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<br />University), UANL (Universidad Autonpma de Nuevo Leon), and UMMZ (Uniwers.3ty
<br />of Michigan Museum of Zoology).
<br />I. Mimbres- River Drainage, New Mexico
<br />(A) Tributary to Mimbres River, ca. 300 to N of confluence of Bear Canyon
<br />(Fig. 1, ~I1), Grant Co., T16S, R11W, Sec. 28, 1840. m, 4 May 1979.- Thzs is
<br />the last; known locality for Gila nigrescens in the Mimbres River, as stated
<br />above. Since. we did not wish. to disturb the-adult population we confined our
<br />efforts to seining and dipnetting young-of--the-year fish in this spring-fed
<br />tributary. Study of this material revealed only Catos-tomus plebeius Baird and
<br />Girard and the exotic A osia chrysogaster. There is no evidence that G.
<br />nigrescens is reproducing successfully in the Mimbres River.
<br />II. Rio Casas Grandes
<br />(A) Rio Casas Grandes, ca. 21 km SW of Ascension (Fig. 1, ~2), 108°09'
<br />N long., 30°53' W lat., 1325 m, 5 May 1979.-Gila nigrescens was not found;
<br />we did collect. two exotics, Cyprinus carpio and Ictalurus melas, and eight adults
<br />of the native shiner, Notropis formosus. Gila nigrescens was last collected
<br />here in 1950 by R.R. Miller and party. Considerable change has taken place
<br />since then --the habitat is now badly polluted, the pools stagnant, with almost
<br />no flow on riffles, and choked by Spirogyra and other algae. About 10 years
<br />ago, Dr. W. L. Minckley, ASU, collected the Chihuahua chub ca. 4 km N of this
<br />station; however, the status of that population is presently unknown, although
<br />it seems unlikely that it still persists downstream from the present station. ~
<br />(B) Rio Casas Grandes just E of Casas Grandes (Fig. 1, !13), 107°56' N long.,
<br />30°23' W lat., 1470 m, 4 June 1979 -The Chihuahua chub was not present here.
<br />The habitat consisted of sluggish, murky water choked with Spirogyra, blue-green.
<br />algae, and narrow-leaf Potamogeton, and strewn with trash. The only fishes
<br />collected were Cyprinus carpio, Ictalurus melas, Notropis formosus, and Campostoma
<br />ornatum. Gila nigrescens was last recorded here in 1964 (ASU 816 and UANL 58$);
<br />the 425 specimens in the latter collections indicate the previous abundance of
<br />the chub .
<br />(C) Rio Piedras Verdes, 6.5-10 km above Colonia Juarez (Fig. 1, ~~4),
<br />ca 108°08' N long., 30°21` W lat., ca. 1550 m, 3 June 1979.-One juvenile Gila.
<br />nigrescens, 77 mm SL, was taken at this station. This 5-10~m wide portion of
<br />the river had clear-flowing water (fast over riffles, slow in deeper portions),
<br />sand, gravel, rocks and boulders (only occasionally mud), often dense patches
<br />of narrow-leaf Fotamo e~ton, ~drocotle, and Spirogyra, undercut banks, good
<br />cover .(downed willows and shrubs), and few pools. The banks were characterized
<br />by sod, shrubs, willows, sycamore, and ash trees. Other fishes collected were
<br />Catostomus plebetus, Campostoma ornatum, Notropis formosus, Fimephales promelas,
<br />Ictalurus melas, and Ambloplites rupestris (exotic). Previous collections made
<br />in t-he o Piedras Verdes near Colonia Juarez indicate that G. nigrescens was
<br />formerly more abundant (e.g., IIMMZ 182401, 85 specimens). In June 1978 the
<br />Rio Yaqui survey team, from ASU, found the chub to be common here. The apparent
<br />change between June, 1978,and June, 1.979, is inexplicable'at this time.
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