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Key Message

This core indicator evaluates the status of the marine environment based on the reproductive status of seals in the Baltic Sea. Good status is achieved when the annual reproductive rate (i.e. the proportion of females pregnant/showing postpartum pregnancy signs per year) is at least 90% for harbour seals of five years and older,  and grey and ringed seals of six years and older. A reproductive rate of 90% is defined as the threshold for each of these parameters as this is indicative of increasing populations. The overall status assessment is evaluated based on the average ratio (pregnancy rate or inferred birth rate % divided by the threshold) for the two parameters, where a value below 1 is deemed to fail the threshold for the combined parameters. However, it should be noted that should a population be defined as at carrying capacity, it may exhibit a lower reproductive rate due to the prevailing conditions and population stability or equilibrium.


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Key message figure 1. Status assessment results based on evaluation of the indicator 'reproductive status of seals'. The assessment is carried out using Scale 2 HELCOM assessment units (defined in the HELCOM Monitoring and Assessment Strategy Annex 4). Click here to access interactive maps at the HELCOM Map and Data Service: reproductive status of seals.


Currently, a full status evaluation has only been carried out for the grey seal based on Finish and Swedish data. The indicator is applicable for all species of seals and marine mammals that occur in the Baltic Sea, however the amount of data have so far been insufficient for an evaluation of other marine mammal species. The evaluation of grey seal reproductive status is based on data from 2011-2016 and for the established threshold in failed for this period.


The confidence in the assessment is low for this period due to the small and spatially restricted data sample used to calculate the current status evaluation.

Further ongoing work is being carried out in association with this reproductive status indicator to develop a new Seal Health Indicator that will provide details about the reproductive health and thus link causes and effects of population level trends in reproductive status.


Grey seals occur in the entire Baltic Sea except for the Kattegat where the species has not been breeding since the 1930s, except for a few observations inrecent years. The grey seal in the Baltic Proper is evaluated as a single unit since they perform long migrations across the sea region, whereas the Kattegat grey seals are evaluated separately. Grey seal reproduction is not in good status with regard to reproductive rate in the entire Baltic when evaluated as one single population.

Ringed seals are evaluated for two management units: 1) the Bothnian Bay and 2) the Gulf of Finland, Archipelago Sea, Gulf of Riga and Estonian coastal waters. A tentative threshold value is set at 90% reproductive rate for six years and older seals but there are insufficient data to carry out an evaluation of status at present.

Harbour seals are confined to the Kalmarsund, Southern Baltic Sea, the Kattegat and the Limfjord, which are all separate management units. Harbour seals are evaluated for four management units. A threshold value is set at 90% reproductive rate for five years and older seals but data have not been analysed yet and hence no evaluation of status has been carried out.

The indicator is applicable in the waters of all the countries bordering the Baltic Sea as the indicator includes all seal species that occur in the Baltic Sea and at least one of the species occurs in all HELCOM assessment units. The indicator is currently only fully operational for some species and in some assessment units due to data availability, resources and additional proposals to develop alternative approaches.


Relevance of the core indicator

Marine mammals are top predators in the marine ecosystem and good indicators of the state of food webs. They have a disposition for the accumulation of fat soluable hazardous substances such as heavy metals and PCBs in their tissues and thus reflect the level of pollution in the environment. Marine mammals are also affected in their reproductive functions by human influences that cause stress and disturbance such as hunting, by-catches, disturbance and noise pollution. The effect of algal toxins on seal reproductive health is so far unknown.

Distributions of different seal species during feeding and annual migrations encompass the entire Baltic Sea although no land-based haul-out sites occur in Germany, Latvia and Lithuania. Monitoring of relevant reproductive rate parameters occurs in all countries where stranded, by-caught or hunted seals are collected.

The reproductive rate provides important information at the average population level and on female health. It will be important to also develop an additional Seal Health Indicator that links trends in reproductive rate to the causes behind its variation, such as disease, food availability or hazardous substances.


Policy relevance of the core indicator

 BSAP segment and objectivesMSFD Descriptor and criteria
Primary link


  • Viable populations of species

D1 Biodiversity

D1C3 Population demographic characteristics of the species are indicative of a healthy population which is not adversely affected due to anthropogenic pressures.

Secondary link


  • Thriving and balanced communities of plants and animals

Hazardous Substances

  • Healthy wildlife

D1 Biodiversity

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 Food-web

D4C4: Productivity of the trophic guild is not adversely affected due to anthropogenic pressures.

D8 Contaminants

D8C2: The health of species and the condition of habitats are not adversely affected due to contaminants including cumulative and synergetic effects.

Other relevant legislation: In some Contracting Parties also EU Water Framework Directive – Chemical quality, Habitats Directive.

Cite this indicator

HELCOM (2018) Reproductive status of marine mammals.  HELCOM core indicator report. Online. [Date Viewed], [Web link].

ISSN 2343-2543


Download full indicator report

Reproductive status of seals HELCOM core indicator 2018 (pdf)