The Baltic assessment model focuses on the state of recruitment, which is defined as the observed recruitment (i.e. observed densities) compared to the potential reference recruitment (i.e. maximal densities that could be expected under the given habitat conditions). The model uses electrofishing data of the individual sea trout populations, together with habitat information collected at the same sites, to calculate the trend in population development over time. The examination of the data is site-specific (several sites can exist in one river) but the evaluation of the state of the stock can be concluded on the river level and also at a sub-area level. Average values of recruitment state are calculated for assessment areas, sub-divisions, and, where more countries have streams in one sub-division, for each individual countries (ICES 2011; ICES 2017).
At the annual meetings of the International Council for the Exploration of the Seas Baltic Salmon and Trout Assessment Working Group (ICES WGBAST), all available data from the Baltic countries on estimated abundance of sea trout parr (per 100 m2) from individual sites are compiled. In 2017, data was available from a total of 257 sites in about 132 streams, which reflects the general situation of data availability. For the evaluation, parr abundance are divided into young of the year (0+) and older trout (>0+). If there is a lack of data, young of the year (0+) and older trout (>0+) can be aggregated instead of analysed separately.
Differences in habitat qualities (e.g. suitability for trout) influence trout abundance. Selected monitoring sites are situated in small rivers, focused on typical habitats of sea trout. To be able to compare trout abundances between sites with different habitat quality, a sub-model has been proposed: the Trout Habitat Score (THS). The THS is calculated by first assigning values (scores) for each habitat parameter for 0+ trout: average/dominating depth, water velocity, dominating substrate, stream wetted width, slope (where available) and shade. Values (scores) are assigned between 0 (for sites with poor conditions) and 2 (for best conditions) by using suitability curves and partly by expert judgement (ICES 2011). All scores are then summed, resulting in a THS between 0 (zero) for sites with very poor conditions and 12 (10 if slope is omitted) for sites with very good conditions for sea trout parr. The THS scores obtained are then combined into Habitat Classes (HC) that range between 0 (poorest) and 3 (best).
Due to the significant climatic (e.g. temperature and precipitation) and geological differences across the Baltic Sea region, the densities of sea trout parr vary between areas. The predicted reference potential densities for sites across the Baltic at full recruitment are determined through a multiple linear regression analysis using the following parameters: stream wetted width, climate (average air temperature), latitude (proxy for productivity due to climate), longitude (proxy for the gradient from oceanic to continental climate) and the habitat score (0-1-2-3; see ICES 2011) with log (0+ trout density + 1) as dependent variable. For this analysis, only sites with the best quality and highest observed densities are used.
Sites judged to have good to intermediate water quality (a prerequisite for trout to fulfil their life cycle) are selected for assessment, irrespective of the habitat quality class (HC) of the site.
Recruitment trend over time is calculated for each site through linear regression of parr density versus years (currently 2000–2014) as Pearson r correlation coefficient, resulting in values from ‐1 to +1. Values close to ‐1 indicate a high correlation to a straight line, representing a negative development.
In addition to parr density data, evaluations of status can be supported by direct counts of ascending spawners in some rivers. This is possible by means of video counting which distinguishes between sea trout and salmon. Also smolt counting is carried out in a couple of rivers. Both spawner and smolt counting data provide complementary input to the estimation of stock status based on the parr densities. However, electrofishing survey monitoring, which measures the parr densities, provides the primary data used for evaluating sea trout stock status in the Baltic Sea area.
Since all the Baltic sub-basins have naturally reproducing stocks of sea trout, this indicator is relevant in the entire Baltic Sea. Sea trout migrate between fresh water river systems and the marine area, mainly feeding within <100 km from their home river. The assessment units most applicable for evaluating sea trout status are therefore the coastal assessment units.
While it appears that most populations of sea trout make relatively short feeding migrations (a few hundred kilometres), it is known that all sea areas have populations with long migration patterns where the sea trout spread into neighbouring coastal areas. For example fish tagged in the Finnish part of the Gulf of Finland are found in Estonia and Russia (and vice versa), and similarly, fish tagged in the Finnish side of Gulf of Bothnia are found in Sweden (and vice versa). According to previous work by SGBALANST (ICES 2008, 2009), the sea trout populations in the Bothnian Sea, Bothnian Bay and Gulf of Finland (ICES sub-divisions 30, 31 and 32) have been pointed out as highly separated units with respect to state of the stock and migration patterns.
For the purposes of this HELCOM core indicator on sea trout spawners and parr, the environmental status is assessed for coastal areas using HELCOM assessment unit scale 3. Some sub-basins may be combined for evaluations at a later stage, if necessary, when the migration patterns of the sea trout populations become better known.
The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.