The assessment of environmental status is based on the comparison of the observed parr densities in rearing habitats with the reference potential parr densities in the specified habitats, which can be based on model estimations or expert judgement.
Parr is a young sea trout living in the river before the smoltification and start of feeding migration to the sea. The parr stage is sometimes subdivided according to age, where parr 0+ are young fish less than one year old. Depending on the river, sea trout parr spend 1-3 years in the river before the sea run.
The site specific reference parr densities exhibit a rather large natural variation between years. In rivers where the abundance of spawners has been estimated to continuously meet the conservation limits, and are thus considered to reflect good status, the parr densities have varied between 60-100% of the estimated reference potential densities. In this evaluation, good status is achieved when the moving parr density average remains above 50% of the reference parr density. Consequently, the threshold value is a moving average of parr densities over 4-5 years (Thresholds figure 1). It should be noted that only rivers accessible to spawners and containing suitable good quality spawning habitat are evaluated.
Thresholds figure 1: Reference parr densities are determined either through model estimations based on actual reference data or expert judgment. Good status is achieved if the site-specific moving parr density average remains above 50% of the site specific reference potential parr density.
The reference potential parr densities are estimated for each river by a rigid Baltic assessment model that takes into account the physical characteristics of the river habitat (see e.g. ICES 2017). The present model is applicable to rivers where the parr density is estimated to have reached the full production level at least once in the available time series. Rivers in Denmark and some other areas in the southern Baltic Sea meet this requirement. For the northern rivers, the Baltic assessment model indicates not good status conditions in accordance with other data, however, there is still some uncertainty as to the precision of the model in this area. The uncertainty is due to the quality of the background data that do not include full production level conditions and since natural conditions differ significantly from other parts of the Baltic, implying that the production levels from the southern areas cannot be used as reference values in the northern areas. Rivers in the northern areas are presumed to have a naturally lower overall productivity due to lower temperatures and other environmental factors. In these areas, the maximum potential parr densities have been provided by expert evaluations. Currently, a list of river specific reference values is not available.
The threshold values have mainly been defined based on expert judgement and long-term data on reference conditions, and therefore the confidence of the target is considered to be moderate to high.