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bottom. The scales are reduced to bony platelets, some of which form longitudinally arranged ridges that may function to direct water over the body surface and assist in maintenance of position in swift runs. Woundfin select areas with rocky substrate in current for spawning, at a temperature of about 250 C. Young grow rapidly, achieving lengths of 20 to 40 mm a month or two after hatching. The species was known from the Gila and lowermost Colorado rivers in the late 1800s but since then has been found only in the Virgin River mainstream. In the Rio Yaqui, the Pacific shad/sardinita del Pacifico (Fig. 69) is a midwater inhabitant of lakes or open parts of rivers. They feed on finely divided detrital material from the bottom or on plankton in the water column, and in doing so act as an avenue for these foodstuffs through the food chain to larger fishes. Shad in freshwaters provide a food base for predatory fishes just as anchovies provide food for larger fishes in the sea. With construction and filling of reservoirs in the Rio Yaqui system, Pacific shad appeared far inland, thus acting as forage for larger fishes in those newly created habitats. The threadfin shad, a similar species from eastern drainages of the United States and Mexico, has recently been introduced into the Rio Yaqui basin, the results of which are not predictable. The Yaqui catfish (Fig. 62) resembles the common channel catfish of eastern North America. They commonly achieve more than 40 cm in length and a kilogram in weight, and live in relatively deep water during the day, only to move onto riffles and runs at night to feed. The few stomachs that have been examined contained aquatic invertebrates, other Figure 69. Pacific shadlsardinita del Pacifico, 17.8 em in total length, from Marina del Rey, Presa Alvaro Obregon, Sonora. Status of this midwater, possibly estuarine species is speculative. It is known from scattered localities near the sea in western Sonora. fishes, and organic debris. Spawning is apparently similar to channel catfish, with the male defending eggs in a depression or hole in the bank and protecting his newly hatched young for a time. Young live in shallower water than adults, often on riffles among cobble and boulders. Channel, blue, and flathead catfishes from eastern North America have now been introduced into the Rio Yaqui system, and there is already evidence of hybridization between the first and the Yaqui catfish. Last among the freshwater fishes characterizing the lowermost Rio Yaqui basin is the colorful Sinaloan cichlid (Fig. 70), which lives in quiet waters of the mainstream, creek mouths, and sloughs and Figure 70. Sinaloan cichlid/mojarra sinaloense, 12.5 em in total length, from Rio Chico, Sonora. Status of this species is unknown. 35