Movement into floodplain habitats by gizzard shad (Dorosoma cepedianum) revealed by dietary and stable isotope analyses

Floodplain habitats have been inferred to provide a variety of functions for aquatic organisms, yet few studies have documented movement between channel and aquatic floodplain habitats. We exploited spatial variation in stable isotope ratios of gizzard shad (Dorosoma cepedianum) to document movement between floodplain lakes and the main river channel of the Brazos River, Texas, during a period of frequent hydrologic connectivity. Additionally, we examined stomach contents of shad to determine if ontogenetic diet shifts or faunal exchange best explained variation in isotopic ratios. Regression analysis indicated significant relationships between gizzard shad size and isotopic ratios in oxbow lakes, whereas these relationships were not significant for the main channel. Plots of individual fish in each habitat suggested that adult shad migrated into oxbow lakes during floods whereas juveniles assimilated material produced in oxbows. Some adults in oxbows had signatures similar to juveniles, and these individuals were probably long-term oxbow residents. The proportion of adults with a river signature was greater in the oxbow with the shortest flood recurrence interval where opportunities for faunal exchange were more frequent. Analysis of stomach contents indicated almost total overlap between adult and juvenile diets indicating that movement between habitats having different isotopic ratios of basal resources rather than ontogenetic dietary shifts best explained patterns of isotopic variation in Brazos River gizzard shad.