This core indicator evaluates the status of the marine environment based on population trends and abundance of the three species of seals that occur in the Baltic Sea. Good status is achieved for each species when i) the abundance of seals in each management unit is has attained a Limit Reference Level (LRL) of at least 10,000 individuals to ensure long-term viability and ii) the species-specific growth rate is achieved indicating that abundance is not affected by severe anthropogenic pressures.
The status evaluation is presented separately for the three seal species. The grey seal population of the Baltic Sea is evaluated as a single unit, excluding Kattegat, where the indicator is not applicable for grey seals. The status of ringed seals is evaluated for two management units. The status of harbour seals is evaluated for three management units. The evaluation of abundance and trends of seals are based on data from 2003-2016, and the period 2011-2016 is assessed for current status. Furthermore, in the context of this independent evaluation it has to be taken into account that, utilising different assessment units, national evaluations in the reporting for the Habitats Directive may differ, and in addition that other HELCOM indicators assessing different aspects of seals may show different evaluation results.
Key message figure 1. The overall status assessment results based on evaluation of the indicator 'population trends and abundance of seals' – Ringed seals (left), Grey seals (centre) and Harbour seals (right). The assessment is carried out using Scale 2 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4), using the one-out-all-out approach. Thus, if a seal management unit, in not good status, has a given assessment unit as part of its range, the assessment unit is marked red. However, for harbour seals, the Kattegat assessment unit is marked as good status, although the Limfjord management unit with doubtful status occupies part of the assessment unit (see below). Click
here to access interactive maps at the HELCOM Map and Data Service: ringed
seal and harbour
Harbour seals differ from grey seals in their movement behaviour and population structure, with harbour seal in the southwestern Baltic and Kattegat management units appearing to form a metapopulation connected by some degree of movements. Thus, when evaluating the status of the two management units, the LRL is estimated for their combined abundance, whereas their growth rates are estimated and evaluated separately. For some isolated harbour seal units, the LRL of 10,000 individuals may not be achievable. This may be the case for the Kalmarsund and the Limfjord units (degree of reproductive isolation of the harbour seals in the inner Limfjord is being resolved by an ongoing project). For these management units, the abundance criterion will be considered achieved when they have attained carrying capacity, even if it is at a level below the LRL.
The abundance of grey seals is above the LRL of 10,000. Grey seals occur in the entire Baltic Sea except for in the Kattegat where the species has not been breeding since the 1930s, except for a few observations from recent years. Grey seals do not achieve good status with regard to population growth rate in the entire Baltic Sea if evaluated against the 7% annual increase during exponential growth. However, they will achieve good status if evaluated against criteria for carrying capacity with population decrease less than 10 % over a 10-year period. When using the latter criteria for population growth rate and the abundance, grey seals overall achieve good status.
Ringed seals occur in the Bothnian Bay (which is considered one management unit) and in the Gulf of Finland, Archipelago Sea, Gulf of Riga and Estonian coastal waters (which are together considered a second management unit). The size of the population exceeds the LRL and achieves good status only in the Bothnian Bay management unit. The sizes of subpopulations in the second management unit add up to only a fraction of the LRL and are stable or declining. The ringed seal population growth rate is below the threshold value for good status (not good status) in both units.
Harbour seals are confined to the Limfjord, the Kattegat (including the Northern part of the Great Belt assessment unit), Southern Baltic Sea (i.e. Bornholm Basin, Arkona Basin, Bay of Meklenburg, Kiel Bay, The Sound and the Southern part of the Great Belt assessment units), and the Kalmarsund.
The Limfjord subpopulation is below LRL, but may nevertheless be approaching carrying capacity since the annual growth rate is fluctuating close to zero. However, at present it is not possible to evaluate the status of the Limfjord population due to limited information about the degree of immigration and interbreeding with the Wadden Sea population.
The growth rate of the core subpopulation in the Kattegat and northern Great Belt is levelling off, which is a sign of that this management unit is approaching its carrying capacity. As no decline exceeding 10% has been detected over the last 10-year period, and the abundance is above the LRL of 10,000 animals, this management unit has achieved good status with regard to both population growth rate and abundance.
The management unit in the Southern Baltic is connected to the Kattegat management unit and thus achieves good status for abundance, but the annual growth rate is just below the threshold value for good status for populations in exponential growth phase. Thus, the overall status of the southern Baltic management unit is not good.
The Kalmarsund population does not meet the criterium for good status with respect to growth rate, and the abundance is well below the LRL, so the status of this population is not good.
Confidence of the indicator evaluation is considered to be high for all seal species regarding applicable assessment units, except for harbour seals in the Limfjord, where the connectivity with the Wadden Sea seals is unknown. Similarly, confidence is not high regarding the evaluation that grey seals and the Kattegat-northern Great Belt harbour seal unit are approaching carrying capacity. A longer series of stagnating trends is needed for the confidence to increase.
The indicator is applicable in the waters of all the countries bordering the Baltic Sea since the indicator includes all species of seal that occur in the Baltic Sea, at least one of which occurs in all HELCOM assessment units.
The population trends and abundance of seals signal changes in the number of marine top predators in the Baltic Sea. Being top predators of the marine ecosystem, marine mammals are good indicators of the state of food webs, levels of hazardous substances and direct human disturbance.
Distributions of different species during feeding and annual migrations encompass the entire Baltic Sea although no terrestrial haul-out sites occur in Latvia and Lithuania.
D1 Species groups of birds, mammals, reptiles, fish and cephalopods
D1C2 The population abundance of the species is not adversely affected due to anthropogenic pressures, such that its long/term viability is ensured
D1C4 The species distributional range and, where relevant, pattern is in line with prevailing physiographic, geographic and climatic conditions.
D4 Ecosystems, including food webs
D4C2 The balance of total abundance between the trophic guilds is not adversely affected due to anthropogenic pressures.
D4C4 Productivity of the trophic guild is not adversely affected due to anthropogenic pressures.
D8 Concentrations of contaminants are at levels not giving rise to pollution effects
D8C2 The health of species and the condition of habitats (such as their species composition and relative abundance at locations of chronic pollution) are not adversely affected due to contaminants including cumulative and synergistic effects.
HELCOM (2018) Population trends and abundance of seals. HELCOM core indicator report. Online. [Date Viewed], [Web link].
Population trends and abundance of seals HELCOM core indicator 2018 (pdf)