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Spatial and temporal patterns in the fish assemblage of the Blanco River, Texas.

Abstract. -- Spatial and temporal patterns in occurrence, abundance, and habitat associations of the Blanco River drainage fish assemblage were examined among ten sites that were sampled quarterly for two years. Cyprinids comprised 78% of the overall assemblage, with Cyprinella venusta (41%), Pimephales vigilax (14%), and Notropis amabilis (11%) being the most abundant species. Variation in the fish assemblage was examined using canonical correspondence analysis. Physical habitat parameters explained 15.3%, followed by site (11.2%), and season (2.3%). The impact of low-head dams on the fish assemblage was also assessed. Low-head dam impoundment assemblage was markedly different from riverine mainstem sites (Analysis of Similarities, P < 0.01) in that the impoundment assemblage was dominated by more lentic species and generally lacked species normally associated with higher velocity runs and riffles.

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Identification of patterns in species diversity and abundance and their causal mechanisms have received much attention (Shmida & Wilson 1985; Brown & Maurer 1989). The causal mechanisms generally are subdivided into abiotic and biotic factors, and are evaluated on recent and localized scales (Brown & Maurer 1989; Matthews 1998). Abiotic factors include both physical and chemical characteristics of a stream (i.e., depth, current velocity, substrate, temperature, pH, dissolved oxygen, and turbidity) and can affect assemblages based on the autecology of species (Whiteside & McNatt 1972; Matthews 1998). Among the many abiotic factors, gradients in current velocity, depth, and substrate often most strongly associate with variation in fish assemblages at a local scale (Gorman & Karr 1978; Schlosser 1982; Cantu & Winemiller 1997; Walters et al. 2003; Williams et al. 2005). Biotic factors affecting fish assemblages include: intra- and interspecific competition, food availability, and predation (Matthews 1998). Understanding which factors are most strongly associated with the distributional patterns of stream fishes can reduce the error in predicting how fish assemblages might change as habitats are impacted by anthropogenic effects (Harding et al. 1998).

Although the southern United States has a highly diverse fish fauna (Burr & Mayden 1992, Warren et al. 1997), 28% are listed as extinct, endangered, threatened, or of special concern (Warren et al. 2000). Williams et al. (1989) list five factors contributing to the demise of North American fishes: habitat degradation, over-exploitation, disease, natural or anthropogenic-induced biotic factors, and restricted range. Warren et al. (2000), however, attribute the decline of native fishes of the southern United States primarily to habitat degradation.

Anthropogenic disturbance in the form of increased sediment and nutrient loads, introduced species, and altered hydrologic regimes associated with dams, is among the greatest threats to the freshwater fauna of the United States (Richter et al. 1997), and is cited as the reason for species declines across the country (Warren et al. 2000). Impoundment of streams reduces the connectivity of upstream and downstream segments (Edwards 1978) and among streams within a drainage (Herbert & Gelwick 2003), decreases the discharge (Bonner & Wilde 2000) and magnitude of floods (Adams 1985) downstream from impoundments, and creates a more lentic habitat within the impounded segment (Taylor et al. 2001). Collectively, these alterations adversely affect fish assemblages. Reduced stream connectivity restricts movement of fishes, resulting in reduced upstream diversity, extirpation of obligate riverine species (Winston et al. 1991; Porto et al. 1999), and dominance of the assemblage by habitat generalists (Winston et al. 1991; Taylor et al. 2001). When variable upstream reaches experience harsh conditions that cause a local extirpation of all fish from a stream reach, these reaches are subsequently repopulated by species surviving in stable, downstream habitats (Whiteside & McNatt 1972). Impoundments serve as a stable source from which upstream habitats are opportunistically colonized and are generally dominated by generalist species (Herbert & Gelwick 2003). With obligate riverine species being absent from reservoirs, even temporary cessation of flow upstream from an impoundment might lead to the permanent extirpation of fluvial specialists and an assemblage dominated by habitat generalists. Downstream from dams, changes in habitat caused by scouring of substrate (Gillette et al. 2005) and reduced peak discharge (Bonner & Wilde 2000) contribute to changes in fish assemblages.

The purpose of this study was to identify factors important in structuring the Blanco River fish assemblage and determine the effects of low-head dams within the watershed. The Blanco River is a stream system typical of the Texas hill country and Edwards Plateau characterized by low turbidity and high dissolved solids. Additionally, these streams possess many endemic taxa, about which, little is known. These stream systems face several threats (Bowles & Arsuffi 1993) including low-head dams as Texas leads the nation with over 6,000 dams constructed in its waters (Shuman 1995). The effects of low-head dams on the fish assemblages of streams of the Texas Hill Country are not known. Description of the current fish assemblage and identification of factors structuring the assemblage will allow for determination of future changes in the Blanco River fish assemblage and prediction of impacts of anthropogenic disturbance within the watershed on regional and drainage endemic species (e.g., Dionda nigrotaeniata, Macrhybopsis marconis, and Micropterus treculii) as well as the overall assemblage. Specifically, the objectives were to determine current habitat and fish assemblage structure and identify habitat associations, longitudinal and seasonal patterns, and effects of low-head dams on the Blanco River fish assemblage.

METHODS

The Blanco River drains an area of 1,067 [km.sup.2] (USGS 2003) in Kendall, Comal, Blanco, and Hays counties, Texas, before its confluence with the San Marcos River (Fig. 1). Little Blanco River and Cypress Creek are the two largest tributaries of the Blanco River. Both tributaries are spring-fed although baseflow in the Little Blanco River is subterranean about 5 km before reaching the Blanco River.

Ten sites in the Blanco River watershed were sampled quarterly from October 2003 through July 2005. Eight sites were located on the mainstem with two upper (sites 1 and 2), two middle (sites 3 and 4), and four lower reach sites (sites 5, 6, 7, and 8). Two sites were established on major tributaries with one on the Little Blanco River (Site 9) and one on Cypress Creek (Site 10).

At each site, fish were collected from available geomorphic units (i.e., runs, riffles, pools, backwaters, reservoirs, and plunge pools; Arend 1999) by a combination of seining (9.5 mm mesh), backpack electrofishing (Smith-Root Model 12-B POW), and experimental gill nets (three nets set for two hours). Seines were used at all sites, backpack electrofishing was used in areas not conducive for seining (i.e., around cover, large woody debris, boulders, and shallow riffles) and gill nets were used at Site 2 (Reservoir Site) in deepwater (>2 m) habitats. Fish were collected from each geomorphic unit until fish were depleted from the geomorphic unit (only a few individuals were captured) and no new species was collected (Williams & Bonner 2006). Fish from each geomorphic unit were isolated in buckets until sampling was completed in all geomorphic units. Fish were identified to species, measured to the nearest millimeter total length (up to 30 specimens per species per site), and released or retained as voucher specimens. Voucher specimens were anesthetized with a lethal dose of tricaine methanesulfonate and preserved in 10% formalin.

[FIGURE 1 OMITTED]

Habitat parameters recorded include geomorphic unit type, length, stream width, percent substrate (silt, sand, gravel, cobble, boulder, and bedrock), percent woody debris, percent vegetation, percent detritus, mean current velocity (m/s), maximum current velocity (m/s), mean depth (m), maximum depth (m), temperature ([degrees]C), pH, conductivity ([micro]S/cm), dissolved oxygen (mg/L), and turbidity (NTU). Geomorphic unit length and width were measured to the nearest meter. Percent substrate, woody debris, vegetation, and detritus were visually estimated for each geomorphic unit (Williams et al. 2005). Depth was recorded to the nearest 0.01 m and current velocity was measured using a Marsh-McBirney FLOW-MATE[TM] model 2000 flow meter. Temperature and chemical parameters were measured using YSI-Model 85 and YSI-Model 650 water quality instruments. Site estimates of physical and chemical data were calculated by weighted averaging by geomorphic unit area. Stream discharge was obtained from USGS gauging stations No. 08171000 (Wimberley, Texas) and No. 08171300 (Kyle, Texas).

Principal components analysis (PCA) was performed using site means of physical habitat data. Qualitative data (i.e., geomorphic units) were represented with dummy variables whereas quantitative data were z-score transformed (Krebs, 1999). The resulting loadings and plots were used to describe habitat present at each site. Fish abundance and habitat data were analyzed using canonical correspondence analysis (CCA; Canoco 4.5, ter Braak 1986) to determine habitat associations as well as seasonal, site, and habitat effects in structuring the Blanco River fish assemblage. A variance partitioning method (Borcard et al. 1992) was used to determine pure site, season, and habitat effects as well as shared (two- and three-way) effects by producing a reduced CCA model for each effect with the additional two effects as covariates. Species richness, Shanon-Wiener diversity indices, and Pielou's evenness indices were calculated in PRIMER (version 5; Primer-E, Ltd., Plymouth, United Kingdom) for each site per each quarter. Bray-Curtis similarity indices were calculated for species abundance data pooled by season for each site. Species abundance data were standardized as relative abundances because sampling effort (i.e., area sampled) was not equal among sites. The resulting similarity matrix was used in analysis of similarities (ANOSIM; Clarke & Green 1988; Clarke 1993) to test for differences in fish assemblage structure among sites within, adjacent to, and distant from impoundments. Sites adjacent to impoundments were defined as those within 1 km of an impoundment whereas sites distant from impoundments were greater than 1 km from an impoundment. Distance to impoundment was determined along the thalweg by examination of aerial photographs and topographical maps. Determination of which species were contributing the greatest amount to the dissimilarity between the impoundment and the other categories was accomplished using the SIMPER function in PRIMER.

RESULTS

The Blanco River, Little Blanco River, and Cypress Creek generally were shallow to moderate depth wadeable streams with substrate dominated by bedrock with some coarse gravel (Table 1). Sites ranged in length from 43 to 325 m depending on heterogeneity of available habitats and accessibility. The tributaries, Little Blanco River and Cypress Creek, were deeper, had slower current velocities, greater percentages of gravel substrate, and more aquatic vegetation and detritus than mainstem sites. Mainstem sites were wider and shallower with swifter current velocities and greater percentages of bedrock substrate. Sand and boulder substrates were uncommon across all sites but were both highest at Site 5. Among mainstem sites, the Reservoir Site (Site 2) and Site 8 had the greatest amount of vegetation (filamentous algae and emergent macrophytes: 22%) whereas no vegetation was present at Site 5. During the duration of the study, median discharge was 4.25 [m.sup.3]/s between sites 4 and 5 (USGS Station No. 08171000) and 3.96 [m.sup.3]/s between sites 6 and 7 (USGS Station No. 08171300). Stream discharge was lowest in late summer and fall and increased sharply in the spring in both years of the study (Fig. 2). Across sites and dates, mean temperature was 20[degrees]C, pH was 9.18, conductivity was 441 [micro]S/cm, dissolved oxygen was 8.77 mg/L, and turbidity was 2.9 NTU. Seasonal and diel water quality and geochemistry measurements were taken concurrently with this study and reported by Cave (2006).

Principal component axes I and II explained 37% of the variation in habitat among sites within the Blanco River watershed. PC I represented a substrate gradient whereas PC II represented a velocity, depth, and substrate gradient (Fig. 3). Strongest positive loadings for PC I were gravel (0.52), woody debris (0.34), and vegetation (0.32); strongest negative loadings were bedrock (-0.52), boulder (-0.16), and sand (-0.16). Strongest positive loadings for PCII were silt (0.56), depth (0.38), and vegetation (0.36); strongest negative loadings were current velocity (-0.58), bedrock (-0.18), and gravel (-0.16). Site 1 and the Reservoir Site had higher percentages of bedrock and silt substrate. The Reservoir Site differed from Site 1 by having greater vegetation and depth. Site 4 had a high percentage of bedrock and gravel and the lowest percentage of silt. Sites 5 and 6 had greater percentages of bedrock and boulder substrates and higher current velocities whereas sites 3 and 8 and Little Blanco River had greater percentages of gravel substrate with sites 3 and 8 having greater percentages of cobble and sites 8 and Little Blanco River having greater amounts of woody debris. Cypress Creek had a greater percentage of bedrock and cobble substrate and detritus.

[FIGURE 2 OMITTED]

A total of 29,265 fishes representing 10 families and 33 species was collected from October 2003 through July 2005 within the Blanco River watershed (Table 2). Overall fish abundance was highest at Site 2 (N = 6,586) and Site 8 (N = 4,319) and lowest at Site 4 (N = 1,213) and Cypress Creek (N = 1,273). Most abundant families were Cyprinidae (78%), Centrarchidae (10%), and Poeciliidae (8%). Lepisosteidae, Catostomidae, Characidae, Ictaluridae, Fundulidae, Percidae, and Cichlidae each comprised less than 2% of the overall assemblage. Cyprinella venusta (41%), Pimephales vigilax (14%), Notropis amabilis (11%), Gambusia affinis (8%), and Notropis volucellus (5%) were the most abundant species. Cyprinidae (N of species = 11) and Centrarchidae (N of species = 10) were the most species rich families. Species richness ranged from 3 to 15 among samples with the lowest richness occurring at Site 4 in January 2004 and the highest richness occurring at Site 8 in October 2003 and at Site 7 in July 2005. Mean Shannon Diversity and Pielou's evenness were highest at the Little Blanco River and Cypress Creek and lowest at Site 5 (Table 3). Highest individual sample diversities occurred at the Little Blanco River in July 2005 (2.37) and July 2004 (2.09). Lowest individual sample diversities occurred at Site 1 (0.33, July 2004) and Site 4 (0.54, January 2005).

[FIGURE 3 OMITTED]

Cyprinids generally were more abundant in the mainstem and Cypress Creek, systems with more persistent flows, whereas centrarchids and poeciliids generally were more abundant in Little Blanco River, a stream with intermittent flows near its confluence with the Blanco River mainstem. Cyprinidae (81%) was the most abundant family in the Blanco River, followed by Poeciliidae (8%) and Centrarchidae (7%). Likewise, Cyprinidae (69%) was the most abundant family in Cypress Creek, followed by Centrarchidae (20%), Poeciliidae (4%), Percidae (4%), and Cichlidae (3%). In the Little Blanco River where pool habitats were common, Centrarchidae (42%) was the most abundant family, followed by Cyprinidae (38%), Poeciliidae (14%), Percidae (4%), and Catostomidae (2%).

Among fishes with a relatively small geographic range, Dionda nigrotaeniata was only present in the tributaries with its greatest abundance occurring in the Little Blanco River (1.8%). Three individuals of Macrhybopsis marconis were collected at Site 8. Notropis amabilis occurred at all sites and was abundant (> 2%) at six sites. Moxostoma congestum occurred at six sites and was most abundant at the Little Blanco River (2.2%) and the Reservoir (0.9%). Fish initially identified as Micropterus treculii based on morphology (Hubbs 1991) were present at four sites with a relative abundance < 0.3% at all sites and 0.04% overall. However, subsequent genetic analyses conducted on the Blanco River population failed to detect pure M. treculii in the population, which suggests that only Micropterus dolomieu x M. treculii hybrids exist in the Blanco River drainage (Littrell et al. 2007).

Physical parameters, site, and season accounted for 40% (P < 0.01) of the variation in the Blanco River drainage fish assemblage. Pure effects of physical parameters accounted for 15.3% (P < 0.01), site accounted for 11.2% (P < 0.01), and season accounted for 2.3% (P < 0.01) of fish assemblage variation. Two- and three-way shared effects among physical parameters, site, and season accounted for 10.7% of fish assemblage variation. Physical parameters with the strongest positive centroids for the first canonical axis (CA I) were riffle (1.57), side channel (0.64), and maximum velocity (0.56). Physical parameters with the strongest negative centroids for CA I were reservoir (-1.28), pool (-0.78), and silt (-0.54). Within the mainstem of the Blanco River, CA I centroids were negative for sites 1 through 3 and positive for sites 4 through 8. Among the tributaries, Little Blanco River had a negative centroid whereas Cypress Creek had a positive centroid. CA I expressed a gradient from upstream sites with slow current velocities, greater depths, silt substrate, and detritus to downstream sites with faster current velocities, shallower depths, and cobble substrate. Physical parameters with the strongest positive centroids for the second canonical axis (CA II) were riffle (1.00), backwater (0.54), and detritus (0.27). Physical parameters with the strongest negative centroids for CA II were sand (-0.72), plunge pool (-0.70), run (-0.52), and boulder (-0.48). CA II expressed a weaker habitat gradient from shallow backwaters to deeper runs with sand substrate. The strongest negative loadings of CA II described the habitat at Site 5. Summer and fall had negative centroids for CA I and winter and spring had positive centroids for CA I, however, these centroids were generally weak.

Species with the strongest positive associations with CA I include Percina sciera, Pimephales promelas, Percina carbonaria, Etheostoma spectabile, and Ameiurus natalis (Fig. 4). Species with the strongest negative associations with CA I include Lepomis microlophus, Micropterus salmoides, Cyprinus carpio, Cyprinella lutrensis, and Lepomis gulosus. Along the habitat gradients expressed by CA I and CA II, P. carbonaria (N = 20) and E. spectabile (N = 540) were strongly associated with riffles having high current velocities, shallow depths, and intermediate-size substrate such as gravel and cobble. Cobble and gravel substrates were dominant at Site 8 where E. spectabile relative abundance was highest among mainstem sites (Table 2). Cyprinella venusta (N = 11,918) and G. affinis (N = 2,419) were associated with intermediate currents and showed no strong substrate affinities. Campostoma anomalum (N = 1,160) and Ictalurus punctatus (N = 110) were associated with intermediate current velocities, shallow depths, and cobble substrate. Notropis amabilis (N = 3,308) and M. dolomieu (N = 33) were associated with intermediate current velocities and coarse substrates. Fish species associated with deep, low-velocity habitats with greater amounts of vegetation and detritus included C. lutrensis (N = 5), C. carpio (N = 3), D. nigrotaeniata (N = 36), P. vigilax (N = 4,136), M. congestum (N = 115), Lepomis cyanellus (N = 165), L. gulosus (N = 6), Lepomis macrochirus (N = 795), Lepomis megalotis (N = 511), L. microlophus (N = 10), and M salmoides (N = 196). Species strongly associated with pool and reservoir habitats included C. carpio, C. lutrensis, P. vigilax, M. congestum, L. gulosus, L. macrochirus, and L. megalotis.

Although pure site effects explained only 11.2% of fish assemblage variation, several species showed strong site affinities. Macrhybopsis marconis (N = 4), Astyanax mexicanus (N = 15), Poecilia latipinna (N = 1), Lepomis punctatus (N = 16), and P. sciera (N = 1) occurred exclusively at sites 7 and 8 (Table 2) downstream from the lowermost low-head dam on the Blanco River and nearest to the confluence with the San Marcos River. Cyprinus carpio was collected only from the Reservoir Site and in a deep pool at Site 4. Etheostoma spectabile, P. carbonaria, and P. sciera were present downstream of the falls at Site 4 and were absent above the falls. Dionda nigrotaeniata was present in the tributaries (Little Blanco River and Cypress Creek) but was absent from mainstem sites.

[FIGURE 4 OMITTED]

Sites were grouped into four categories to test for influence of low-head dams on fish assemblages. Site categories were "adjacent to low-head dams (<1 km)" (sites 1 and 7); "impounded by a low-head dam" (Reservoir Site); "distant from a low-head dam (>1 km)" (sites 3, 4, 5, 6, and 8), and "tributaries" (Little Blanco River and Cypress Creek). Analysis of similarities (Table 4) indicated that differences existed (P < 0.01) among fish assemblages relative to low-head dams. Pair-wise tests indicated significant differences between impounded and adjacent site assemblages (P < 0.01), impounded and distant site assemblages (P = 0.04), adjacent and tributary site assemblages (P < 0.01), and distant and tributary site assemblages (P < 0.01). Fish assemblage differences were not detected between impounded and tributary site assemblages (P = 0.07) or between adjacent and distant site assemblages (P = 0.70). Fish assemblage differences between impounded and adjacent and between impounded and distant assemblages were attributed to the large number of P. vigilax (61%) and lower number of C. venusta (20%) in the Reservoir Site relative to riverine sites.

DISCUSSION

Physical habitat parameters, site effects, and seasonal effects explained significant amounts of variation within the Blanco River fish assemblage. However, the amount of variation explained by season was small whereas physical habitat parameters explained a relatively large amount of variation. Few species (i.e., N. volucellus, P. vigilax, G. affinis, and M. salmoides) showed strong seasonal trends in relative abundances. Relative abundance was highest for G. affinis and N. volucellus in the fall. Mean length was lowest for both species in fall, thus the higher relative abundances may represent an abundance of juveniles prior to significant seasonal mortality (Matthews 1998). The peak in P. vigilax relative abundance occurred in winter and resulted from a single seine haul at the Reservoir Site. Micropterus salmoides abundance was highest in summer when age-0 fish were small and unable to escape from seines. This presumably resulted in a greater capture rate for smaller individuals (Weinstein & Davis 1980).

The extent of site associations was highly variable among species. Dionda nigrotaeniata typically inhabits spring-influenced headwaters (Hubbs et al., 1991) and showed a strong site affinity for the Little Blanco River and Cypress Creek. Ameiurus natalis was collected exclusively in Cypress Creek and is often associated with small, clear, rock- or gravel-bottomed streams (Robison & Buchanan 1988). All percids (i.e., E. spectabile, P. carbonaria, and P. sciera) were absent above Site 4. This observation is consistent with reports by the Texas Game and Fish Commission (now Texas Parks and Wildlife; 1957), indicating that the falls at Site 4 present an apparent barrier to the upstream movement of fishes, and contribute to an abrupt discontinuity in assemblage variation. Macrhybopsis marconis, A. mexicanus, P. latipinna, and P. sciera occurred only at sites 7 and 8. These fishes were rare, and likely represent vagrants from the San Marcos River, as larger, more stable water bodies serve as species pools from which less stable upstream habitats are colonized (Whiteside & McNatt 1972). Spatial variation within the Blanco River fish assemblage across geographically distant sites (i.e., tributary vs. mainstem and upstream vs. downstream) likely represents differences in stream processes (Wilkinson & Edds 2001). The relatively small amounts of spatial and seasonal variation suggest adequately stable habitats with assemblages primarily structured by local habitat parameters (Meador & Matthews 1992).

Physical habitat parameters were found to be the primary factors structuring fish assemblages. The first two canonical axes of CCA both represented gradients best described as velocity and substrate gradients. Along these gradients, centrarchids were generally more abundant in lentic type habitats with lower velocities and greater percentages of silt substrate and vegetation whereas percids were more abundant in shallow lentic habitats such as riffles dominated by cobble and gravel substrate. Cyprinids, however, exhibited a much wider range of habitat associations. For example, C. anomalum was strongly associated with shallow riffles whereas N. amabilis was associated with runs and C. venusta did not exhibit any strong associations. Gorman & Karr (1978) noted that stream depth, current velocity, and substrate are important in structuring stream fish assemblages. Greater variability in these components results in increased habitat complexity which regulates local assemblages of fish as stream fish are commonly habitat specialists (Mendelson 1975).

The low-head dam at the Reservoir Site created a distinctly lentic habitat in which centrarchids were abundant, as were the ubiquitous C. venusta and P. vigilax. Santucci et al. (2005) reported major differences in habitat quality between impounded and free flowing reaches such as higher turbidity, lower dissolved oxygen minima (as low as 2.5 mg/L), and homogenization of habitats. Homogenization of habitats can lead to increased abundance of generalist native species (Scott & Helfman 2001) and loss of native stream specialists (Boet et al. 1999). In addition to habitat alterations, fish assemblages within reservoirs may be altered by purposeful introductions of sportfish species as well as incidental introductions via bait-buckets (Taylor et al. 2001). The Reservoir Site ranked sixth in species richness with 17 species collected, while the mean species richness across all sites was 17.3 species. Although species richness at the Reservoir Site was nearly average for the sites, the structure of the assemblage at this site was significantly different from that of sites both adjacent to and distant from reservoirs. Campostoma anomalum, M. dolomieu, and M. treculii, which typically inhabit swifter waters, were rare or absent from the Reservoir Site, but were present and more abundant at Site 3. Gillette et al. (2005) and Taylor et al. (2001) reported similar shifts in assemblages with lower abundances of fishes normally associated with higher current velocities, such as percids and stream dwelling micropterids. Additionally, the absence of percids at the Reservoir Site is attributed to the natural barrier present at Site 4 and is likely not the result of the low-head dam.

The Little Blanco River consisted mostly of a series of pools due to low-flow conditions during the study, and was dominated by lentic species (i.e., centrarchids). In contrast, the consistently flowing Blanco River and Cypress Creek were dominated by cyprinids. Periods of low flow and increased centrarchid abundance likely resulted in the relatively high degree of similarity between tributaries and the Reservoir and a lack of a significant difference between the two treatment groups in analysis of similarities. Although no differences were detected among assemblages at sites either adjacent to or distant from impoundments, it cannot be concluded that low-head dams do not impact the fish assemblage as there is no comparison to the pre-impoundment assemblage structure.

Although no pre-impoundment assemblage data are available, several changes in occurrence have taken place since the Texas Parks and Wildlife Department survey in 1957. Dorosoma cepedianum, Carpiodes carpio, Notemigonus crysoleucas, Notropis buchanani, Ameiurus melas, Pomoxis annularis, and Etheostoma lepidum were not collected in our sampling efforts, but were present, though relatively rare, in the 1957 collection. Species present in our collections but absent from the 1957 collection include the introduced species P. promelas, P. latipinna, and M. dolomieu, as well as P. sciera which is likely a vagrant from the San Marcos River.

Physical habitat parameters, site, and season all influenced Blanco River fish assemblage. However, there is a substantial amount of variability in the assemblage that is not explained by the present model. This variability may reflect unmeasured or inestimable factors such as land use, riparian vegetation, or biological interactions that act to structure the fish assemblage at the local and watershed level. Although these unmeasured factors are likely to influence the structure of the fish assemblage, current velocity, depth, and substrate often adequately account for variation in fish assemblages at a local scale (Gorman & Karr 1978; Schlosser 1982; Cantu & Winemiller 1997; Walters et al. 2003; Williams et al. 2005). Such factors, comprising major aspects of stream morphology, are likely to shift in response to anthropogenic disturbance (Odemerho 1984; Golladay et al. 1987), and the fish assemblage can be expected to track with such shifts.

Among the threats to the Blanco River and other Texas Hill Country streams, excessive groundwater pumping is the greatest (Bowles & Arsuffi 1993). Continued excessive pumping, especially during drought, will likely result in a loss of spring associated headwater specialists such as D. nigrotaeniata. As rapid urbanization continues, changes in geomorphology and hydrology will likely include increased impervious cover resulting in higher but shorter-duration hydrograph peaks and changes in substrate composition resulting from siltation, similar to changes in nearby and urbanized Waller Creek in Austin, Texas (Swezey 1991). Such changes in stream characteristics will likely influence changes in fish assemblage structure as several species in the Blanco River are strongly associated with particular substrate types (e.g., percids).

ACKNOWLEDGMENTS

The River Systems Institute at Texas State University, The Nature Conservancy of Texas, and Peter Way (Way Ranch) provided partial funding for this project. We thank Megan Bean, Casey Williams, and Jackie Watson for assistance in field collections and Peter Way, Hall and Pat Hammond, Jerry and Diane Turner, Dorothy Gumbert, Bill Buchanan, and Don and Nelle Still for river access. A. W. Groeger and D. G. Huffman provided comments on earlier versions of this manuscript.

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PTB at: preston.bean@txstate.edu

Preston T. Bean, Timothy H. Bonner and Bradley M. Littrell

Department of Biology/Aquatic Station

Texas State University-San Marcos, 601 University Drive

San Marcos, Texas 78666
Table 1. Mean ([+ or -] SD) physical habitat parameters across sampling
dates for eight sites on the Blanco River, one site on the Little Blanco
River, and one site on Cypress Creek sampled between October 2003 and
July 2005.

 (Reservoir) Main Stem
 1 2 3 4 5
 Mean SD Mean SD Mean SD Mean SD Mean SD

Width (m) 15.1 4.8 35.0 0.0 11.9 3.3 13.9 2.9 44.1 3.2
Depth (m) 0.4 0.3 0.8 0.0 0.5 0.1 0.6 0.2 0.5 0.2
Current 0.2 0.3 0.0 0.0 0.2 0.1 0.2 0.2 0.3 0.2
 Velocity
 (m/s)
Substrate
 Type (%)
 Silt 25.9 31.9 43.3 6.2 25.0 23.0 0.9 2.4 2.8 3.6
 Sand 3.8 7.4 0.0 0.0 1.4 2.6 0.0 0.0 6.3 10.3
 Gravel 2.8 4.4 1.7 1.5 24.9 20.6 12.1 12.6 15.9 11.3
 Cobble 1.3 2.3 1.7 1.5 31.5 23.9 1.3 2.3 8.1 6.9
 Bedrock 64.9 37.2 51.7 1.5 13.8 12.7 83.1 16.4 53.4 22.9
 Boulder 1.3 2.3 1.7 1.5 3.5 3.5 2.7 5.7 13.4 14.6
Detritus (%) 12.5 35.4 7.5 9.8 0.8 2.3 0.0 0.0 0.0 0.0
Woody Debris 0.8 0.7 0.8 0.6 1.8 1.3 2.4 4.8 1.7 1.5
 (%)
Vegetation 2.6 3.5 22.3 21.2 6.7 8.0 11.1 26.9 0.0 0.0
 (%)

 Little Cypress
 Main Stem Blanco Creek
 6 7 8 9 10
 Mean SD Mean SD Mean SD Mean SD Mean SD

Width (m) 19.6 5.6 16.1 4.7 12.5 8.6 11.5 2.3 11.3 3.0
Depth (m) 0.4 0.1 0.4 0.1 0.5 0.1 0.9 0.3 0.3 0.0
Current 0.6 0.2 0.4 0.2 0.4 0.4 0.1 0.1 0.2 0.2
 Velocity
 (m/s)
Substrate
 Type (%)
 Silt 3.3 3.2 20.9 29.4 17.8 24.3 11.6 27.7 13.3 17.1
 Sand 0.0 0.0 0.5 1.4 0.4 0.5 0.1 0.4 0.5 1.4
 Gravel 45.4 11.4 31.2 30.2 47.9 31.4 77.8 26.4 9.5 13.6
 Cobble 8.3 7.9 9.1 8.8 33.2 30.2 8.9 12.8 29.6 20.4
 Bedrock 42.2 18.0 38.2 27.3 0.2 0.4 0.0 0.0 38.2 21.3
 Boulder 0.9 1.6 0.0 0.0 0.5 1.1 1.5 1.8 9.0 14.0
Detritus (%) 3.2 8.3 0.0 0.0 0.0 0.0 10.8 20.3 18.8 24.7
Woody Debris 2.8 5.8 1.7 1.8 9.3 13.3 3.5 1.6 4.9 2.3
 (%)
Vegetation 1.6 2.6 5.8 4.6 21.6 29.4 40.6 44.4 5.1 13.1
 (%)

Table 2. Relative abundances of fishes collected from the Blanco River,
Little Blanco River, and Cypress Creek across all sampling dates.

 Site
 1 Res. 3 4 5

Lepisosteus osseus - - - - -
Campostoma anomalum 7.8 0.1 4.4 13.4 0.2
Cyprinella lutrensis - 0.1 0.1 - -
Cyprinella venusta 70.0 20.2 41.1 37.6 61.6
Cyprinus carpio - 0.03 - 0.1 -
Dionda nigrotaeniata - - - - -
Macrhybopsis marconis - - - - -
Notropis amabilis 1.7 11.5 28.3 19.7 8.4
Notropis stramineus - - - 1.4 16.9
Notropis volucellus - - - 3.0 6.1
Pimephales promelas - - - - -
Pimephales vigilax 0.03 61.3 0.3 0.5 -
Moxostoma congestum - 0.9 0.2 0.1 -
Astyanax mexicanus - - - - -
Ameiurus natalis - - - - -
Ictalurus punctatus 0.6 0.1 1.5 2.1 0.1
Fundulus notatus - - - - -
Gambusia affinis 3.9 1.2 2.4 13.5 3.0
Poecilia latipinna - - - - -
Lepomis auritus 3.0 2.0 9.9 2.6 1.1
Lepomis cyanellus 0.8 0.03 3.5 - 0.03
Lepomis gulosus 0.1 0.03 - - -
Lepomis macrochirus 4.5 2.0 0.2 - 0.4
Lepomis megalotis 4.0 0.3 6.1 4.5 0.9
Lepomis microlophus 0.1 0.02 0.2 - -
Lepomis punctatus - - - - -
Micropterus dolomieu - - 0.2 0.6 0.3
Micropterus salmoides 3.5 0.3 0.5 0.5 -
Micropterus treculi - - 0.3 - 0.1
Etheostoma spectabile - - - - 0.6
Percina carbonaria - - - 0.6 -
Percina sciera - - - - -
Cichlasoma cyanoguttatum - 0.1 1.0 - 0.4
N = 3,303 6,586 1,766 1,213 2,928

 Site
 6 7 8 LBR CypCr

Lepisosteus osseus 0.1 - - - -
Campostoma anomalum 5.2 5.0 2.2 4.9 12.3
Cyprinella lutrensis - - - - -
Cyprinella venusta 36.2 65.2 39.7 5.8 33.9
Cyprinus carpio - - - - -
Dionda nigrotaeniata - - - 1.8 0.4
Macrhybopsis marconis - - 0.1 - -
Notropis amabilis 1.4 5.2 19.5 11.3 21.8
Notropis stramineus 4.2 0.3 0.2 - -
Notropis volucellus 1.3 7.7 20.8 9.3 0.2
Pimephales promelas 0.03 - 0.02 - -
Pimephales vigilax - - 0.3 4.5 -
Moxostoma congestum 0.03 0.1 0.1 2.2 0.4
Astyanax mexicanus - 0.3 0.2 - -
Ameiurus natalis - - - - 0.2
Ictalurus punctatus 0.4 0.2 0.2 - 0.1
Fundulus notatus - 0.9 0.1 - 0.4
Gambusia affinis 38.6 2.6 6.1 14.3 3.8
Poecilia latipinna - - 0.02 - -
Lepomis auritus 1.3 5.0 2.1 11.4 16.0
Lepomis cyanellus - 0.1 - 4.0 0.1
Lepomis gulosus - - - - 0.2
Lepomis macrochirus 3.2 0.8 1.3 17.9 0.2
Lepomis megalotis 0.6 0.3 0.2 6.0 2.7
Lepomis microlophus - - - 0.2 -
Lepomis punctatus - 0.6 - - -
Micropterus dolomieu 0.1 0.04 - 0.2 0.5
Micropterus salmoides 0.1 0.04 - 2.4 0.2
Micropterus treculi - - 0.1 0.1 -
Etheostoma spectabile 5.4 3.3 3.3 3.8 3.6
Percina carbonaria - 0.04 0.3 0.1 -
Percina sciera - - 0.02 - -
Cichlasoma cyanoguttatum 2.0 2.3 3.2 - 3.1
N = 3,326 2,820 4,319 1,731 1,273

Table 3: Mean seasonal species richness (S), Shannon diversity (H'), and
Pielou's evenness (J') for eight Blanco River sites, the Little Blanco
River, and Cypress Creek between October 2003 and July 2005.

 Fall Winter
Site: S H' J' S H' J'

Site 1 10.5 1.08 0.46 9.5 1.32 0.61
Reservoir 10.0 1.67 0.73 8.5 1.16 0.54
Site 3 10.0 1.48 0.64 9.0 1.30 0.62
Site 4 6.5 1.27 0.70 3.5 0.66 0.53
Site 5 7.0 0.88 0.45 11.0 1.20 0.50
Site 6 7.0 1.03 0.55 8.5 1.25 0.58
Site 7 12.0 0.87 0.35 11.5 1.77 0.73
Site 8 14.0 1.71 0.65 12.0 1.03 0.42
LBR 10.5 1.74 0.74 9.0 1.73 0.79
CypCr 10.0 1.50 0.65 11.0 1.81 0.76

 Spring Summer
Site: S H' J' S H' J'

Site 1 7.0 1.38 0.76 9.0 0.91 0.41
Reservoir 9.0 1.35 0.62 9.5 1.21 0.55
Site 3 9.5 1.21 0.54 12.5 1.38 0.55
Site 4 6.5 1.34 0.72 9.5 1.49 0.68
Site 5 9.0 1.18 0.56 9.0 1.02 0.47
Site 6 11.5 1.37 0.56 11.0 1.25 0.52
Site 7 11.5 1.17 0.48 12.5 1.29 0.51
Site 8 11.0 1.25 0.52 8.0 0.82 0.40
LBR 10.0 1.77 0.77 12.5 2.23 0.88
CypCr 8.5 1.54 0.72 7.5 1.20 0.58

Table 4: Results of ANOSIM global and pair-wise tests for differences in
assemblage between sites impounded by a low-head dam, adjacent to a low-
head dam, distant from a low-head dam, and tributaries.

 R P value

Global Test 0.264 < 0.01
Pairwise Tests
 Near vs. Impounded 0.654 < 0.01
 Near vs. Distant -0.063 0.70
 Near vs. Tributary 0.585 < 0.01
 Impounded vs. Distant 0.346 0.04
 Impounded vs. Tributary 0.267 0.07
 Distant vs. Tributary 0.386 < 0.01
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Author:Bean, Preston T.; Bonner, Timothy H.; Littrell, Bradley M.
Publication:The Texas Journal of Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2007
Words:7702
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