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Changes in the fish assemblage of an intermittent prairie stream upstream from a Texas impoundment.

Abstract. -- Comparisons of fish collections made from the Double Mountain Fork of the Brazos River before and after impoundment of Lake Alan Henry show that the stream fish assemblage upstream from the lake has changed dramatically. Overall, numerical dominance in the fish assemblage has shifted from minnows (family Cyprinidae) to pupfishes and killifishes (family Cyprinodontidae). The cyprinid species Hybognathus placitus and Notropis buccula, which were abundant in pre-impoundment collections, have become extirpated, or nearly so, since the river was impounded.


Dams and impoundments dramatically alter the physical and chemical characteristics of streams and rivers (Baxter 1977; Stanford & Ward 1979). Post-impoundment changes in water temperature, concentrations of dissolved gases, substrate composition, and the timing and volume of stream discharges, may affect stream fish assemblages (Edwards 1978; Holden 1979; Anderson et al. 1983; Bain et al. 1988). Holden (1979) distinguished between two broad classes of stream fishes according to the degree to which they required riverine conditions and thus, were affected by impoundments. Obligate riverine species, such as the darters (Etheostoma and Percina spp.), include those species that require riverine conditions for all, or some (=partially obligate species, i.e., salmon, Oncorhynchus spp.) of their ecological needs. Dams and impoundments almost invariably negatively affect obligate riverine species (Holden 1979). Facultative riverine species, such as gizzard shad (Dorosoma cepedianum) and largemouth bass (Micropterus salmoides) are less impacted by impoundments and actually may increase in abundance in, and downstream from, impoundments (Holden 1979).

The effects of impoundments generally are most apparent in downstream areas and, consequently, the greatest attention has been focused on these areas (e.g., Holden 1979). However, effects of dams and impoundments also extend upstream. Facultative riverine species may increase in abundance in impoundments and move upstream where they may impact obligate riverine species (Ruhr 1956; Erman 1973). Dams and impoundments also may act as barriers to upstream migration by fishes that historically occupied, but either did not, or could not, maintain permanent populations in upstream areas (Hatch et al. 1985; Reyes-Galivan et al. 1996). Finally, in headwater and intermittent streams, impoundments may affect upstream fish assemblages by preventing recolonization following periods of drought, or other natural disturbances, that periodically extirpate local populations (e.g., Winston et al. 1991).

In this paper, changes in the fish assemblage of an intermittent prairie stream, the Double Mountain Fork of the Brazos River, upstream from a recently constructed impoundment are described. At the initiation of this study, two changes in the assemblage were expected: (1) the extirpation of the obligate riverine species Notropis buccula and Hybognathus placitus, which were believed to be unable to maintain permanent populations upstream from the impoundment in the absence of periodic immigration from downstream sites; and (2) a decline in relative abundance and species richness of minnows (family Cyprinidae), compared with pre-impoundment collections, as a result of the loss of N. buccula and H. placitus.


The Double Mountain Fork of the Brazos River rises in a break in the Llano Estacado in eastern Lynn County, Texas. The river flows approximately 264 km to its confluence with the Salt Fork of the Brazos River. The confluence of these two streams, just west of the Stonewall/Haskell county line, forms the Brazos River proper.

In its upper reaches, the Double Mountain Fork is an intermittent stream and flows for only a few days after rain events. After flows cease, stream fishes are confined to isolated pools for up to several months at a time. Prior to 1991, the Double Mountain Fork was unregulated. Construction of John T. Montford Dam was begun in that year, and impoundment of the Double Mountain Fork to form Lake Alan Henry began in October 1993.


Fishes were collected from the Double Mountain Fork, Garza County, Texas, from August 1996 through December 1998. All collections were made at the U.S. Highway 84 crossing at river km 52, approximately 14 km upstream from Lake Alan Henry and 22 km upstream from Montford Dam. This site was chosen because of its accessibility and because two pre-impoundment collections had been made there in March 1978 (G. P. Garrett, pers. comm.) and in May 1992 (Moss & Mayes 1993). During August 1996 through December 1998, fish were collected with a 3.7 by 1.2 m minnow seine. All fishes collected were preserved in 10% formalin and returned to the lab for identification and enumeration. Fish were collected from flowing waters, when present, or isolated pools in an attempt to sample all possible habitat types and obtain a representative sample. An effort was made to collect a minimum of 200 specimens per sampling event. During sampling, width of wetted stream ranged from 0.4 to 7.4 m. Length of stream sampled ranged from 0.4 to 3.2 km.

Although fishes were sampled from a single site on the Double Mountain Fork, this site is located in the lower reaches of the unimpounded river. During extended dry periods, permanent pools are absent for at least several hundred meters upstream and downstream from Highway 84. Pools in this area are recolonized, from upstream and downstream sites, during flood events. Therefore, collections from this site should be adequate to document species presence and relative abundance in the Double Mountain Fork upstream from Lake Alan Henry.


Eleven species of fish, representing six families, have been collected from the Double Mountain Fork at Highway 84 (Table 1). Eight species were collected before impoundment of Lake Alan Henry and one additional species, Ameiurus melas, probably occurred in this area even though it was not present in pre-impoundment collections. The facultative riverine species Dorosoma cepedianum and Micropterus salmoides are native to the Brazos River and its tributaries, but probably did not occur this far upstream in the Double Mountain Fork prior to impoundment of Lake Alan Henry. Their presence in post-impoundment samples is probably due to immigration from the lake.

In the earliest collection (1978) from the Double Mountain Fork, the fish assemblage was dominated by cyprinids, primarily N. buccula and H. placitus, which together represented 49% of all fishes captured. Two species of cyprinodontids, Cyprinodon rubrofluviatilis and Fundulus zebrinus, comprised 34% of the fish assemblage. In 1992, Notropis buccula and Hybognathus placitus represented 21% of all fishes captured, with other cyprinids comprising an additional 15% of the fish assemblage. Cyprinodontids represented 64% of the assemblage. Differences in the fish assemblage between 1978 and 1992 may represent natural variation associated with differences in stream volume, water temperature, salinity, and the length of time that fish had been restricted to pools when sampled. However, one cannot discount the possibility that these differences are due to modifications of the Double Mountain Fork associated with construction of Montford Dam.

In collections made from August 1996 though December 1998, after impoundment of Lake Alan Henry, cyprinodontids increased in relative abundance and comprised 40 to 99% of fishes captured, whereas cyprinids comprised only 1 to 29% of the assemblage. In contrast to earlier collections, cyprinids were primarily represented by Cyprinella lutrensis. Notropis buccula and Hybognathus placitus comprised less than 1% of the fish assemblage following impoundment of Lake Alan Henry.


Stream fish assemblages throughout Texas have changed considerably since the 1950s (Anderson et al. 1995; Hubbs et al. 1997). Many of these changes are associated with reservoir construction and related environmental changes (Anderson et al. 1995). Since the 1950s, several reservoirs have been constructed in the Brazos River drainage and, compared with other river drainages in the state, the Brazos River is among those that show the greatest temporal changes in fish assemblages (Anderson et al. 1983; Anderson et al. 1995; Hubbs et al. 1997). This study documents changes in the fish assemblage of the upper Double Mountain Fork that are attributable to reservoir construction. This is of particular interest because several water-development projects, including at least one additional reservoir and three chloride-control projects, have been proposed for construction on the upper Brazos River (Texas Water Development Board 1990).

Since the 1950s, there has been a general decline in the relative abundance of cyprinids in stream fish assemblages throughout Texas (Anderson et al. 1995). In this study, there was a considerable decrease in the relative abundance of cyprinids in the Double Mountain Fork upstream from Lake Alan Henry; however, there was a complementary increase in the relative abundance of cyprinodontids, which was not described by Anderson et al. (1995). Results of this study also suggest an apparent increase in the relative abundance of Gambusia affinis in the Double Mountain Fork after impoundment of Lake Alan Henry. Anderson et al. (1995) reported that the relative abundance of this opportunistic species had increased statewide since the 1950s.

Cyprinella lutrensis comprised 9 to 15% of the pre-impoundment fish assemblage and was a persistent, but variable element (1 to 29%) of the post-impoundment assemblage. The relative and absolute abundance of C. lutrensis commonly increases in disturbed environments, especially where stream discharges have been reduced in magnitude and variability (Cross and Moss 1987) including areas downstream from reservoirs (Edwards 1978; Anderson et al. 1983). There was no evidence of any increase in abundance of C. lutrensis in the Double Mountain Fork upstream from Lake Alan Henry, possibly because stream discharge and other abiotic conditions in this reach of the river were unaffected by impoundment of the lake.

Results of this study suggest that Hybognathus placitus has become extirpated from the Double Mountain Fork upstream from Lake Alan Henry and that Notropis buccula either has, or soon will, experience a similar fate. Hybognathus placitus has not been collected upstream from Lake Alan Henry since 1996 and was represented in that sample by a single specimen. Notropis buccula was infrequent, and irregular in occurrence, in collections made since 1996. Loss of these species represents a 50% decrease in cyprinid species richness in the Double Mountain Fork upstream from Lake Alan Henry.

Both Notropis buccula and Hybognathus placitus are obligate riverine species. Spawning appears to occur in response to flood events in H. placitus (cf. Lehtinen & Layzer 1988; Taylor & Miller 1990) and N. girardi (cf. Moore 1944), which is closely related to N. buccula (cf. Cross 1953; Cross et al. 1983). In the upper reaches of the Double Mountain Fork, fishes are confined to isolated pools for extended periods. Notropis buccula and H. placitus may be unable to reproduce in these pools, which would explain their disappearance from the river upstream from Lake Alan Henry. Prior to impoundment of Lake Alan Henry, these species could recolonize the upper Double Mountain Fork once flows returned, however, the lake now prevents this. Winston et al. (1991) described the similar extirpation of two cyprinid species, Macrhybopsis aestivalis and Notropis potteri, upstream from an impoundment of the North Fork of the Red River, Oklahoma. They also noted that two additional cyprinids, Hybognathus placitus and Notropis bairdi, appeared to be periodically extirpated and reintroduced as bait-bucket introductions. Winston et al.'s (1991) observations and results of the present study suggest that attempts to reestablish N. buccula and H. placitus upstream from Lake Alan Henry would not be successful.

Notropis buccula is endemic to, and historically was widespread throughout, the Brazos River drainage (Cross 1953; Hubbs et al. 1991). Presently, it is restricted in occurrence to the Brazos River and its tributaries upstream from Possum Kingdom Reservoir (Moss & Mayes 1993). Because of recent decreases in the distribution and abundance of N. buccula and plans for continued development of the upper Brazos River drainage, Moss & Mayes (1993) suggested that N. buccula might warrant protection as a threatened species. In light of the decreasing distribution of the species, the eventual extirpation of N. buccula upstream from Lake Alan Henry must be considered as a significant loss.
Table 1. Relative abundance (%) of fishes captured from the Double
Mountain Fork of the Brazos River at US Highway 84, Garza County, Texas,
during 1978 through 1998.

 Pre-impoundment Post-impoundment
 Mar May Aug May Aug Nov Jan
 1978 1992 1996 1997 1997 1997 1998

Dorosoma cepedianum 0.0 0.0 0.0 0.0 0.0 0.4 0.0
Ameiurus melas 0.0 0.0 0.5 0.0 0.0 0.4 0.8
Cyprinella lutrensis 9.1 14.8 5.9 26.3 28.9 20.0 16.2
Hybognathus placitus 22.0 14.5 0.5 0.0 0.0 0.0 0.0
Notropis buccula 26.5 6.8 0.0 0.0 0.0 0.4 0.0
Pimephales promelas 0.8 0.0 0.0 0.0 0.0 0.0 0.0
 rubrofluviatilis 15.2 14.8 50.5 48.8 19.3 16.1 48.5
Fundulus zebrinus 18.9 48.7 42.6 22.5 21.1 39.8 34.6
Gambusia affinis 7.6 0.0 0.0 1.3 27.2 22.8 0.0
Lepomis cyanellus 0.0 0.3 0.0 1.3 0.0 0.0 0.0
Micropterus salmoides 0.0 0.0 0.0 0.0 3.5 0.0 0.0
Total Number
 Collected 132 310 202 80 114 460 130

 May Sep Oct Dec
 1998 1998 1998 1998

Dorosoma cepedianum 0.0 0.0 0.0 0.0
Ameiurus melas 1.1 0.3 0.0 0.0
Cyprinella lutrensis 2.2 11.7 15.0 1.0
Hybognathus placitus 0.0 0.0 0.0 0.0
Notropis buccula 0.0 0.3 0.0 0.0
Pimephales promelas 0.0 6.6 0.0 0.0
 rubrofluviatilis 11.8 66.5 66.1 97.0
Fundulus zebrinus 83.9 14.6 18.9 1.8
Gambusia affinis 1.1 0.0 0.0 0.2
Lepomis cyanellus 0.0 0.0 0.0 0.0
Micropterus salmoides 0.0 0.0 0.0 0.0
Total Number
 Collected 93 349 301 595


We thank Chris Smith and Derek Marks for help in making field collections and Gary Garrett, Texas Parks and Wildlife Department, for providing information on the fish assemblage during March 1978. This is contribution T-09-834 of the College of Agricultural Sciences and Natural Resources, Texas Tech University.


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Gene R. Wilde and Kenneth G. Ostrand

Department of Range, Wildlife and Fisheries Management

Texas Tech University, Lubbock, Texas 79409

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Author:Wilde, Gene R.; Ostrand, Kenneth G.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Aug 1, 1999
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