Diel Turnover of Assemblages of Fish and Shrimp on Sandbanks in a Temperate Floodplain River

Seven sandbanks in the main channel of the Brazos River, Texas, were sampled a total of 35 times during both day (1800-1900 hours) and night (2200-2300 hours) in June and July 2008 to examine the diel turnover of fish assemblages. Day samples had an average of 10.1 species, whereas night samples had an average of 12.9 species. Average abundance (catch per unit effort, here defined as the number of individuals per 10-m seine haul) for day samples was 41.5, compared with 80.5 for night samples. Species richness and abundance were significantly greater on sandbanks at night. Additionally, nonmetric multidimensional scaling analysis and cluster analysis indicated consistent differences in assemblage structure between day and night samples. Most of the diel change in assemblage structure was due to ictalurids and palaemonids (freshwater decapod crustaceans) that were only common on sandbanks nocturnally. Catastomids, clupeids, cyprinids, and poeciliids were present on sandbanks both diurnally and nocturnally. Species that exclusively use sandbank habitats may be foraging during the day and seeking refuge from predators at night. Nocturnal species apparently move onto sandbank habitats at night for foraging and then migrate to deeper, more structurally complex habitats during the day to escape predation. Diel turnover of sandbank fish assemblages may be a general phenomenon in lotic ecosystems, but it is apparently not as pronounced in temperate rivers as in tropical rivers. Diel changes in habitat use deserve greater study because species assemblage structure and habitat use are the basis for ecological niche models used to predict species distributions based on climate change scenarios as well as physical habitat simulation models designed to determine instream flows.