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Article Excerpt
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...
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