The relationship of environmental factors to spatial and temporal variation of fish assemblages in a floodplain river in Texas, USA

Abstract –  Spatial and temporal variation of species–environment relationships were evaluated for shallow-margin and deep-water fish assemblages in the Brazos River, a large floodplain river in Texas, USA. Total variation among the deep-water assemblages (11 species, 86% turnover across gill net samples) was greater than for shallow-margins (38 species, 64% turnover across seine samples). For both shallow-margin and deep-water assemblages, variation was greater among sites than between winter and summer seasons. Shallow-margin assemblage structure was related to depth, velocity and substrate, whereas for deep-water assemblages river discharge, temperature and velocity were important. Season itself accounted for little of the variation among either shallow (6.7%) or deep-water (2.3%) assemblages. Overall temporal patterns of shallow-margin samples appeared to show responses to juvenile recruitment, spates and migration of coastal fishes, whereas for deep-water samples, patterns related to use of reproductive habitats, juvenile recruitment and seasonal activity levels. Brazos River assemblages were less variable overall in comparison with studies along similar length of reach in headwater streams and wadeable rivers. The residual variation in species distribution (54% for shallow-margin and 67% for deepwater) that was not explained by instream variables and season suggests a greater influence of biotic interactions in rivers, particularly those across the spatially dynamic interface of main channel habitats and shallow river margins.