Figure 2. Graphic analysis of head-depth, postorbital length of head and predorsal length of Hybopsis gracilis, expressed as thousandths of standard length. Numbers in parenthesis = number of specimens examined from each locality. In each symbol, horizontal line = range, vertical line = mean, open rectangle = one standard deviation on each side of mean, black rectangle = twice the standard error on each side of mean. The dash-lines represent drainage patterns of rivers in which this species occurs. All measurements are of specimens 70 to 100 mm in standard length.
In the Pecos and Arkansas basins, species commonly taken with H. g. gulonella are Catostomus commersonnii, Hybognathus placita, Notropis lutrensis lutrensis, Notropis stramineus missuriensis, Pimophales promelas, and Campostoma anomalum plumbeum. The only spiny-rayed fishes that we have found with H. g. gulonella are Lepomis cyanellus and L. humilis, both of which are scarce. Associates of H. g. gracilis include the same species, plus other ostariophysan fishes such as species of Carpiodes, Ictiobus, and silt-adapted species of Hybopsis and Notropis.
We failed to find the flathead chub at any of 11 localities in the South Platte drainage, where we collected in September, 1959. Dr. George Baxter, of the Department of Zoology, University of Wyoming, told us that he has never found H. gracilis in that drainage. The fauna of the South Platte includes Catostomus catostomus, Semotilus atromaculatus, Hybopsis biguttata, Hybognathus hankinsoni, Notropis cornutus frontalis, Etheostoma nigrum and E. exile--species rarely if ever found with H. gracilis.
Ecologically, H. g. gulonella seems to be the counterpart of Semotilus atromaculatus in streams where the latter species is absent. Observations of H. g. gulonella in the Purgatoire River indicated that loosely-organized groups of flathead chubs congregated one to four inches above the bottom of pools, and near or under protective cover such as roots of vegetation or debris lodged against shore. Individuals moved about independently within the group (rather than as schools), and occasionally rose to the surface, perhaps for food.
The flathead chub is chiefly carnivorous, but its food includes some aquatic vegetation (Table 1). Most organisms found in specimens (both subspecies) were terrestrial insects (Coleoptera, Diptera, Orthoptera); all insects were adult stages, except those designated as larvae in Table 1. Roundworms probably were parasites, rather than food.
Hubbs (1927:76) states that the food of young flathead chubs that were obtained from the Arkansas River System in New Mexico consisted "almost entirely of crustaceans (small ostracods and cladocerans to the exclusion of all else but an occasional larval or adult insect, etc.)."