Abiotic filters like high gradients or hydraulic drops play an integral role in excluding species from interacting with some headwater communities. When humans manipulate the ecosystems to install small ponds in the headwaters of river networks, they allow stocked species of fishes to escape and bypass these filters creating novel communities already under stress from other environmental changes. We investigated a novel headwater community to assess if and how escaped Lepomis spp. compete with a native headwater fish, Chrosomus tennesseensis (Tennessee Dace). We designed an ex situ mesocosm study to test the effects of exploitative and interference competition by two species—L. macrochirus and L. cyanellus—on C. tennesseensis. We observed strong intraspecific competition that exceeded the effects of both Lepomis spp. on C. tennesseensis. Although one individual was always a clear winner in intraspecific interactions, morphology at the beginning of the experiment could not explain why one individual was more successful than another. We also observed marginally higher growth rate in C. tennesseensis when Lepomis spp. were caged rather than free-swimming indicating that introduced Lepomis spp. likely impact headwater fishes through exploitative and interference competition in addition to the potential of predation at large size differences. More research is needed to understand the breadth and magnitude of potential problems posed by the unintentional introduction of stocked fishes to low-order streams.

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