Optimizing sampling design for monitoring non-indigenous species in coastal recreational marinas: Insights from the Tagus estuary case study

The absence of effective monitoring and management measures will lead to a larger impact of Non-Indigenous Species (NIS). To develop appropriate monitoring programs to assess NIS spatial and temporal trends it is crucial to understand the influence of different local scale factors for their establishment and detection. This study aims to identify the best sampling design for monitoring NIS and detecting new introductions in transitional areas using the Tagus estuary (Portugal) as a case study by comparing results obtained i) in four different marinas located along an estuarine gradient, ii) collected in two different years (2016 and 2022), iii) using different sampling methods (scraping and Rapid Assessment Survey - RAS and iv) using a different number of replicates (24 and 4). Results showed an increase in NIS number and abundance from 2016 to 2021, but seasonal effects cannot be excluded. Although the marina size, used as proxy of the propagule pressure, influenced NIS richness and abundance, the location of these marinas in the estuarine gradient seems to have the highest influence on invasiveness levels. The results based on different sampling efforts indicate that the minimum number of replicates required will depend on the goal but four replicate samples collected by scrapping in different areas of the estuarine gradient seem to be a balanced sampling effort to detect NIS trends in 6 years evaluation periods, as required by the Marine Strategy Framework Directive (MSFD).