Feeding Ecology of Early Life Stage Razorback Sucker Relative to Other Sucker Species in the San Juan River, Utah

Low survival of early life stage (ELS) Razorback Sucker Xyrauchen texanus in the Colorado River basin, USA, is thought to cause a recruitment bottleneck. Conversely, two other native sucker species, the Bluehead Sucker Catostomus discobolus and Flannelmouth Sucker C. latipinnis, successfully recruit to adulthood. To explore a potential factor leading to this survival discrepancy among species, we used museum‐catalogued specimens collected from the San Juan River, Utah, to investigate trophic resource use of co‐occurring ELS suckers. We evaluated diet diversity and composition, expecting high overlap in diets among sucker species. All species were significantly different in median SL and varied in the ranges of developmental stages present in collections, with Razorback Sucker generally being the smallest and least developed. Size differences among sampled individuals coincided with a 1.7× higher mean diet richness for Bluehead Sucker and Flannelmouth Sucker compared to Razorback Sucker. The Bluehead Sucker was the only species for which diet richness increased significantly with fish size. Discriminant function analysis was able to distinguish Razorback Sucker from the other two species based on a low frequency of occurrence of all diet items. Nine of twelve diet item categories differed significantly in frequency of occurrence among species. Bluehead Sucker and Razorback Sucker had higher mean carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) relative to Flannelmouth Sucker, and variation in δ¹³C was high among individual Flannelmouth Sucker across sites. The δ¹³C and δ¹⁵N of Bluehead Sucker decreased significantly with fish size, but no significant relationship was found for Flannelmouth Sucker or Razorback Sucker. The confounding issue of fish size and species identity in this data set prevented us from completely isolating either of these two effects on differences in diet richness, δ¹³C, and δ¹⁵N. Nevertheless, differences in diet item occurrence across sizes and species reflect differences in resource acquisition among ELS suckers that might be related to recruitment bottlenecks.