Abiotic determinants of fish assemblage structure in upper Brazos River, Texas
AuthorOstrands, Kenneth Gerard
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I studied potential mechanisms by which environmental conditions affect fish species distribution, abundance, and survival in the upper Brazos River drainage during 1997 and 1998. 1 examined spatial and temporal variation in fish abundance along the upper Brazos River drainage and determined important abiotic variables associated with fish assemblage patterns. There was considerable spatial variation in environmental conditions among the 13 sites sampled in the upper Brazos River that culminated in a downstream pattern of increasing species diversity and a change in species composition from primarily cyprinodontids upstream to an assemblage dominated by cyprinids downstream. Changes in species diversity and species composition were the result of environmental variability in specific conductance and current velocity among sites and streams. Relative abundance of cyprinids was inversely related and that of cyprinodontids was positively related, to increasing specific conductance. Conversely, cyprinid abundance was positively related and cyprinodontid abundance was inversely related to current velocity. In this system the erratic localized flow events and accompanying environmental conditions resulted in a mosaic of individual stream reaches with distinct environmental conditions and therefore distinct predictable species assemblages. Fish assemblage patterns were examined in isolated streambed pools in the upper Brazos River to examine fish assemblage structure and its association with environmental conditions. I examined 199 isolated pools sampled seasonally during 1997 and 1998 and focused on two questions. Are fish distributed in isolated pools by chance? If not, how is fish distribution related to environmental conditions? Then I examined 35 isolated pools during 1998 from tiie period of pool isolation until surface connections were reestablished following a rain event or pools completely dried and focused on two questions. Do environmental conditions change in an expected fashion as isolated pools shrink? If so, do fish assemblages change in response to changing environmental conditions in an expected manner? Fish assemblage patterns existed among pools and were strongly related to environmental conditions. Logistic regression identified pool volume and specific conductance to be important in determining species presence and absence in pools. The probability of Red River pupfish occurring in pools increased as specific conductance increased. Conversely, the probability of mosquitofish, red shiner, plains minnow, and sharpnose shiner occurring in pools decreased as specific conductance increased and pool volume decreased. Pools sampled through time showed abiotic changes to be deterministic. As isolated pools shrank specific conductance progressively increased and volume progressively decreased. Decreases in cyprinid abundance occurred concurrently with specific conductance increases and volume decreases; however, cyprinodontid abundance remained constant. Finally, I examined the physiological tolerance of the five most common species to maximum temperatures, maximum specific conductance, and minimum dissolved oxygen concentrations. Laboratory tests showed that cyprinodontids were more tolerant of lower dissolved oxygen concentrations, high temperatures, and high specific conductance than were cyprinids. Tolerances of cyprinids to abiotic factors precluded successful maintenance of populations in upstream reaches and isolated pools as they shrank, whereas cyprinodontids were limited to downstream reaches resulting m a species distribution that was strongly associated with physiological tolerances.