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I I I I I I I I I I I I I I I I I I I thought that an increase in sballmouth bass abundance will severely worsen the adverse I effects that nonnatives alre~y have on the lower Yampa's distressed native fauna. Concern for susceptible humpback and Colorado pikeminnow to small mouth bass predation mounted at the RI!P's nonnative fish control workshop in 2003 and small mouth bass were recognized to po~e the greatest threat to endangered and native fishes in the lower Yampa River. Therefore, the Vernal Field Office proposed an incidental take of I channel catfish and a primary focus of control on smallmouth in Yampa Canyon. , Identification of Desert and Sonora Sucker Larvae and Early Juveniles. ! Snyder, Darrel E., Kevin R.IBestgen, Sean. C. Seal, and C. Lynn Bjork (illustrator). Larval Fish Laboratory, Department of Fishery and Wildl((e Biology. Colorado State University, Fort Collins, cq i Desert sucker (Catostomus ~larJd, subgenus Pantosteus) and Sonora sucker (Catostomlls insignis, subgeneus Catostotnus) are common native fishes of the Gila River Basin in southern Arizona and south~estern New Mexico. Appearance and development are very similar to that of blue head s~cker (c. discobollls) and tlannelmouth sucker (c. latipinnis), respectively, in upstream pqrtions of the Colorado River Basin. Hatching at 8-10 mm SL, completing yolk absorption by 12-14 mm SL and becoming juveniles by 23-24 mm SL, desert sucker are gener~lly 1-2 mm smaller than Sonora sucker at comparable states of development. However, gut loop formation proceeds much more rapidly in desert sucker with prominent cros~-wise folds developing by or shortly after transition to the metalarval phase whereas i~ Sonora sucker such folds don't appear until well after transition to the juvenile period. Desert sucker proto larvae and flexion meso larvae are characterized by broadly an~ evenly scattered melanophore pigmentation over the dorsal surface, gradually extending onto lateral surfaces of the body, and a highly variable but usually very extensive line dr band of melanophores on the ventral midline between heart i and vent. In contrast, dorsal surface pigmentation in Sonora sucker is limited to a line or band of grouped, obliquely ~ligned, melanophores parallel to each side of the midline, and ventral midline pigmen(ation is usually absent or sparse. Desert sucker metalarvae and juveniles have 8-12 principal dorsal-fin rays, distinct notches separating upper and lower lips at the corners of tpe mouth, broadly connected lower-lip lobes, a well-folded gut, and a dark peritoneum. r Sonora sucker metalarvae and early juveniles have 10-12 principal dorsal-fin rays, Iip~ continuous at the corners of the mouth, deeply divided lower-lip lobes, a simple s-s~aped gut until well after transition to the juvenile period, and little if any ventro-later~l to ventral peritoneal pigmentation. 25