Invasion potential and ecosystem-level impacts of non-native fishes in a riverine system

Background/Question/Methods

The introduction of species to novel habitats is often cited as one of the largest and most pervasive anthropogenic impacts on ecosystems.  Upon introduction, some species become invasive, dominating community biomass and alter community and ecosystem processes.  In particular, introduction of non-native fishes is a global concern, with invasive fishes leading to impacts in many ecosystems.  However, predicting when and where non-native fish populations become established and whether these species become invasive is poorly understood.  It is also not well-known how impacts of non-native fishes vary with environmental conditions.  Suckermouth catfishes (Loricariidae) have invaded aquatic systems throughout the world and are of concern because they affect trophic and nutrient dynamics. We present a results of a multi-part study in which we examined the abundance, distribution, and impacts of Hypostomus spp. in the San Marcos River, TX, USA.  We assessed whether the growth and impacts of Hypostomus were independent from nutrient enrichment in a replicated stream channel experiment in which the presence and absence of catfish was cross-classified with the presence and absence of added nutrients.  We also examined whether the abundance and distribution of Hypostomus in the San Marcos River was related to a pronounced enrichment gradient in the river. 

Results/Conclusions

In the stream channel experiment, Hypostomus decreased algal biomass, altered algal nutrient ratios, and increased sediment transport and OM decomposition rates.  The presence of nutrient enrichment had fewer effects on ecosystem characteristics and the effects of catfish were largely independent from those of nutrients, indicating that nutrient enrichment may have a limited role in mediating or exacerbating the impacts of catfishes.  However, at the end of the experiment, Hypostomus exhibited higher lipid content and body mass in the presence of nutrients.  In the field study, contrary to our predictions, the density of catfishes was lowest at the most enriched site on the river.  Field survey results indicate that Hypostomus densities were most influenced by water depth and substrate size gradients in the river rather than nutrient availability.