Thresholds and Status evaluation

Good status is achieved when the abundance of piscivores is above a specified threshold value and the abundance of cyprinids is within an acceptable range. 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 (<15 years) are available, a trend-based approach 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 sheet for further background). 

Estimates of the relative abundance and/or biomass are used to determine whether coastal fish key functional groups in the Baltic Sea achieve good status or not. These estimates are derived from fishery independent monitoring 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 fish key functional groups 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.png 

Thresholds figure 1. Determination of acceptable deviation from baseline (>15 years) for piscivores (left) and acceptable range from baseline for cyprinids (right). See description in the Assessment protocol.

 

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.png

Thresholds figure 2. Application of the trend-based approach for evaluating environmental status for piscivores (left) and cyprinids (right). 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.

 

The functional groups used in this indicator are piscivorous fish species and members of the cyprinid family. In areas where cyprinids do not exist naturally, mesopredatory fish species could be used e.g. any mid-trophic level species that are not piscivorous. Due to the absence of cyprinids in one area, mesopredators assessed in this single area (Monciskes/Butinge in Lithuania). Piscivorous coastal fish species are typically represented by perch (Perca fluviatilis), pike (Esox Lucius), pikeperch (Sander lucioperca) and burbot (Lota lota) in the less saline eastern and northern Baltic Sea (Sweden, Finland, Estonia, Latvia and Lithuania) and in sheltered coastal areas in Poland and Germany. In the more exposed coastal parts of the central Baltic Sea and in its western parts, piscivores are typically represented by cod (Gadus morhua) and turbot (Scophthalmus maximus). A similar division can be made for members of the cyprinid family (Cyprinidae), e.g. roach (Rutilus rutilus) and bream (Abramis sp.) that are the most abundant species in the less saline eastern and northern parts of the Baltic Sea, whereas mesopredatory fish are representative of the more exposed coastal parts of the central Baltic Sea and in its more saline western region.