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The present study aimed to develop monitoring methods for shallow water sessile and mobile epifauna with the main focus on enhancing the chance of early detection for new non-indigenous species (NIS) invasions. The field sampling was conducted between June and September in 2012, in the Archipelago Sea (Finland). The tested monitoring methods included baited traps that capture organisms and habitat collectors that provide habitat and refuges for organisms, as well as fouling plates. Catch efficiency of a trap/collector was defined as the number of NIS and all species caught, including their abundances. The American collector with oyster shells (habitat collector) caught the highest number of NIS, and their use is recommended in all places, where oyster shells are easily accessible. Sampling of all habitats of interest between 1 and 2 m depth is recommended with at least three habitat collectors per site. Highlights • Sampling of mobile and sessile epifauna with habitat collectors is recommended. • Habitat traps should provide attachment surfaces, habitats and refuges to organisms. • Sampling of all habitats present between 1 and 2 m depth is recommended. • Deployment of 3 collectors per site is suggested for routine coastal monitoring. • Timing and duration of sampling may differ significantly between geographical areas.

Long-term population data of marine invaders are rarely collected although it provides fundamental knowledge of the invasion dynamics that is important for evaluating the impacts, interactions and range expansion of the invader and for management purposes. During a 6-year monitoring period, we studied the dynamics and population demographic characteristics of the newly introduced mud crab Rhithropanopeus harrisii. The overall abundance of R. harrisii appeared to follow the boom and bust pattern with a rapid initial abundance increase and subsequent decline. The recruitment and growth of juveniles were correlated with the temperature during the larval period indicating that the increase in water temperature caused by climate change could have a positive effect on the recruitment and have potentially facilitated the establishment of R. harrisii in the northern Baltic Sea. The changes in the survival of reproductive females influence most the growth rate of the studied population. Hence, native predators feeding on benthic fauna such as fish could regulate the population growth of R. harrisii in the study area by reducing female survival. Although the population size seems to be stabilized at the monitoring locations, R. harrisii continues to expand its distribution range, and the rapid initial population increase is likely occurring at newly invaded sites.

The need to detect and monitor introduced marine species has increased with the increasing number of marine invasions. To complement standard detection and monitoring techniques, new approaches using environmental DNA (eDNA) have recently been developed. However, most of the eDNA work has focused on vertebrate species in spatially limited freshwater habitats while benthic invertebrates in coastal environments have received much less attention. Here, we evaluated the suitability of the eDNA approach for detecting benthic, hard-shelled, crustacean mud crab species in a brackish water environment. We demonstrated for the first time that eDNA from an introduced mud crab Rhithropanopeus harrisii can be successfully amplified in aquarium water samples and detected in the brackish water environment. However, the detection rate was rather low. This suggests that in contrast to freshwater vertebrates, it may be more challenging to develop a highly sensitive eDNA method for detecting crustacean species in a marine environment.