Relevance of the Indicator

Biodiversity assessment

The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of the complex issue. In addition to providing an indicator-based evaluation of the reproductive status of seals, this indicator will also contribute to the next overall biodiversity assessment to be completed in 2018 along with the other biodiversity core indicators.


Policy relevance

The core indicator on reproductive status of seals addresses the Baltic Sea Action Plan's (BSAP) Biodiversity and nature conservation segment's ecological objective 'Viable populations of species'.

The core indicator is relevant to the following specific BSAP target:

  • 'By 2015, improved conservation status of species included in the HELCOM lists of threatened and/or declining species and habitats of the Baltic Sea area, with the final target to reach and ensure favourable conservation status of all species'.

The HELCOM Recommendation 27/28-2 'Conservation of seals in the Baltic Sea area' outlines the conservation goals which the indicator's threshold value is based on. The explicit long-term objectives of management plans to be elaborated are: Natural Abundance, Natural Distribution, and a health status that ensures the persistence of marine mammals in the Baltic.

The core indicator also addresses the following qualitative descriptors of the MSFD for determining good environmental status (European Commission 2008):

Descriptor 1: 'Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions'

Descriptor 4: 'All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity'

Descriptor 8: 'Concentrations of contaminants are at levels not giving rise to pollution effects'

Descriptor 10: 'Properties and quantities of marine litter do not cause harm to the coastal and marine environment' and

Descriptor 11: 'Introduction of energy, including underwater noise, is at levels that do not adversely affect the marine environment'

and the following criteria of the draft Commission Decision on GES criteria (2016):

  • D1C3 Population demographic characteristics of the species
  • D1C2: The population abundance of the species
  • D1C4: The species distributional range
  • D4C4: Productivity of the trophic guild
  • D8C2: The health of species and the condition of habitats are not adversely affected due to contaminants

Marine mammals were recognized by the MSFD Task Group 1 as a group to be assessed.

In some Contracting Parties the indicator also has potential relevance for implementation of the EU Water Framework Directive (WFD, Chemical quality) and Habitats Directive. The WFD includes status categories for coastal waters as well as environmental and ecological objectives, whereas the EU Habitats Directive (European Commission 1992) specifically states that long-term management objectives should not be influenced by socio-economic considerations, although they may be considered during the implementation of management programmes provided the long-term objectives are not compromised. All seals in Europe are also listed under the EU Habitats Directive Annex II (European Commission 1992), and member countries are obliged to monitor the status of seal populations.


Role of marine mammals in the ecosystem

Being top predators in the Baltic ecosystem, seals are exposed to ecosystem changes in lower trophic levels, but also to variations in climate (length of seasons and ice conditions) and impacts of human activities. These pressures can affect fish stocks, levels of harmful substances, boat traffic, noise pollution, as well as direct mortality caused by hunting or by-catch. The vulnerability of seals to these pressures makes them good indicators for measuring the environmental status of ecosystems.


Ecological background to the indicator concept

An adult female seal bears at most one pup annually in healthy growing seal populations. The mean values of fecundity for entire populations will always be lower than the theoretical maximum for an individual, also for populations which live under favourable conditions. Chance events such as failed fertilization or early abortions reduce annual pregnancy rates. Mean pregnancy rates rarely reach 0.96 in samples of reasonable sizes in American (Boulva & McLaren 1979; Bigg 1969), and European harbour seals (Heide-Jørgensen et al. 1992) in age classes >4 years of age. Maximum life span is about 35-45 years in Baltic seal species (e.g. Heide-Jørgensen et al. 1992). Another factor that will decrease mean pregnancy rates is senescence (Heide-Jorgensen et al. 1992), however due to annual mortality rates, only a small fraction of the population becomes older than about 24 years old. Further, extrinsic factors will reduce pregnancy rates. In evaluating changes in mean pregnancy rate among years in this core indicator, it is important to separate the causes into (1) natural decline due to density dependent effects and (2) anthropogenic effects from environmental pollution, this will be linked in the new Seal Health Indicator. The HELCOM core indicator 'Population trends and abundance of seals' will signal when the populations reach carrying capacity. But at population abundances below carrying capacity, a change in pregnancy rate can be an early warning of unwanted changes in the ecosystem.

Natural decline in fertility due to limited food supply

As seal populations approach carrying capacity and food limitation becomes an issue, body growth rate in sub-adult seals declines and the age at sexual maturation is delayed. In poor nutritive conditions, age at sexual maturity in phocid seals can be delayed up to three or four years (Kjellqvist et al. 1995; Harding & Härkönen 1999). Other stressors such as infectious disease and stress can also delay sexual maturity. Another response to poor nutritive conditions is so called 'year skipping', i.e. the female does not become pregnant when her fat stores are too low (Kjellqvist et al. 1995). Seals have delayed implantation and the fertilized egg does not attach to the uterine wall unless the female is well fed.  Decreased pregnancy rate due to food shortage at carrying capacity is thus a natural phenomenon and shall not be confused with reproductive failure caused by disease or xenobiotics.


Reproductive failure caused by disease or xenobiotics

The Baltic ringed and grey seal populations became the main subjects in the PCB scandal. The mean level of PCB in seals from the northern Baltic Proper was about 450 parts per million (PPM) lipid in the early 1970s, which eventually declined to considerably lower values in accordance with lower concentrations in their prey (Jensen et al. 1969; Olsson 1977; Bignert et al. 1998). A sample of 225 adult ringed seal females revealed an alarmingly low pregnancy rate of 30% which dropped further to 20% during the period 1973-1979 (Helle 1980). The low reproductive rates were largely explained by occlusions in the uterine horns. The prevalence of this pathological change increased from 35% to 59% during the same time period (Helle 1980). The occlusions caused permanent sterility in ringed seals and the frequency of occlusions also increased with the age of the animals (Helle 1979; 1980). Also in grey seals, severe reproductive disturbances were documented (Bergman & Olsson 1986; Bergman 1999). An underlying cause of some of the toxic effects of PCBs may be alterations in hormonal levels (Bäcklin et al. 2003). Experiments carried out on the American mink (Neovison vison) showed that the early formation of the placenta is disrupted in animals exposed to PCBs, which leads to the death of the foetus (Bäcklin et al. 2003).

In populations of harbour seals, concentrations of PCBs vary with the level of industrialization and the extent of water exchange of different sea regions. This is demonstrated by mean values of concentrations of different PCB fractions in harbour seals in the Atlantic, where Icelandic harbour seals have the lowest concentrations of about 1.5-5.0 PPM lipid, while seals in the heavily industrialized and enclosed St. Lawrence Estuary show concentrations of about 17.1 PPM (Safe 1984). The harbour seals in the Baltic Sea and Wadden Sea had mean concentrations of 85 PPM lipid (Bernt et al. 1999) in the late 1970s. The effects of high levels of PCBs are generally very difficult to quantify. One reason is that levels of PCBs vary substantially depending on which part of the season, which age groups, individuals and which parts of the body are sampled (Safe 1984; Bignert et al. 1993). However, a controlled feeding experiment revealed lowered pregnancy rates in captive seals fed with Baltic herring compared to the control group that got North Sea herring (Reijnders 1986). The most likely candidate responsible for the former low gynaecological health among Baltic seals was high concentrations of PCB (Helle 1979; Bredhult et al. 2008; ). Levels in the Wadden Sea harbour seal populations are still quiet high (Siebert et al. 2012), nevertheless the populations have recovered very quickly after each die-off.

In 2008-2009, the pregnancy rate was 88% in 4-20 years old grey seal females hunted in the Bothnian Sea and the Baltic Proper. The last case of uterine obstruction in grey seals investigated in Sweden was seen in 1993 (Bergman 1999). And in 2009, one unilateral occlusion was seen in a 13-year old female grey seal in Finland. In the 2000s, about 20% of examined Baltic ringed seals still suffered from uterine obstructions, which likely explain the 68% pregnancy rate in ringed seals in 2001-2009, which is lower than "normal" (Helle et al. 2005; Kunnasranta 2010). After the year 2000 there are 62 females which are at least four years old (data from Finland and Sweden), and 8.1% of these had occusions. The last observed case is from 2011. There are no observations or reports of uterine obstructions in Baltic harbour seals or harbour porpoises.

It is important to distinguish between pregnancy rate, birth rate, pup production (= pups that survive until weaning), and the role of pregnancy/birth rate rate for the population growth rate. Even if a female weans her pup successfully, a study on individually branded harbour seals showed a delayed response to poor nutritive conditions (Härkönen & Harding 2001; Harding et al. 2005). Winter survival in the young of the year was highly dependent on the autumn weight. Consequently, pregnancy/birth rate is an important indicator of status of the population, but in evaluations for population consequences also other information is needed, the new Seal Health Indicator aims to assess the causes behind shifting trends in pregnancy rates.


Human pressures linked to the indicator

  General MSFD Annex III, Table 2
Strong link

Contamination by hazardous substance

Fisheries and food availability

Ecosystem changes (food web, introduction of pathogens and non-indigenous species)

Noise pollution


Theme: Biological

  • Disturbance of species (e.g. where they breed, rest and feed) due to human presence
  • Extraction of, or mortality/injury to, wild species (by commercial and recreational fishing and other activities)

Theme: Substances, litter and energy

  • Input of other substances (e.g. synthetic substances, non-synthetic substances, radionuclides)
Weak linkHunting

Theme: Substances, litter and energy

  • Input of litter (solid waste matter, including micro-sized litter)
  • Input of anthropogenic sound (impulsive, continuous)

Historically, hunting of seals has been a major human pressure on all the seal species in the Baltic Sea. A coordinated international campaign was initiated in the beginning of the 20th century with the aim of exterminating the seals (Anon. 1895). Bounty systems were introduced in Denmark, Finland and Sweden over the period 1889-1912, and very detailed bounty statistics provide detailed information on the hunting pressure. The original population sizes were about 180,000 for ringed seals, 80,000 for Baltic grey seals and 5,000 for the Kalmarsund population of harbour seals (Harding & Härkönen 1999; Härkönen & Isakson 2011). Similar data from the Kattegat and Skagerrak suggest that populations of harbour seals amounted to more than 17,000 seals in this area (Heide-Jørgensen & Härkönen 1988). Changes in population density will affect pregnancy rates.

By-caught grey seals are significantly leaner as compared with hunted seals (Bäcklin et al. 2011, Kauhala et al. 2015), which may suggest that food is a limiting factor for by-caught grey seals. It is possible that food limitation is becoming an important factor also for the entire population since data on blubber thickness in Baltic grey seals show a significant decline during the last decade (Bäcklin et al. 2011). Food limitation is expected to lead to declining pregnancy rates in all species.