The ultimate goal is to minimize anthropogenic introductions of NIS to zero. The threshold value between good status and not good status is 'no new introductions of NIS per assessment unit through human activities during a six year assessment period' (Thresholds figure 1). As a mid-term goal a decrease in the rate of new introductions should be considered. The evaluation against the baseline species list is carried out and all new species introduced to the Baltic Sea per assessment unit over a six year period are listed and counted.
The focus in the indicator is on human-mediated introductions and not secondary spread by natural means (migration, water currents etc.). There are large regional inconsistencies in the assessment of introductions to vectors/pathways due to different knowledge level and information availability in different sub-basins. Therefore the indicator considers only new introductions into the Baltic Sea where we have a better level of confidence for the vector/pathway and not the spread inside the Baltic although part of this spread is for sure due to human actions (certainly for some bivalve species e.g. Mytilopsis leucophaeata and Rangia cuneata).
Thresholds figure 1. Schematic representation of the threshold value, where the threshold is achieved if no new species appear in the Baltic Sea due to human activities during the six year assessment period.
The confidence in the applicability of the threshold value is moderate as the concept is broadly considered to be valid and the deleterious effects of NIS are in general well known. As monitoring data is not readily available, the applicability has not been sufficiently tested. Furthermore the 6-year evaluation period has been selected based on management cycles and may not be the most ecologically relevant assessment period. However, a recent study conducted by ICES on the temporal adequacy of a three year period assessment states that this is likely to be a too short period and considers a six-year assessment period to be more appropriate (ICES, 2013).
Eradication of already introduced NIS species has proved not to be feasible in aquatic environments (Sambrook et al. 2014) after establishment and spread. No knowledge of eradication of alien invertebrates or marine organisms has been recorded in Europe. This proves the difficulty from the operative and economic perspective of implementing such measures (Genovesi, 2005). Thus, reaching a pristine status cannot be used as a relevant threshold value.
To enable an evaluation of status, the indicator requires a baseline in the form of a list that specifies which NIS/CS were already present in each assessment unit at a certain point in time. The baseline list has been made for the year 2010. Altogether 160 NIS and cryptogenic species have been observed in the Baltic Sea by 2010 (based AquaNIS 2018) (see Metadata for details). The number of species present in 2010 varies between assessment units. Some flexibility in the indicator evaluation against the baseline list should be ensured if a NIS is later found to have invaded an area during a previous assessment period.