Detection of Larval Remains after Consumption by Fishes

In southwestern North America, consumption of native fish larvae by nonnative predators has imperiled native populations. Field‐acquired dietary analyses have provided little evidence of this cause‐effect relationship. In this study, small, nonnative green sunfish Lepomis cyanellus, bluegills L. macrochirus, red shiners Cyprinella lutrensis, fathead minnow Pimephales promelas, and yellow bullheads Ameiurus natalis were each fed a single larva of the native razorback sucker Xyrauchen texanus. Gut content analysis revealed that prey detection generally became increasingly difficult over a short postconsumption time period under laboratory conditions. For green sunfish, bluegills, and yellow bullheads, significant relationships between prey detection and time were revealed; the probability of prey identification was initially 50% or greater for about 30 min postconsumption, whereas few prey (3%) were identifiable at 60 min postconsumption. For red shiners and fathead minnow (pooled for analysis), no relationship was evident; these two species completely masticated their prey, thus hindering identification. Green sunfish and bluegills swallowed prey whole, and yellow bullheads damaged larvae during consumption. Many larvae were discovered in the foregut, and 25% were regurgitated during predator fixation. Use of gut content analysis as evidence of predation on native fish larvae by small, nonnative fish is problematic and unreliable due to rapid mechanical and chemical digestion of fragile larval tissues.