The assessment of environmental status is based on the smolt production in rivers with wild salmon stocks. The estimated smolt production is compared to an estimated potential smolt production capacity (PSPC) of rivers and the threshold value is defined as 75% of the PSPC (see Thresholds figure 1). This level of production compares to a stock size at maximum sustainable yield (MSY) practically for all stocks.
Thresholds figure 1: The estimated smolt production is compared to an estimated potential smolt production capacity (PSPC) of rivers and the threshold value is to reach 75% of the PSPC.
The PSPC is estimated using a life history model developed by the International Council for the Exploration of the Sea Assessment Working Group on Baltic Salmon and Trout (ICES WGBAST). Some uncertainty in the method still exists, and thus the potential production capacity may be over-estimated for some river areas. Accordingly, the precautionary principle is applied when making estimates of the PSPC against the threshold value, and there is a small risk of falsely evaluating a river as being below the threshold value. Hence, when evaluating the status of an assessment unit that includes several rivers, a one-out-all-out approach is considered unsuitable and a weighted evaluation is applied instead.
The adult spawners ascend the spawning rivers after the feeding period and the number of smolts (estimated based on the measured parr densities or smolt counts from the rivers) reflect the abundance of the adult spawners and the success of recruitment. The number of adult spawners is used as a supporting parameter in the indicator for areas where such monitoring data are available. Changes in the level of pressures affecting the salmon populations are expected to be noticeable, with a shorter time lag, in the number of spawners compared to the smolt production capacity, and thus any significant changes in trend are to be considered as early warning signals.
Mainly due to the use of the model and good availability of data from several areas, the confidence of the accuracy of the threshold is considered to be moderate to high.