Prioritizing management of non-indigenous species in the Lagoon of Venice: mission impossible?

The Lagoon of Venice (432 km2 surface) is a major hotspot of non-indigenous species (NIS) introductions in the Mediterranean Sea and represents the site of first-introduction for several NIS that eventually have spread elsewhere. In order to suggest priority areas for management we surveyed the entire Lagoon for sites of high NIS occurrence. Benthic assemblages colonising artificial substrates (wooden poles) were compared from harbours, marinas, sites of live seafood import, shellfish farms as well as control sites, in all the three sub-basins of the Lagoon (for a total of 45 samples analysed). Nineteen species out of the 214 taxa identified in the Lagoon were NIS. The isopod Ianiropsis serricaudis and the bryozoan Tricellaria inopinata were very abundant and widespread, occurring in 90-100% of samples. Single samples displayed an average number of 8 NIS (range: 2-11), accounting for 20% (range: 6%-30%) of the taxonomic richness of benthic assemblages. The central sub-basin, where shipping activities are more intense, displayed the highest number of NIS and NIS/all taxa ratio. The differences in NIS composition among sub-basins were mainly due to dissimilar abundances of dominant NIS. Interestingly, no significant differences in number and composition of NIS were found among sites: even control sites, where no human activities related to NIS introduction take place, displayed a rich NIS assemblage. These results suggest that eradication actions are virtually impossible to actuate in Venice, and that pre-border management should integrate multiple vectors of introductions. The monitoring of pathways departing from the Lagoon is also advised.