Assessment Protocol

This core indicator evaluates whether good status is achieved by determining the growth rate of the population as well as the population size over a specified time period. The data collected and used in this indicator are based on national aerial surveys described in Galatius et al. (2014).

Each assessment unit is evaluated against two threshold values, for population growth rate and the Limit Reference Level (LRL). The overall status of seals in each management unit only achieves good status if both threshold values are met.

Time series of data for each seal species and each management unit are used as input values in Bayesian analysis with uninformative priors, where it is evaluated whether observed data support the set threshold value. In this process, 80% support for a growth rate ≥ the threshold value is required. If the unit fails to achieve good status, the probability distribution is used to evaluate the confidence of the assessment. The package 'bayesm' in the program R has been used for the analysis. The following is an example of the procedure using survey data on harbour seals in the Southern Baltic Sea over the period 2002-2014:

Assessment table.png

In this example, there is 80% support for a growth rate ≥0.072 (read at 20%). Thus, the unit does not achieve good status (threshold for harbour seals is 0.09).

Assessment units and management units

This core indicator evaluates the population trends and abundance of seals using HELCOM assessment unit scale 2 (division of the Baltic Sea into 17 sub-basins). The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4.

The existing management plans for seals operate according to management units that are based on the distribution of seal populations. The management units typically encompass a handful of HELCOM scale 2 assessment units. Evaluations are therefore done by grouping HELCOM assessment units to align with the management units defined for each seal population.

  • The Baltic grey seal is evaluated as a single management unit, although genetic data show spatial structuring (Fietz et al. 2013). The total numbers of counted seals in the entire Baltic Sea during moulting surveys since 2014 were above 30,000. The proportion of the population hauled out during moult has been estimated to about 60-80% (e.g. Hiby et al. 2013). Coordinated aerial surveys encompassing the entire Baltic started in 2000, which is why only data after that year are used in analyses.
  • The Baltic ringed seal is distributed in the Gulf of Bothnia (first management unit) and the Archipelago Sea, Gulf of Finland and Gulf of Riga (second management unit). This sub-division is justified by ecological data that indicate separate dynamics of the stocks. Since ringed seals from both areas show a high degree of site fidelity, as seen in satellite telemetry data (Härkönen et al. 2008), it is unlikely that extensive migrations occur at current low population numbers, although some individuals may show more extensive movements (Oksanen et al. 2015).
  • Harbour seals in the Kalmarsund constitute a separate management unit and is the genetically most divergent of all harbour seal populations in Europe (Goodman 1998). It was founded about 8,000 years ago, and was close to extinction in the 1970s as a consequence of intensive hunting, and possibly also impaired reproduction (Härkönen et al. 2005). The genetic diversity is substantially reduced compared with other harbour seal populations.
  • Harbour seals in the southern Baltic (Danish Straits, Danish, German, Polish, and Swedish (Öresund region including Skåne county) waters) form a subpopulation which should be managed separately, although some migrants are exchanged with neighbouring populations, as this stock is genetically distinct from adjacent populations of harbour seals (Olsen et al. 2014). Combined with harbour seals from the subpopulation in the Kattegat, they form a metapopulation. The combined abundance of the two subpopulations is assessed with respect to the LRL threshold, while the abundance trends of the two subpopulations are assessed separately. Thus, different assessments are possible for subpopulations of the same metapopulation.
  • Harbour seals in the Limfjord form the third management unit and are genetically distinct from the Kattegat harbour seals, but are influenced by migrants from the Wadden Sea (Olsen et al. 2014).