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<br />(D) Rio Piedras Verdes, near Colonia Hernandez (Fig. 1, 465), 108° 18' N.
<br />long., 30° 03' W lat., ca. 1975 m, 6 May 1979.-Gila nigrescens was very abundant
<br />at this locality, and a graded series of young to breeding adults was preserved.
<br />Many of the adults were in breeding color; 2 females (175 and 142 mm SL) had
<br />mature eggs but were not quite ready to spawn. The presence of many juveniles
<br />less than 30 mm SL (estimated to be 4-6 weeks old), and the condition of the
<br />females, indicate that spawning may occur over a protracted period or that
<br />bimodal spawning takes place (e.g., spring and fall). The habitat could be
<br />characterized as pristine - clear-flowing water, deep pools, sand and gravel
<br />bottom, large boulders, and crevices in the canyon ~va11s. Many adult chubs,
<br />were observed among the boulders in the deeper pools; juveniles were collected
<br />and observed in shallow water over gravel. Only indigenous species were taken
<br />with the chubs: Notropis _formosus, Campostoma ornatum, and Catostomus plebeius.
<br />Gila nigrescens was last collected in 1953 3.2 km NE of Colonia Garcia, ca. 7
<br />airline km upstream (SW) of Colonia Hernandez.
<br />(E) Arroyo del Aguila, ca. 5 airline km ESE of Colonia Hernandez (Fig. 1,
<br />466), 108° 15' N long., 30° 03' W lat., ca. 2020 m, 6 May 1979.-This stream is a
<br />major tributary to the Rio Piedras Verdes and joins that river at Colonia
<br />Hernandez. The collection was taken in the mountains ca. 50 m above the town.
<br />The upper reaches of the arroyo were intermittent and G. nigrescens was abundant
<br />in many of the pools and rheocrenes. The habitat comprised sand and gravel
<br />bottom with rocks and large boulders; little if any aquatic vegetation was
<br />present. Only three individuals were preserved for comparative purposes. Also
<br />taken with the Gila were the same three native species caught at Colonia
<br />Hernandez. This is the first known collection at .this locality.
<br />(F) Rio San Miguel at Ignacio Zaragoza (Fig. 1, #7), 107° 46' N long.,
<br />29° 34' W lat., 2088 m, 30 May 1979.-- Although the Chihuahwa chub was known
<br />from this locality as of 1975 (UANL uncatalogued, ASU 6375, 6381), it was not
<br />present in 1979. The habitat has been drastically modified-- the channel
<br />has been dredged and levees built (we estimate this was done within the last year
<br />because the. levees were not vegetated and the gravel was still loose). The
<br />water was murky, without current, and not more than 5 cm deep anywhere. The
<br />bottom consisted o~ sand, gravel, and cobble, and some areas had dense stands
<br />of narrow-leaf Potamogeton, Hydrocotle, and Myriophyllum. Only Cyprinodon sp.
<br />was encountered.
<br />III. Rio Santa Maria Drainage.
<br />(A) Rio. Santa Maria, .3-1.5 km above Buenaventura (Fig. 1, 468), 107° 29'
<br />N long., 29° 47' W lat., ca. 1510 m, 30 May 1979.- The Chihuahua chub was common
<br />in some parts of the river. Notes on their microhabitat distribution are
<br />instructive. At the time of collection, the water was ca. 1 m deep, brown,
<br />turbid, and flowing swiftly. The bottom consisted o~ sand, gravel, boulders,
<br />and some mud closer to shore. Vegetation was not noted in the main channel,
<br />but was present in fords and small bays; the major species were narrow-leaf
<br />Potamogeton, Hydrocotle, Marsallia, and Nasturtium. Strong current made the
<br />main channel very difficult to work, and only some small shiners and a poeciliid
<br />were collected there. Gila nigrescens, including adults in breeding color,
<br />was collected only in flooded road crossings, bays, and eddies. Most chubs
<br />were taken in one small embayment with an undercut bank (the significance of
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