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194 MINNOW FAMILY, CYPRINIDAE Colorado squawfish, Prychocheilus lucius Giard ,~' n Identification. Colorado squawfish are similar in shape and color to Sacramento squawfish, except that the head and eye are smaller in relation to the rest of the body, the body is more slender, and the head is more elongate. There are 9 rays in the dorsal fin, 9 rays in the anal fin, 80 to 95 scales in the lateral line, and the pharyngeal teeth are 2,5-4,2. Names. The trivial name lucius means pike, referring to the superficial resemblance of squawfish to true freshwater pikes (Esocidae). Jordan and Evermann (1423) listed its common name as "white salmon of the Colorado" or "whitefish." Other names are as for Sacramento squawfish. Distribution. Formerly found throughout the Colorado River and its major tributaries, it appears to be absent today from the lower Colorado River and rare elsewhere, although it is in the process of being cultured and reintroduced. Life History. The Colorado squawfish is a big-river species. The large adults are typically found in deep, fast-flowing waters of the Colorado River and in large pools of its tributaries. Smaller fish frequent quiet water of the river's edge or shallow pools with sand or silt bottoms (Vanicek and Kramer, 1969). Unlike Sacramento squawfish, Colorado squawfish seem to do well in turbid, silty water which occurs naturally in the Colorado River system. Like Sacramento squawfish, they are predatory. Fish less than 50 mm TL feed mostly on cladocerans, copepods, and chironomid midge larvae. Aquatic insect larvae are the most important food fer fish between 50 and 10(1 mm TL, while fish, especially other minnows, are the dominant food for those greater than 100 mm TL (Vanicek and Kramer, 1969). Colorado squawfish over 200 mm TL feed almost exclusively on other fish, but do so sporadically; Vanicek and Kramer found that 39 percent of the stomachs of large squawfish they examined were empty. Presumably because they occupied the position of chief piscivore in a major river system, Colorado squawfish achieved large sizes. Jordan and Evermann (1896) indicate that they occasionally exceeded 1.8 m TL and 45 kg, making this species the largest native North American cyprinid. However, few weighing more than 18 kg have been captured since 1930 (Sigler and Miller, 1963). Vanicek and Kramer (1969) collected none larger than 61 cm TL and 2:3 kg. Like the Sacramento squawfish, Colorado squawfish are slow growing. The Green River, Colorado-Utah, population studied by Vanicek and Kramer was 44 mm TL at year I, 95 mm TL at year II, 162 mm TL at year III, 238 mm TL at year IV, and 320 mm TL at year V. After the fifth year, the growth rate decreased, so that a 61 cm TL squawfish was age XI. Vanicek and Kramer also found that the growth rate of squawfrsh in a stretch of river below a new dam slowed down considerably following impoundment, apparently because the tempera- ture regime of the river had changed, being either colder or warmer on the average than it had been prior to the construction of the dam. Colorado squawfish mature between ages V and VII. Spawning takes place in July and August in the Green River, Wyoming, and seems to depend on water temperatures rising above 18°C coupled with dropping water levels (Vanicek and Kramer, 1969). In other areas, spawning may take place in the Late spring (Sigler and Miller, 1963). Details of spawning habits have not been recorded but they are undoubtedly similar to those of the Sacramento and northern squawfishes. There is some evidence that