| Summary: | Red drum (Sciaenops ocellatus) is an estuarine-dependent species capable of survival in fresh and low salinity habitats. Standardized sampling by Texas Parks and Wildlife Department (TPWD) for various life stages of red drum occurs in estuaries, but not in tidal creeks and rivers. The goal of this study was to examine the individual variability of red drum low salinity occupancy patterns within the Mission-Aransas and Nueces estuaries using natural chemical tracer approaches. TPWD personnel obtained age 0-2 red drum using gill nets between November 2016 and June 2017. Stable isotope analysis of muscle tissue (n=201) and otolith microchemistry (n=99) were conducted to obtain migratory and dietary histories of individuals. Ward’s Hierarchical clustering analysis of muscle tissue δ13C and δ15N values was employed to determine distinct groupings of fish according to isotopic niche occupancy and derived partition coefficients for otolith chemistry (Sr:Ca and Ba:Ca ratios) were derived from the literature to identify movements into low salinity habitats. Based on analyses of muscle tissue stable isotopes and otolith microchemistry, two groups were found. The first group had an isotopic signature with lower δ13C and higher δ15N values compared to the second group. However, spatial analysis indicated that unique stable isotope compositions of bays explained differences between groups, and therefore all sampled red drum were most likely feeding within an estuarine environment. Ba:Ca otolith chemistry threshold values indicated 2-35% of individuals showed low salinity movement during life. Stable isotope signatures were not directly correlated with otolith microchemistry. The primary pattern of habitat use by red drum appears to be residency within specific bays of the estuary. However, the potential for red drum to move into areas of low salinity could be a useful facultative behavior for populations in a region experiencing inter-annual flood and drought events. Individuals that move into the tributaries of south Texas ...
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