7010 - Laserfiche WebLink

Home Browse Search

.. . the Colorado and Green rivers. This conclusion cannot be confidently applied to certain popula- tions that we have available but have not fully studied from Lake Powell, however. Furthermore, populations of Gila robusta from certain tribu- taries were elegans and cYP4a are absent parallel these species in many characteristics. We choose a taxonomic treatment of this situation that em- phasizes the specific distinction of the three populations in the big-river habitat, but admit the possibility that the species isolating mecha- nisms may break down under disturbed (reservoir) conditions and that the populations in tributary streams may not be completely independent or iso- lated from the three central forms. We refer to the entire complex as the Gila robusta super- species, including robusta, cypha, and elegans as defined above, and including subspecies or races of robusta, seminuda of the Virgin River, a similar population in the San Juan River, and a number of isolated populations in northern and western Mexico. The complex in the Gila River basin includes Gila intermedia and forms inter- mediate between that species and robusta, i.e., G. r. "grahami" (cf. Rinne, 1976). Gila inter- media is inferred to be a part of the robusta super species on the basis of shared characters and possible gene exchange through "grahami", but may be a sister phyletic line by its origins, as indicated by its similarity to several of the large-scaled, small-finned subgenera of Gila (cf. Miller, 1945). In spite of the uncertainty regarding peripher- al populations, it is clear that robusta (s.s.), elegans, and cypha have diverged in a number of characters, and can be discriminated on the basis of counts of fin rays, vertebrae, and gill rakers, the relative depth and length of the caudal pe- duncle, snout shape, fin position, and form of the nuchal hump. The multivariate analysis shows that cypha is intermediate between robusta and elegans in general, but is extreme in several respects. This suggests the possibility that cypha and elegans were derived from robusta by separate speciation events. Gila robusta shows the most primitive characters of the three, and the sepa- rate ways in which elegans and cypha are extreme suggest that neither is likely to be ancestral to either of the other two. T~ack of coexistence of the three species in s ler tributaries is in accordance with the expect relationship between diversity and spatial hetero neity. The species seem to be omnivorous carnivo es with specializations related to habitat --chacters associated with food processing (jaws, teeth) are similar, except that robusta has fewer gill rakers and elegans more (Table 2). In small tributaries, such as the Virgin and possibly the San Juan, a single species with intermediate morph- ology seems to be selected for; in the larger tributary systems of the Gila River, a bewilder- ing mosaic of generalized forms occur (Rinne, 1976), where in former times robusta and elegans lived in the main channel. The close apparent control of habitat size and diversity over morphology and species diversity suggests that if the habitat could be experimen- tally changed, the populations should show pre- dictable responses. Rather unfortunately, the destruction of main-river habitat by Glen Canyon Dam has created such an experiment. Early indi- cations are that the two more specialized species will not exist in the Lake Powell environment. The three species seem to be breaking down locally by hybridization. We predict that a single- species of mixed origins and generalized charac- teristics will appear. Given long enough, some re-oriented diversification would develop, but the reservoir is apparently silting in too rapidly for the necessary stability to persist. In a period of controversy following the poi- soning of the upper Green River in 1962 (Miller 1963b), erroneous statements about the classifi- cation, biology, and distribution of Colorado River chubs appeared which have been destructive to the understanding and management of these fishes, and require correction. For example, Stroud (1963:7) irresponsibly claimed that the construction of Flaming Gorge Reservoir resulted in making available "large numbers" of humpback chubs, all of which were supposedly males. The agency (U.S. Bureau of Sport Fisheries and Wild- life) that "confirmed the presence and ready availability of numerous humpback chubs in [the Green River below Flaming Gorge Dam] . . ." subsequently reported (Vanicek 1967; Vanicek et al. 1970; Holden and Stalnaker 1975) the absence of Gila cypha in the area indicated and great rarity of this species below the mouth of the Yampa River; only three specimens were taken in 1963, and none during 1964-1966, in the Green River in Colorado and Utah (Kramer 1967:Table 2; Vanicek et al. 1970:Table 4). Vanicek and Kramer (1969:194-195) stated, . . . c~iteria are not available for distinguish- ing between young fish of the two morphological variants" (Le., between what were then called Gila r. robusta and G. r. elegans). "Consequent- ly, specimens shorter than 200 rom total length were combined in the present study under the general taxon, Colorado chub.- Collections of Gila robusta and G. elegans at The University of Michigan, containing young as small as 22 to 40 rom S.L., were identified as early as 1926; by 1968, humpback chubs as small as 43 rom S.L. had been determined. By the criteria reported here, we have identified Gila robusta robusta (UMMZ 162818) to 20 rom, Gila cypha (UMMZ 182415) to 54 rom, and Gila elegans (UMMZ 162846) to 22 rom in total length. Juveniles of three species are shown in Figure 10. Holden and Stalnaker (1975) (citing Minckley and Deacon, 1968, as authority) reported Colorado squawfish (Ptychocheilus lucius Girard) in the Grand Canyon, but the two specimens actu- ally came from between Glen Canyon Dam and Lees Ferry,. well above Gran d Canyon. The only valid record (based on preserved material) known to us of this species from the Colorado River in Grand Canyon is represented by an adult (about 320 rom S.L.) caught in 1975 by an unknown fisherman at the mouth of Havasu Creek (ASU7087). However, squawfish formerly moved up the Little Colorado River in Grand Canyon to the base of Grand Falls (Miller, 1963a:l, ftn.l). 622 In our distribution map of the Gila robusta complex (Figure 1), the record for G. elegans in the Little Colorado River (at base of Grand Falls) is based in part on the statement referred to above,' in part on our conclusion as to the true type locality for Gila elegans (and Gila robusta), and in part on the absence of the bony tail from the Little Colorado River above Grand Falls. The holytype (USNM 251), as well as the three syntypes of Gila robusta (USNM 246), were said to have come from the "Zuni River, New Mexico" (Baird and Girard, 1853, 1854; Girard, 1858:286-287). How- ever, at the time of collection during the summer (rainy season) of 1852, Zuni River was described ". . . as a mere rivulet, and not entitled to the name of river; in most parts of our country it would not be dignified with that of creek" (Sitgreaves, 1854:5). This is hardly the habitat of Gila elegans and, moreover, that species is unknown from the Little Colorado River basin (to which Zuni River is tributary in floods) above Grand Falls, an impassible barrier 56 meters high (Dryer, 1965). Furthermore, careful examination of the channel of Zuni River in New Mexico con- vinced us that (at least in recent centuries)