Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—The pumpkinseed <em>Lepomis gibbosus </em>was introduced to Europe, including England, more than 100 years ago, but relatively little is known of its potential ecological impacts on native species and ecosystems. In England, the pumpkinseed is currently established in ponds of the River Ouse (Sussex) and its flood plain. Escapee pumpkinseeds are found in some small tributaries that contain native species of conservation interest (brown trout <em>Salmo trutta</em>, brook lamprey <em>Lampetra planeri</em>, European eel <em>Anguilla anguilla</em>, European bullhead <em>Cottus gobio</em>). We used using electrofishing surveys and telemetry methods to examine the interactions between pumpkinseeds and native stream fishes (mainly brown trout), including predator–prey relationships, home range size, microhabitat preferences, home range fidelity, and habitat overlap/repartition. To assess impacts of pumpkinseed on stream food webs, a preliminary study quantified trout growth and food-web structure of a stream ecosystem (abundance of primary and secondary producers, fish, and riparian spiders) in reaches with and without pumpkinseed. Where pumpkinseeds were in high density, differences were observed in stream food-web structure, in proportional representation of fish species traits and in riparian spider community composition, but these differences cannot be attributed solely to pumpkinseed presence. From the available evidence, there appears to be little direct or indirect adverse impact of pumpkinseed on native species and the stream ecosystem when in low densities, including as a host of nonnative infectious agents. However, this could change under conditions of climate warming, which are likely to favor pumpkinseed reproduction, potentially to the detriment of native species. In a context in which freshwater ecosystems are impacted by several human and climate-induced factors acting synergistically, our results underline the need to study nonnative species impacts through a series of experimental and long-term studies of stream ecosystems.