Assessment Protocol

Currently, this core indicator assesses the nutritional status of only grey seals due to limited data and developmental stages of appropriate methodologies for other species.

Each management unit is evaluated against a threshold value, the threshold value for exponentially growing populations, and a secondary threshold value for populations at carrying capacity is provisionally provided (see Thresholds table 1).

The current analysis is made using samples from sub-adult seals (1-3 years old). The blubber thickness of 1-3 year old grey seals shows a seasonal flux as illustrated in the Assessment Protocol figure 1. A polynomial model fitted to data could be used in future developments of the analysis in order to merge data from all months, by recalculating each data point to the month of October. This month is suggested because there is a reasonable amount of data collected in October. Thus data can be used in this analysis regardless of which month the sample is taken.


Nutritional status seals Assessment figure 1.jpg 

Assessment protocol figure 1. Finnish and Swedish data of sub-adults (means ± SE) from 2012–2015 with month, area (ICES SD), sex and reason for death as covariates.

Observed data is merged for 3-5 year intervals, depending on sample size, to be used as input values in Bayesian analyses with uninformative priors, where it is evaluated if observed data from an assessment unit achieve the threshold value. In this process, 80% support for a blubber thickness ≥ the threshold value is required. If the unit does not achieve the threshold value, the probability distribution is used to evaluate the confidence of the evaluation. The package Bayesian in the program R is used in the analysis.

 

Assessment units

This core indicator evaluates the nutritional status 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. 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. For the current indicator evaluation, grey seals spatial units in the Baltic Sea have been merged and are treated at the scale of the whole Baltic Sea (HELCOM scale 1), with the exclusion of the Kattegat and Limfjord unit.

  • The Baltic grey seal (excluding the Kattegat and Limfjord) is a single management unit, although genetic data show some spatial structuring (Fietz et al. 2013). Data is available both from land-based surveys starting in the mid-1970s and later aerial surveys.
  • The Baltic ringed seal is distributed in the Gulf of Bothnia (one unit - northerly) and Southwestern Archipelago Sea, Gulf of Finland and Gulf of Riga (second unit – more southerly), representing two different management units. 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 can show more extensive movements (Kunnasranta 2010, Oksanen et al. 2015).
  • Harbour seals in the Kalmarsund, Sweden, 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 southwestern Baltic (Danish Straits, Danish, German, Polish Baltic and the Öresund region including Skåne county in Sweden) should be managed separately as this stock is genetically distinct from adjacent populations of harbour seals (Olsen et al. 2014).
  • Harbour seals in the Kattegat are also genetically distinct from adjacent populations (Olsen et al. 2014).
  • Harbour seals in the Limfjord form the fourth management unit and is genetically distinct from the Kattegat harbour seals (Olsen et al. 2014) .