Four marine mammal species are resident in the Baltic Sea: the grey seal, harbour seal, ringed seal and the harbour porpoise. These mobile top predators have an important role in regulating the food web, but are also sensitive to pressures in all their areas of distribution, as well as to changes in the food web. Their exposure to accumulated pressures make marine mammals important indicators of the health of the ecosystem. The overall status of marine mammal species is unfavourable. However, at species level, grey seals and harbour seals show increasing population sizes. Of particular concern are the local population of harbour porpoise in the Baltic Proper, with a population size recently estimated at around 500 animals. Ringed seal is in a critical state in the Gulf of Finland, where it is currently only represented by around 100 animals and has a decreasing abundance.
Out of the four species of marine mammals in the Baltic Sea, grey seal (Halichoerus grypus) occurs in the whole region, whereas harbour seal (Phoca vitulina) is restricted to the southwestern Baltic Sea and the Kattegat, and ringed seal (Pusa hispida) to the eastern and northern Baltic Sea. Harbour porpoise (Phocoena phocoena) occurs mainly in the Kattegat and the southern parts of the Baltic Sea.
Hunting has been a major pressure on marine mammals in the Baltic Sea historically. The populations were severely reduced due to hunting in the beginning of the 1900s. Environmental contaminants caused further decimation of the populations in the 1960s and 1970s, by severely reducing the fertility of ringed and grey seals (Helle 1980). The harbour seal sub-populations in Kattegat and the Danish Straits have experienced two cases of mass mortality in recent times, caused by the ‘Phocine distemper virus’, resulting in more than 50 % of the sub-population dying in 1988 and about 30 % in 2002 (Härkönen et al. 2006). For harbour porpoise, drowning in fishing gear is a main pressure of concern. In all, these events have resulted in severe reduction of the abundance of marine mammals in the Baltic Sea, although today, the situation has improved for several seal populations.
Indicators for assessing marine mammals
The status of seals was assessed within population-specific management units, which are jointly agreed on in HELCOM. The following two indicators were applied to all seal species:
- ‘Population trends and abundance of seals’ is assessed in relation to that the population size in each respective management unit needs to be above the limit reference level (10,000 individuals) in order to have good status, and that a species specific growth rate should be Seals are counted as the numbers of hauled-out individuals during moult (HELCOM 2018an).
- ‘Distribution of Baltic seals’ reflects the occurrence of seals at haul-out sites and the range of seals at sea. Good status is achieved when the distribution of the species is close to pristine conditions. If pristine conditions cannot be achieved due to irreversible long term environmental changes, good status is achieved when all currently available haul-out sites are occupied (HELCOM 2018ao).
Grey seals were additionally assessed by two core indicators reflecting nutritional and reproductive status of the population.
- ‘Nutritional status of seals’ evaluates the blubber thickness of a specimen of the population in relation to a minimum threshold value (HELCOM 2018ap).
- ‘Reproductive status’ measures the proportion of adult grey seal females being pregnant or giving birth over the age of 6 years during July to February in relation to a minimum threshold value (HELCOM 2018aq).
There is currently no operational core indicator for harbour porpoise. HELCOM is developing indicators on the abundance and distribution of harbour porpoise, as well as on the number of drowned mammals caught in fishing gear. However, at present there are no defined threshold levels against which the status can be assessed, and these aspects are presented descriptively.
Integrated assessment results for seals
The status of seals is not good in most parts of the Baltic Sea, according to the integrated assessment. Seals show good status in the Kattegat, where the harbour seal population is assessed based on indicators of abundance and distribution (Figure 5.4.1). The assessment approach for seals requires that all included indicators and populations should achieve their threshold values in order for seals to have good status in the assessed spatial unit. Confidence in the integrated assessment of seals is classified as intermediate. Results for each species in separate are presented further below.
The three Baltic seal species have also been evaluated under the EU Habitats Directive in 2013. The results may differ from those presented here, as the Habitats Directive assessment is bounded by national borders, and the HELCOM assessment is carried out based on populations or sub-populations equivalent to regionally agreed management units. Another difference is that species are evaluated in comparison to a modern or historic baseline under the Habitats Directive, while threshold values in the HELCOM assessment are set in relation to the future viability of the management unit (Härkönen et al. 2017).
The number of grey seals counted in the whole Baltic Sea region in 2016 is 30,000 individuals, compared to the limit reference level of 10,000 individuals, and the population trend is assessed as achieving the threshold value. However, the overall status of grey seal is estimated as not good, since the indicators on reproductive and nutritional status do not achieve the threshold values (Figure 5.4.2).
The low reproductive and nutritional condition of grey seal may be connected to density dependent effects, if the seal population is approaching its ecological carrying capacity, which is likely the case for the grey seal population (See also Figure 5.4.3).
The grey seals of the Baltic Sea all belong to the same management unit, as they forage across the entire region. However, the abundance of grey seals varies between sub-basins. The number of grey seals in their core area of moulting distribution (covering the Bothnian Sea, Archipelago Sea and Western Estonian waters), is counted at over 25,000 in 2016. Around 1,300 grey seals are estimated for the other parts of the Gulf of Bothnia, 2,000 for the southern Baltic Sea and less than 1,000 for the Gulf of Finland. Monitoring along the Polish coast show a count of less than 200 individuals in a recently established haul-out. Some known historic grey seal haul-outs in the southern Baltic Sea are currently not used, and some have vanished due to exploitation of sand. According to the core indicator on the distribution of grey seals, good status is not achieved in the southwestern Baltic Sea.
The status of the ringed seal is assessed as not good (Figure 5.4.6). Ringed seals in the Gulf of Bothnia management unit are at a population size above the Limit Reference Level of 10,000 seals, but the threshold values for growth rate or distribution are not achieved (See also Figure 5.4.7). In the southern management unit, the status of ringed seal is critical. In this area, covering the Archipelago Sea, Gulf of Finland, Gulf of Riga and Estonian coastal waters, the population is decreasing. The eastern part of the Gulf of Finland has only around 100 animals.
Despite the weak results (Figure 5.4.6), the status of ringed seal in the integrated assessment is likely overestimated for the southern management unit. Due to a lack of estimates for population size, this parameter was included qualitatively in the assessment tool, which likely gave a stronger result than if quantitative estimates had been available.
The breeding of ringed seal is restricted by the availability of suitable sea ice. The ringed seal needs compact and very close pack ice where snow can accumulate, which makes it particularly sensitive to climate change (Sundqvist et al. 2012). The ringed seal is categorised as vulnerable on the HELCOM Red List (HELCOM 2013a).
A major study conducted in 2011–2013 using passive acoustic recorders supports the presence of two sub-populations of harbour porpoise in the Baltic Sea: one mainly occurring east of Bornholm in the Baltic Proper and the other one occurring in southern Kattegat, the Belt Sea, and the southwestern parts of the Baltic Sea (Anonymous 2016; Figure 5.4.8). A recent population genomics approach also emphasised notable differences between the Kattegat, Belt Sea, Western Baltic and the Baltic Proper (Lah et al. 2016).
Due to the lack of indicator, harbour porpoise was not included in the integrated assessment. However, the Baltic Proper sub-population is categorised as critically endangered in the HELCOM Red list (HELCOM 2013b). The number of animals is estimated to be around 500 animals (95 % confidence range 80 to 1,091). A large part of this sub-population occurs around the shallow offshore banks south of Gotland in summer during calving and mating.
The Kattegat-Belt Sea-Western Baltic sub-population is also assessed as threatened (HELCOM 2013b), albeit with the lower threat status ‘vulnerable’. The population is estimated at around 40,500 animals (95 % confidence range 25,614 to 65,041) using a visual line transect survey (Viquerat et al. 2014). Based on a later survey of small cetaceans in European Atlantic waters and the North Sea, Kattegat and Belt Sea (SCANS) there is no statistical support for a change in abundance over the period 1994 to 2016 (Hammond et al. 2016).
By comparing the age structure with the average age at sexual maturity, it has been estimated that around 28 % of the female harbour porpoises found dead along the German Baltic coast of Schleswig-Holstein had lived long enough to reach sexual maturity. In comparison, about 45 % of the dead females from the North Sea had reached sexual maturity. About 30 % of the animals were suspected to be by-caught, based on pathological findings. The low proportion of harbour porpoises reaching sexual maturity in the Baltic Sea supports the need to reduce the magnitude of bycatches (Kesselring et al. 2017; see also Box 5.4.1).
The harbour porpoise requires strict protection under the EU Habitats Directive as a species listed under Annex IV (concerning Animal and plant species of community interest in need of strict protection). For the Habitats Directive’s reporting period 2007 to 2012, the conservation status of harbour porpoise in the Baltic region (which includes both the Belt Sea population and the Baltic Proper population) is assessed as in the worst status class (‘unfavourable–bad’) by all countries that reported on the species in the Baltic Sea region: Denmark, Germany, Poland, and Sweden.
The situation of the status for Baltic Proper harbour porpoise is recognised by the agreement on the conservation of small cetaceans in the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) and is reflected in the ASCOBANS recovery plan for Baltic harbour porpoises (Jastarnia plan, ASCOBANS 2016) and HELCOM Recommendation 17/2 (HELCOM 2013f).
Recognizing the importance of ensuring the long term survival of the Baltic Sea seals, HELCOM agreed in 2006 on a Recommendation of the ‘Conservation of seals in the Baltic Sea’ (HELCOM 2006). The Recommendation is a regional agreement on joint management principles, management units for the different seal populations, limit reference levels for the respective management unit, and coordinated monitoring programmes. Today, the population trends are indicating recovery of most populations.
However, the overall status of the seal populations is still of concern, particularly for the ringed seal. Future perspectives are species specific, due to different habitat preferences and different pressures. Current ongoing pressures affecting marine mammals include climate change, fish stock depletion and contamination. Decimated populations are also threatened by mortality resulting from incidental by-catch, and harbour seals have previously been vulnerable to viral epidemics (1988, 2002 and 2014). In addition, underwater sound and chemical pollution, food depletion and disturbance are continuous pressures on harbour porpoises. For ringed seals available breeding sites in ice lairs are expected to decrease with climate change.
To protect the harbour porpoise, in particular the Baltic Proper population, the aim is to minimize incidental by-catches in fishing gear to close to zero, as agreed in the Baltic Sea Action Plan, but there is a lack of data for proper assessments. The HELCOM Marine Protected Areas are important to protect harbour porpoise, particularly when relevant management measures are in place.