Small-sized Fish Assemblages Do not Differ Between a Native and a Recently Established Non-indigenous Macrophyte in a Neotropical Ecosystem

Abstract

Hydrilla verticillata recently invaded a protected area in the Upper Parana River floodplain. In a rare opportunity, when this non-indigenous species co-occurred with the native Egeria najas in a secondary channel of the river, we assessed its effects on assemblage of small-sized fish. Traps were used to catch fish inside macrophyte patches in periods of low and high water. We assessed fish abundance, biomass, diversity, richness and the community composition (using a DCA). The number of fish species did not differ between plants. Fish abundance and biomass were significantly higher during the low water period but plant species did not affect these attributes. Similarly, fish assemblage composition differed only between seasons. Our results indicate that the oscillations in the water level had a greater effect on the small-sized fish assemblage than the presence of the non-indigenous macrophyte. In addition, H. verticillata favors small fish assemblages such as the native does; however, we contend that our conclusions are valid only for habitats scarcely colonized by native plant species. The invasion of other habitats colonized by a high diversity of native species of macrophytes, which are key for small-sized fish, is a matter of concern.