The value of regular monitoring and diverse sampling techniques to assess aquatic non-native species: a case study from Orkney

A monitoring programme for marine and brackish water non-native species (NNS), initiated by Orkney Islands Council, has produced comparable data from multiple sites since 2012. Sampling was performed at both natural habitats and areas under anthropogenic influence, such as marinas, and has included rapid assessment, wall scrape, settlement panel, benthic grab and phytoplankton samples, from which 15 NNS and 12 cryptogenic species have been recorded, of which three NNS (Boccardia proboscidea, Asterocarpa humilis and Melanothamnus harveyi) and one cryptogenic (Ctenodrilus serratus) represent new records for Orkney. A historical bibliographic and database review, conducted also within this study, shows these results to represent 71% of all non-native and 60% of all cryptogenic species ever found to have been identified for Orkney (total 41 non-native or cryptogenic species). The most widespread non-native species found in the present study were red algae (Melanothamnus harveyi and Bonnemaisonia hamifera), the bryozoan Schizoporella japonica and the Japanese skeleton shrimp (Caprella mutica). Many of the benthic non-native species recorded were found in multiple sample types but some of the smaller species were missing from rapid assessment samples. Additional methods and locations would be necessary to produce a complete inventory of non-native species in Orkney, as evidenced by comparison with records from other sources. The programme has provided a valuable baseline, including new Orkney records for some non-native species. Continuity and comparability of future surveys will be essential to monitor changes in the distribution and abundance of current non-native species and for tracking new arrivals.