To estimate Common Indicator 6, a trend analysis (time series analysis) of the available monitoring data needs to be performed, aiming to extract the underlying pattern, which may be hidden by noise. A formal regression analysis is the recommended approach to estimate such trends. This can be done by a simple linear regression analysis or by more complicated modelling tools (when rich datasets are available), such as generalized linear or additive models. To monitor trends in temporal occurrence, two indicators are estimated on a yearly basis. The first is about the number of non-indigenous species at the current year that were not present at the previous year. To calculate this indicator the non-indigenous species lists of both years are compared to check which species were recorded in year n, but were not recorded in year n-1 regardless of whether or not these species were present in earlier years. The second indicator is estimated as the total number of known non-indigenous species at Tn minus the corresponding number of non-indigenous species at Tn-1, where Tn stands for the year of reporting. It is recommended to use standard monitoring methods traditionally being used for marine biological surveys, including, but not limited to plankton, benthic and fouling studies described in relevant guidelines and manuals. Standard methods for monitoring marine populations include plot sampling, distance sampling, mark-recapture, removal methods, and repetitive surveys for occupancy estimation. As a complimentary measure and in the absence of an overall IAS NIS targeted monitoring programme, rapid assessment studies may be undertaken, usually but not exclusively at marinas, jetties, and fish farms. The compilation of citizen scientists input, validated by taxonomic experts, can be useful to assess the geographical ranges of established species or to early record new species.
The current assessment is based on literature, recent projects and initiatives in the Mediterranean, as work is still ongoing for all Mediterranean countries to update their national monitoring plans to be aligned with the IMAP decision (UNEP/MAP 2016) and begin reporting comparable data.