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The Influence of Environmental Factors on Spatial and Temporal Variation of Fish Assemblages in the Lower Brazos River, Texas

Raymond Li

Large floodplain rivers are spatially heterogeneous and temporally dynamic ecosystems. However, few studies have quantified the variation or species-environment relationships of fish assemblages in the main-channel of large rivers. Fishes were collected along a 10-km reach of the lower Brazos River, a large floodplain river in Texas. Collections targeted the 15th, 30th, and 50th percentile discharge rates of summer and winter seasons. My objectives were: (1) to compare fish assemblage structure in shallow river-margins versus deepwater habitats, (2) to evaluate the spatial and temporal variability of fish assemblages in these two habitats, (3) to identify species-environment relationships that likely structure these assemblages, and (4) to quantify the relative variation in assemblage structure as related to environmental versus seasonal sources. A total of 41 species and 28,469 individual fishes were collected. Assemblages were less variable than levels typically reported for streams and had weak species-environment relationships. Temporal variability of the shallow river-margin fish assemblage was primarily the result of juvenile recruitment, displacement of individuals following spates, or seasonal immigration by Mugil cephalus. Among the deepwater assemblages, increased movement associated with reproductive activities increased temporal variation. Spatial variation was detected only among deepwater assemblages and was related to velocity. Eighteen commonly collected species were evaluated for relationships with environmental variables and season.

Shallow river-margin assemblages were dominated by habitat-generalists and were most strongly differentiated by season, discharge and conductivity. Deepwater samples were dominated by Lepisosteus osseus and L. oculatus and were most strongly differentiated by velocity. For shallow river-margin and deepwater assemblages, environmental variables uniquely explained more of the total variation than season. Results of this study point to biotic factors as probably explaining a large proportion of the unexplained variation.

ACKNOWLEDGMENTS - I thank the many people who made the completion of this thesis possible. This thesis was completed with funding from the Texas Water Development Board, the Department of Wildlife and Fisheries Sciences at Texas A&M University, and through a Mill's Scholarship awarded by the Texas Water Resources Institute. Jeff Coleman of “Big Nuts Tree Farm” provided and maintained access to the Brazos River. Christine Burgess, Mike Kjelland, Michael Miller, and Virginia Shervette of the Department of Wildlife and Fisheries Sciences and Adam Cohen of the Texas Commission on Environmental Quality provided countless hours in the field with collections. I thank my committee chair, Dr. Fran Gelwick, for her guidance and enthusiastic support throughout this project. Drs. Kirk Winemiller and Anne Chin provided advice with field techniques and editorial comments that greatly improved this thesis. Thanks also to the students of the Department of Wildlife and Fisheries Sciences who contributed greatly to my education, both inside and outside the classroom. Finally, I thank my parents for all their support, and especially for fostering my ever-inquisitive mind and helping to me to appreciate the importance of 'ever-education'. My sincerest of thanks to one and all, and now let's go fish.

Note: This research was largely funded by the Texas Water Development Board. The research was assisted by a TWRI Mills Scholarship awarded to Raymond Li. The Chair of Raymond's committee was researcher Frances Gelwick of the TAMU Wildlife and Fisheries Sciences Department.

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