Fish Population Community Structure in Brazos
River Oxbow Lakes

KIRK O. WINEMILLER, SONER TARIM, JAMES B. COTNER, AND DAVID E. SHORMANN (Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843-2258)

The Brazos River is meandering lowland river that drains nutrient-rich forested and agricultural landscapes in central Texas. We have identified over 30 oxbow lakes in the broad floodplain of the river’s middle-lower reaches. These oxbows provide important habitat for flora and fauna of the region. Oxbow fish populations, community composition, and food-web structure was investigated for 3 yrs along a 160 km reach of the middle-lower Brazos River floodplain corridor. Several species, such as white crappie and spotted gar, were more common in oxbows, whereas others, such as red shiners and alligator gar, were most abundant in the active river channel. The age and size structure of several fish species showed significant between-lake variation that can be partially explained by a combination of habitat and biotic parameters. Old, shallow lakes dry out periodically and these unstable environments favor small fishes, like silversides and mosquitofish, that rebuild stocks rapidly whenever these lakes refill. Young lakes are deeper, more frequently flooded by the river, and contain a greater diversity and biomass of fishes. Oxbows with many long-lived piscivorous fishes tend to have lower densities of smaller planktivorous fishes and higher densities of zooplankton during the warmer months (possible evidence of a trophic cascade). Flooding history and species life-history strategies influence within-year variation in community structure, and oxbow communities undergo more or less predictable changes over the course of a year, plus longer-term successional changes associated with hydrology and sedimentation.