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 area 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. Overall, the status of marine mammal species is assessed as unfavourable. However, at species level, grey seals and harbour seals show increasing population sizes. A particular concern is the local population of harbour porpoise in the Baltic Proper, with a population size recently estimated at around 500 animals. Also, the population of ringed seals in the Gulf of Finland is of concern, as the population (which is sensitive to climate change) is decreasing, currently only represented by around 100 animals.
Hunting has been a major pressure on marine mammals in the Baltic Sea historically. Populations of seal were severely reduced due to hunting at the beginning of the 1900s. Environmental contaminants in the 1960s and 1970s caused further decimation of the populations by severely reducing the fertility of ringed and grey seals (Helle 1980).
The harbour seal sub-populations in Kattegat and the Danish Straits have also 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).
These events resulted in severe reduction of the abundance of mammals in the Baltic Sea, but today the situation has improved for several of the populations.
Indicators included in the assessment
The status of the seal species was assessed by core indicators reflecting population trends and abundance, as well as their distribution (Core indicator reports: HELCOM 2017aa-ab). Grey seals were also assessed using core indicators reflecting changes in nutritional status and reproductive status (Core indicator reports: HELCOM 2017ac-ad, Box 5.4.1). The seal populations in the Baltic Sea are managed and assessed according to management units that have been jointly agreed in HELCOM. There is currently no operational core indicator for harbour porpoise.
For threats on marine mammals from incidental by-catch, see Box 5.4.2, for hunting on seals, see Chapter 4.6 Species removal by fishing and hunting.
Integrated status assessment of seals
Seals are not in good status according to the integrated assessment, with exception of the Kattegat where only the harbour seal population was assessed (Figure 5.4.1). Good status would require all populations for all species to reach good status for all indicators. All four core indicators were used in the assessment, but those reflecting reproduction status and nutritional status are currently only applied to grey seals. The confidence in the assessment was higher for grey seals than for the other seal species due to the lack of indicators reflecting population conditions for harbour seals and ringed seals.
All three species of seal have also been evaluated under the EU Habitats Directive in 2013, where the assessment is bounded by national borders. The HELCOM assessment is carried out based on populations or sub-populations, which are equivalent to regionally agreed management units. Another difference is that evaluation is made against a modern or historic baseline under the Habitats Directive and against thresholds set to ensure future viability of the management unit in the HELCOM assessment (Härkönen et al. 2017). Due to these differences, the evaluation results may differ between the EU Habitats Directive and the HELCOM assessment.
Results for species
The number of grey seals counted in the whole Baltic Sea region in 2015 was 30 000 individuals, which is above the limit reference level of 10 000 individuals, and the population trend is assessed as being in good status (Figure 5.4.3). However the status of the grey seal in the overall assessment is not good (Figure 5.4.2). This is due to the inadequate reproductive and nutritional status, although the values in the assessment period are relatively close to the threshold values for the respective indicator (Core indicator report: HELCOM 2017aa). The reasons for the inadequate condition of the grey seal population have not yet been established.
All grey seals in the Baltic Sea belong to the same management unit and they forage across the entire Baltic Sea. However, their abundance varies between sub-basins; in 2015 about 22 000 grey seals were counted in the Gulf of Bothnia, Åland and Archipelago Seas (including Stockholm county), while counts along the Polish coast were only a few tens of animals. With regard to distribution, some known historic grey seal haul-outs in the southern Baltic Sea are not used, and some have vanished due to exploitation of sand, and according to the definition of the core indicator the distribution of grey seals is thus not achieving good status in the southwestern Baltic Sea (Core indicator report: HELCOM 2017ab).
Of the three management units of harbour seals in HELCOM area, only the Kattegat population shows good status (Figure 5.4.4).
The harbour seals in the southwestern Baltic and the Kattegat are connected and are assessed as one so called metapopulation with respect to abundance. However, they are assessed as separate sub-populations in terms of growth rate. The metapopulation was about 16 000 animals in 2015 and achieves the threshold value for abundance, but the sub-population in the southwestern Baltic does not achieve threshold value for growth rate (Figure 5.4.5). However growth rate is close to the threshold value. Hence, the core indicator on trends and abundance achieves good status in Kattegat but not in the southwestern Baltic Sea (Core indicator report: HELCOM 2017aa).
The Kalmarsund population is genetically divergent from the other populations of harbour seal. The population meets the threshold value for population growth rate, but the total abundance was still only about 1 100 seals in 2015 (total abundance estimate). The Kalmarsund population is also categorised as vulnerable in the HELCOM Red List (HELCOM 2013b).
The status of the ringed seal is not good (Figure 5.4.6). In areas where ringed seals occur, namely the Gulf of Bothnia, as well as the management units consisting of the Archipelago Sea, Gulf of Finland, Gulf of Riga and Estonian coastal waters, the distribution is restricted compared to pristine conditions. The size of the population is above the limit reference level of 10 000 seals in the Gulf of Bothnia (where around 20 000 ringed seals reside), but the growth rate is below threshold values in both managements units (Figure 5.4.7, Core indicator report: HELCOM 2017aa). The status of the ringed seal population in the southern management unit is critical; the population is decreasing, and the eastern part of the Gulf of Finland has only around 100 animals.
Breeding distribution is confined to suitable breeding ice that is compact and very close pack ice where snow can accumulate, making the ringed seal particularly sensitive to climate change (Sundqvist et al. 2012). The ringed seal is categorised as vulnerable on the HELCOM Red List (HELCOM 2013b).
A major study conducted in 2011–2013 using passive acoustic recorders support that there are 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).
The Baltic Proper sub-population was categorised as critically endangered in the HELCOM Red list (HELCOM 2013b). The number of animals in this sub-population is estimated to be around 500 animals (95% confidence range 80 to 1091). A large part of this sub-population occurs around the shallow offshore banks southwest of Gotland in summer during calving and mating.
The Kattegat-Belt Sea-Western Baltic sub-population was estimated at around 40 500 animals (95% confidence range 25 614 to 65 041) using a visual line transect survey (Viquerat et al. 2013). This sub- population was also assessed as threatened by HELCOM albeit with the lower threat status ‘vulnerable’. However, based on a later survey of small cetaceans in European Atlantic waters and the North Sea (SCANS) the population has been stable over the past twenty-two years (Hammond et al. 2016).
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 was 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 was 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 2009) 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 (Figures 5.4.3, 5.4.5, 5.4.7).
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). 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, minimizing incidental by-catches in fishing gear is crucial. The HELCOM Marine Protected Areas (see Figure 7.3 in Chapter 7 – HELCOM actions to improve the Baltic Sea) are also important to protect these species in the Baltic Sea region.