Natural recruitment contributes to high densities of grass carp ctenopharyngodon idella (valenciennes, 1844) in Western Europe

Introductions of grass carp, well known for their potentially negative ecosystem effects, have been performed in several countries around the world. As the species was considered unable to reproduce naturally under non-native environmental conditions, little attention was initially given to its invasive potential. We studied an area in northern-Italy where, contrary to expectations, introductions that were performed in the early 80s still exert a considerable pressure on aquatic macrophytes. In order to reveal whether the observed population dynamics are the result of natural events or stocking we analysed the density, age- and size-structure of the grass carp population and the migration pathways available to it. Telephone surveys were also used to check for fish transport from national and international suppliers. We also sampled potential spawning and nursery areas for young individuals and, when some were captured, we applied stable isotope analyses to discriminate their origin. We found that the population of large individuals likely originated solely from early stocking. We also documented the first analytical evidence of grass carp recruitment in the study area and, to our knowledge, in Western Europe. Therefore the species has the potential to become invasive in these areas and more detailed studies are needed to assess this potential. Further management should account at least for natural recruitment and potential negative environmental effects, controlling the species where needed.