<p>American eel (<i>Anguilla rostrata</i>) life history strategies and food web interactions are highly variable in temperate climates but not well understood in sub-tropical hydrologic systems. Intraspecific variability in isotopic niche breadth and overlap can have major implications for species of conservation concern by revealing dietary plasticity of populations across varied habitats. Muscle tissue stable isotope ratios of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) were analyzed to assess geographic variance in recent trophic histories within and among American eels collected in tributaries of the northwestern Gulf of Mexico and to quantify isotopic niche breadths and overlaps of capture groups. A total of eight specimen groupings were designated according to the riverine and coastal basins where they were collected, with one group comprised of only individuals caught within a wastewater treatment plant. Univariate analyses revealed highly variable isotopic ratios within and between groups. The probability of groups being found in the niche region of another group was low, with only six instances in which the probability of overlap was over 10%. The Colorado (C) group showed the highest probability of overlap of any group, being found in 86.57% of the Brazos-San Jacinto (B-SJ) group’s niche region. Collectively, these results illustrate the high degree of geographic variation in isotope ratios within and between regional habitats in an understudied portion of the species’ range.</p>