Good Status is achieved when key species abundance is above a specified threshold value. The quantitative threshold values for coastal fish are based on location-specific baseline conditions where time series covering more than 15 years are available (ten year baseline + five or more years evaluation period). In areas where shorter time series are available, a trend based approach (<15 years) is used. The specific approach used in the various monitoring locations is presented in the Results section.
A baseline needs to be defined for determining the threshold value. The period used to define the baseline needs to cover at least ten years in order to extend over more than twice the generation time of the typical species represented in the indicator and thus cater for natural variation in the indicator value, due for example to strong and weak year classes. For the period used to determine the baseline to be relevant, it must also be carefully selected to reflect time periods with stable environmental conditions, as stated within the MSFD (European Commission 2008). Substantial turnovers in ecosystem structure in the Baltic Sea were apparent in the late 1980s, leading to shifts in the baseline state (Möllmann et al. 2009), and for coastal fish communities substantial shifts in community structure have been demonstrated in the late 1980s and early/mid 1990s (Olsson et al. 2012; Bergström et al. 2016a). In some areas, there have also been minor shifts in fish community structure later (see environmental fact sheets for further background).
Estimates of the relative abundance and/or biomass of key coastal fish species are used to evaluate whether the threshold value is achieved or not. These estimates are derived from fishery independent monitoring, recreational catch registration and/or commercial catch statistics. Since there are strong environmental gradients in the Baltic Sea and coastal fish communities and stocks are typically local in their appearance and respond mainly to area specific environmental conditions, the evaluations for coastal key fish species are carried out on a relatively local scale.
The evaluation period applied when using the baseline approach should cover at least five years to cater for natural variability. Good status is evaluated based on the deviation of the median value of the indicator during the assessment period in relation to the threshold value (Thresholds figure 1).
Thresholds figure 1: Acceptable deviation from baseline is used to define the threshold value between good status and not good status.
When using the trend-based approach, environmental status is evaluated based on the direction of the trend towards good status, over the time period of the indicator assessment (Thresholds figure 2).
Thresholds figure 2: Application of the trend-based approach for evaluating environmental status where the status is defined based on the direction of the trend of the indicator compared to the desired direction of the indicator over time. GS = good status, nGS = not good status. See description in the assessment protocol.
Typical species considered in the context of this indicator are perch (Perca fluviatilis), flounder (Platichthys flesus) and cod (Gadus morhua), depending on the sub-basin. Perch is generally the key species in coastal fish communities in the less saline eastern and northern Baltic Sea (Sweden, Finland, Estonia, and Latvia), and in more sheltered coastal areas in Lithuania, Poland and Germany. In the more exposed coastal parts of the central Baltic Sea and in its western parts the abundance of perch is generally lower and flounder is used as key species. Cod is the representative species in the western and more saline parts of the region. In the current assessment, however, cod is not included.