Only half of the truth: Managing invasive alien species by rapid assessment Sven Rohde a, * , Peter J. Schupp a , Alexandra Markert b, c , Achim Wehrmann b, d a University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment, Schleusenstr. 1, 26382, Wilhelmshaven, Germany b Senckenberg am Meer, Marine Research Department, Südstrand 40, 26382, Wilhelmshaven, Germany c University of Oldenburg, Institute for Biology and Environmental Sciences, 26111, Oldenburg, Germany d Biodiversity and Climate Research Centre, 60325, Frankfurt, Germany article info Article history: Received 9 November 2016 Received in revised form 12 April 2017 Accepted 29 May 2017 Keywords: Management strategies Species inventory Neobiota Monitoring Bioinvasion hot spots HELCOM/OSPAR guidelines Invasive species Wadden Sea abstract Marine environments are threatened by biological invasions that are transported via ship hulls or ballast water at increasing rates around the globe. Most management strategies that should prevent or reduce the impact of alien species include prevention and eradication programs that both rely on an early detection of introduced species. Early detection requires an up to date inventory of present species and efficient monitoring programs to identify newly arrived species. This study surveyed potential invasion ‘hot spots’ in the coastal waters of the central Wadden Sea, including commercial ports (‘port’ stations) as well as fairways, roads, groynes and oyster reefs (‘Wadden Sea’ stations) to establish lists of macro- benthic species as baseline for future monitoring. In the ports we also applied simultaneously the HELCOM/OSPAR protocol (H/O) and a regional established rapid assessment (RA) to enable the evaluation of the effectiveness and comprehensiveness of both methods. While the RA is mainly based on the visual screening of species in easy accessible habitats in the field, the H/O includes an extensive sampling by operation of scientific divers and boat times and a 4-times higher time effort for sample processing. We identified among all sites 273 species including 37 alien species. The fauna was composed of 210 native and 32 alien species, while the flora was composed of 26 native and 5 alien species. The ports were a diverse habitat with species numbers from 65 to 109. At the Wadden Sea stations, exclusively monitored by the less extensive RA, only half of the species numbers (36e66 species) were detected. However, on the oyster reefs the relative occurrence of alien species compared to natives was similar to the ports and one oyster reef (Nordland) even had the highest ratio (25%, 14 species) of alien species of all investigated sites. The H/O protocol identified 179 species compared to 114 species identified by the RA. Concerning to alien species, the H/O provided 85% of alien species, the RA only 67%. Especially with regard to the actual discussion on harmonized monitoring protocols within the Northeast Atlantic and Baltic regions, we recommend that future port surveys should apply the more extensive H/O protocol, since restrictions to the RA decrease drastically the chance of an early detection of alien species. We also recommend complementing the survey of ports with natural invasion ‘hot spots’ like the oyster reefs by rapid assessment. This combined monitoring design would increase significantly the early detection of alien species as demonstrated in our study. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Invasion by alien species is one of the major threats to the sta- bility of marine communities and the functioning of ecosystems (World Resources Institute, 2005). Facilitated by global trade the number of alien species that were accidentally or deliberately introduced into new regions has increased rapidly during the last decades (McGeoch et al., 2010; Pysek et al., 2012; Simberloff et al., 2013). Alien species from all major animal and algal phyla have established in most coastal regions worldwide with serious con- sequences for marine ecosystems (e.g. Carlton, 1979; Castilla and Neill, 2009; Davidson et al., 2015; Hewitt et al., 2004a). Marine environments are especially accessible to biological invasions since it is estimated that at any time several thousand marine species are * Corresponding author. E-mail address: sven.rohde@uni-oldenburg.de (S. Rohde). Contents lists available at ScienceDirect Ocean & Coastal Management journal homepage: www.elsevier.com/locate/ocecoaman http://dx.doi.org/10.1016/j.ocecoaman.2017.05.013 0964-5691/© 2017 Elsevier Ltd. All rights reserved. Ocean & Coastal Management 146 (2017) 26e35