Geographic Variation in the North American Cyprinid Fish, Hybopsis gracilis Part 2

NATURAL HISTORY

_Habitat_

The species inhabits alkaline streams with shifting sand bottoms where the waterlevel fluctuates considerably with heavy rains and melting snow. The flathead chub is found in silty water and often is the predominant species in streams that have high turbidity. The remarkable ability of this fish to withstand exceedingly high turbidity is ill.u.s.trated by its predominance in the Little Missouri River, which has an average concentration of suspended silt two and one-half times that of the Missouri River at Kansas City (Personius and Eddy, 1955:42).

[Ill.u.s.tration: FIGURE 1. Graphic a.n.a.lysis of lateral line scales, pectoral rays and post-Weberian vertebrae in _Hybopsis gracilis_.

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.

_H. g. gracilis_ is found in large rivers throughout its range, occasionally migrating into smaller streams, especially in the sp.a.w.ning season. It prefers the main channel of rivers in moderate to strong current. All series examined are from elevations lower than 3,000 feet.

Numbers to left of symbols = number of specimens examined from that locality; combined collections indicated by brackets. The dash-lines represent drainage patterns of rivers in which this species occurs.]

_H. g. gulonella_ occupies small rivers and creeks, preferring pools with moderate currents. In fall, dense concentrations of this subspecies have been found in small pools, where brush, driftwood or other debris deflects the current and prevents filling with drifting sand. Hundreds of flathead chubs were collected in such pools in the Purgatoire and Arkansas rivers. Specimens were also collected with ease in Beaver Creek, Colorado, from pools with murky water and slight flow, over bottoms of gravel and bedrock. No brush or other debris was near the pools. In each case the streams carried little water, although they undoubtedly carry greater volumes of water in spring and early summer after rains and spring thaws. The preferred bottom-type for this subspecies seems to be gently shifting sand.

_H. g. gulonella_ is found in warm-water streams, whereas _H. g.

gracilis_ occurs in cooler water. The southwestern subspecies was taken in August in the Mora River drainage at Sapello (temperatures above 80 F.) but not at Mora (temperatures below 70 F.). In the Purgatoire River, a thriving population was found where the water temperature was 92 F., on September 6, 1959. In the Arkansas and Pecos rivers and the Rio Grande this subspecies is most abundant below the mountainous parts of the stream-courses, but at elevations higher than 4,000 feet on the plains.

_a.s.sociated Species_

[Ill.u.s.tration: FIGURE 2. Graphic a.n.a.lysis of head-depth, pos...o...b..tal 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.

a.s.sociates 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 sh.o.r.e. Individuals moved about independently within the group (rather than as schools), and occasionally rose to the surface, perhaps for food.

_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.)."

_Sp.a.w.ning Season_

Specimens of _H. g. gulonella_ that have been examined reach s.e.xual maturity at approximately 65 mm standard length. Most specimens of _H.

g. gracilis_ less than 85 mm in standard length are immature, but larger specimens probably are mature.

The sp.a.w.ning season is in late summer, beginning in July and extending into September. Specimens from the Peace River, collected on August 10, 1952, include females that were mostly spent and tuberculate males.

Males and females in sp.a.w.ning condition were collected in the Milk River in August of 1955. A large presp.a.w.ning female was obtained in Red Deer River in June of 1952. A male from Fort McMurray had fairly well developed tubercles on August 9, 1955. A presp.a.w.ning female was taken from the Saskatchewan River at Clarkboro Ferry on June 7, 1957.

Tuberculate males were collected in the Powder River on June 30, 1957.

Specimens from the White River in South Dakota, collected on July 7, 1934, include tuberculate males. The specimens discussed above are _H.

g. gracilis_ or intergrades tending toward that subspecies.

Specimens of _H. g. gulonella_ collected in the Arkansas River at Pueblo and Florence, Colorado, on September 7, 1959, include some tuberculate males, although most females are spent. On August 8, 1957, a series of flathead chubs that includes tuberculate males was collected in the Redwater River, Montana. In the Pecos River on August 25, 1958, sp.a.w.ning seemingly had been completed, although a few males still bore tubercles.

TABLE 1. ORGANISMS FOUND IN STOMACHS OF HYBOPSIS GRACILIS FROM VARIOUS LOCATIONS, EXPRESSED AS PERCENTAGE OF TOTAL VOLUME.

A: S. Saskatchewan R., Clarkboro Ferry, Sask.

B: Milk R., Alberta C: Missouri R., S. D.

D: Missouri R., Neb.

E: Arkansas R., Fremont Co., Colo.

F: Arkansas R., Pueblo Co., Colo.

G: Pecos R., San Miguel Co., N. M.

==============================+=====+=====+=====+=====+=====+=====+===== A B C D E F G ------------------------------+-----+-----+-----+-----+-----+-----+----- No. specimens examined 1 7 6 10 10 10 10 No. specimens containing food 1 6 1 2 1 3 7 ------------------------------+-----+-----+-----+-----+-----+-----+----- KIND OF ORGANISM Aphasmidia 10.0 00.7 03.0 Arthropoda Araneae Argiopidae 04.0 Theridiidae 04.0 Insecta Ephemeroptera (nymph) Baetidae 05.0 Heptagenidae 08.0 Hemiptera Corixidae 35.0 00.3 Hymenoptera Formicidae 21.0 60.0 Coleoptera Staphylinidae 01.7 07.0 Scolytidae 13.3 70.0 Tenebrionidae 05.7 70.0 Carabidae 05.7 01.0 Curculionidae 01.0 Coccinellidae 09.0 Trichoptera (case) 01.7 Diptera Mymaridae 00.3 Empididae 01.3 Cecidomyiidae 04.0 Trachinidae 00.7 Simulidae 06.7 20.0 Tabanidae 06.0 Chironomidae 06.0 Not identified to family 01.0 Orthoptera Locustidae 07.7 Tettigoniidae 03.0 70.0 09.0 Tetrigidae 06.0 h.o.m.optera Fulgoridae 05.0 01.0 Insect egg 00.7 Plants Cyanophyceae 09.0 99.0 20.0 Cyperaceae 02.0 01.0 Zannich.e.l.lia pal.u.s.tris 00.3 Vascular remains 55.0 27.0 Miscellaneous Sand 00.7 Pharyngeal tooth 00.3 +-----+-----+-----+-----+-----+-----+----- Total (%) 100.0 99.8 100.0 100.0 100.0 100.0 100.0 ------------------------------+-----+-----+-----+-----+-----+-----+-----

Sp.a.w.ning apparently occurs when river levels recede to the seasonal lows. In late summer, temperatures of these rivers probably are maximal, their turbidities are reduced, and their sandy bottoms are stable.

Underhill (1959) reports that this species is rare in the Vermillion River, a northeastern tributary of the Missouri River, except in autumn when large numbers occur near the mouth of the river. We suspect that this is a.s.sociated with sp.a.w.ning.

[Ill.u.s.tration: PLATE 21

Distribution of collections examined.]

[Ill.u.s.tration: PLATE 22

_Hybopsis gracilis gracilis._ Missouri River, Thurston County, northeast of Macy, Nebraska. Largest specimen 87.5 mm standard length.]

[Ill.u.s.tration: PLATE 23

_Hybopsis gracilis gulonella._ Pecos River, San Miguel County, 3 miles south of town of Pecos, New Mexico. Largest specimen 91 mm standard length.]

[Ill.u.s.tration: PLATE 24

FIG. 1. Top: _Hybopsis gracilis gracilis_, 230.0 mm standard length, one of the largest specimens examined. Missouri River, Carson County-Walworth County line, 3 miles northeast of Mobridge, South Dakota, at mouth of Grand River.

Bottom: _Hybopsis gracilis gulonella_, 121.6 mm standard length, the largest specimen examined of this subspecies. Beaver Creek, Fremont County, 10 miles northeast of Florence, Colorado, on Highway 115.]

[Ill.u.s.tration: FIG. 2. Regression of head-depth on standard length in _Hybopsis gracilis gracilis_ from the Saskatchewan River, and in _H. g.

gulonella_ from Beaver Creek, Arkansas River Drainage (KU 4769).]

DISCUSSION