anal rays 

 10 to 12; scales small, 20 or more between the base of the dorsal fin and the 

 lateral line; 120 or more cross-rows of scales. 



This genus includes both marine and fresh-water species. The Atlantic 

 Salmon, S. salar Linnaeus, the Sebago or Landlocked Salmon, S. sehago Girard, 

 and the Trout of the Rocky Mountain and Pacific regions are the North American 

 representatives. All of the species, particularly the fresh-water forms, are closely 

 related, yet many rather definite types are found native in independent rivers 

 or drainages. Some of these trout, as the Loch Leven Trout, S. fario levenensis 

 (Walker), have been shown to be merely well-defined races resulting from peculiar 

 local conditions, forms which when placed under slightly different conditions 

 return to the original type from which they were derived.' Other types appar- 

 ently maintain their identity although closely resembling other trout with which 

 they may be associated. As a result some ichthyologists believe the species of 

 trout to be few and the varieties numerous, while others hold each form to be a 

 distinct species. The fact of large importance is that differences do exist between 

 the trout of separate streams and drainages, and this should be recognized. The 

 trinomial nomenclature is used in this report since it points out both the rela- 

 tionships and the differences for the forms considered. 



Eleven types of trout are to be found in Colorado and excepting two species, 

 the Eastern Brook Trout, Salvelinus fontinalis (Mitchill), and the Mackinaw 

 Trout, Cristivomer namaycush (Walbaum), all are members of the genus Salmo. 

 Four of the forms belonging to this genus are native in Colorado, the Green- 

 backed Trout, S. clarkii stomias (Cope), the Rio Grande Trout, S. clarkii spilurus 

 (Cope), the Colorado River Trout, S. clarkii pleuriticus (Cope), and the YeUow 

 Finned Trout, S. clarkii macdonaldi (Jordan and Evermann), the last-mentioned 

 species occurring only in Twin Lakes. These four trout are known collectively 

 in Colorado as "Native Trout" or "Natives." In addition to the indigenous 

 trout, five other forms may be taken, the Landlocked Salmon, S. sehago Girard, 

 the Rainbow Trout, S. irideus shasta (Jordan) and S. rivularis Ayres, the Steel- 

 head Trout, introduced from various parts of North America; and the Brown 

 Trout, 5. Jario Linnaeus, and the Loch Leven Trout, S. fario levenensis (Walker), 

 introduced from Great Britain. 



Both native and introduced trout have been carried from one drainage to 

 another so that often several forms occur in the same stream. It is at once seen 

 that this condition makes possible both competition and hybridization between 

 species which would not be associated under natural conditions. In general the 

 introduced Eastern Brook Trout is the most successful form, although the demand 

 for this species and the resultant interest in its artificial propagation must be 

 considered in this connection. As regards the hybridization of the trout there 



■Jordan, Science, N.S., Vol. XXII, p. 714, 1905. 



