Results and Confidence

Ringed seal 

In breeding and moulting times the ringed seal occurs in the Bothnian Bay, Archipelago Sea, Western Estonia (Gulf of Riga and Estonian coastal waters), and the Gulf of Finland. Breeding distribution is confined to suitable breeding ice in all subpopulations. Currently the area of occupancy is more restricted compared to pristine conditions, and thus the status of ringed seal distribution does not achieve the threshold value and indicates not good status (Results figure 1).

 

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Result figure 1. Status evaluation outcome for ringed seal distribution.


Distribution during breeding and moult

The winter distribution of ringed seals is tightly linked to the extent of sea ice fields suitable for building lairs (Results figure 2). Highest concentrations of seals are seen in broken consolidated ice that trap snow heaps. Females give birth to their pups in the lairs, which protect the pups against the elements and predators. Formation of this type of ice is critical for the breeding success of this species.

 

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Results figure 2. Access to broken consolidated sea ice is critical for ringed seal breeding success (Sundqvist et al. 2012).

 

The extent and quality of ice show considerable inter-annual variation, but there has been a significant reduction of the formed sea ice area since the 1970s, when compared to historical data (Results figure 3). Climatological modelling shows that the situation is predicted to result in diminishing ice fields and shorter ice covered seasons in the future. This will likely result in the extirpation of the ringed seal sub-population in the Gulf of Riga and severely reduce the population growth rate in the Gulf of Finland and the Bothnian Bay (Sundqvist et al. 2012).

 

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Results figure 3. Extent of maximum annual sea ice fields in thousands of square km since 1650 (Sundqvist et al. 2012). A significant drop occurred after 1970. Predicted future changes will reduce suitable breeding ice for ringed seals and grey seals.

 

Ringed seals have been surveyed during moult via aerial surveillance since 1988 in the Bothnian Bay, and the distribution has been very similar when ice fields extended down to the northern Quark area. Highest concentrations have always been in the central northern part of the Bothnian Bay (Results figure 4), which is similar to the situation in 1930 (Olofsson 1933). In 1996 also the Gulf of Finland and the Gulf of Riga were covered by ice, permitting the first comprehensive survey of ringed seals in the entire Baltic (Results figure 4). In years when ice fields are more limited the distribution of seals also changes.

 

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Results figure 4. Winter distribution of ringed seals hauled out on ice during the 18th to 25th of April 1996, when ice fields extended to the northern Quark area in the Bothnian Bay, much of the Gulf of Finland and the Gulf of Riga.

 

The breeding distribution is identical to the distribution during moult in years when the ice fields are vast and remain intact up to late April. However, when ice fields break up early, ringed seals show a completely different pattern of distribution and gather in larger groups along ice cracks or leads, and when ice is scarce they haul out on rocks. Consequently, ringed seal breeding distribution is closely linked to the extent and composition of the ice cover, which has deteriorated over the past four decades and is predicted to do so also in the future.

Since breeding and moulting areas are being reduced, ringed seals cannot be seen as having achieved good status as the threshold value is failed in this evaluation.

 

Distribution during the ice free period

Some Baltic ringed seals have been equipped with satellite transmitters which have provided data on distribution during the ice free period as well as area of occupancy. During the summer, ringed seals spend about 85% of their time in water - feeding, travelling and resting. Studies have shown that ringed seals mainly stay in the basins where they were tagged (Results figure 5; Härkönen et al. 2008), although some animals can move long distances (Oksanen et al. 2015).

Data from data loggers also show that ringed seals regularly return to the same rocks to haul out during the night. The distribution of these haul-out sites is well known in Estonia and Russia, but not to the same extent in Sweden and Finland.

Ringed seals have free access to haul-out sites and foraging areas, which is why they can be evaluated as having achieved the threshold value and indicating good status with regard to area of occupancy.

 

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Results figure 5. Positions of ringed seals tagged with satellite transmitters in the Bothnian Bay (blue), the Gulf of Finland (red), and Estonian coastal waters (green) during the ice free period of the year.

 

Grey seal

The grey seal population in the Baltic Sea area, excluding the Kattegat, achieved the threshold values for the three parameters evaluated and indicate good status, namely the parameters breeding and moulting sites and area of occupancy in all of the Baltic except for the Southwestern areas (Arkona basin, Bay of Mecklenburg, Kiel Bay, Great Belt and the Sound) (Results figure 6). In the Southwestern areas some sites formerly used for reproduction have not been recolonized.


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Results figure 6. Status evaluation outcome of grey seal distribution in the Baltic Sea.

 

The distribution of grey seals has been expanding to the south over the last decades and this species is now present at Christiansø and Rødsand in Denmark and the Polish coast, and many former haul-out sites have thus been colonized (Results figure 7). The colonization process follows a specific repeatable pattern, where new sites are visited by single animals for up to ten years, after which numbers slowly increase. At some point numbers of sub-adult and adult seals start to increase sharply and few pups are being born. After some five to ten years, the numbers of pups start to increase dramatically. Different phases of this process can be seen at colonized sites along the North Sea coast, Måkläppen in southern Sweden and Rødsand in southern Denmark (Härkönen et al. 2007). Some of the land sites are used for both breeding and moulting, where a majority of pups are born at "older" sites.

The grey seals have achieved the threshold value for the breeding sites component of the indicator except for the Southwestern Baltic. A modern baseline is used in the status evaluation since some breeding sites in the southern Baltic have disappeared as a consequence of exploitation of sand for industrial use. No baseline can be identified for grey seals in the Kattegat which mainly consists of visiting animals from the North Sea population (Fietz et al. 2016).


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Results figure 7. Grey seal haul-out sites in the Baltic Sea. The map includes all currently known haul-out sites, but seals were historically known to use haul-out sites Southwest of Samsø and around Fyn in Southwestern Baltic.

 

The area of occupancy encompasses the entire Baltic Sea ecosystem and grey seals can freely access sites and foraging grounds. Grey seals are evaluated as having achieved good status with regard to area of occupancy. Large numbers of satellite and GSM transmitters have been deployed on Baltic grey seals, and it is evident that they forage and travel in the entire Baltic Sea, although no haul-out sites occur along the Latvian and Lithuanian coasts (Results figure 8). 


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Results figure 8: Movements of grey seals (white) and harbour seals (red) tagged with GSM transmitters at Måkläppen in Southern Sweden. Grey seals travel extensively in the Baltic whereas harbour seals are more sedentary.

 

Harbour seal

In the areas of Kalmarsund, Kattegat and Limfjord the harbour seal populations are evaluated as having achieved the threshold value with regard to distribution on land sites, but the population in the Western Baltic (Arkona basin, Bay of Mecklenburg, Kiel Bay, Great Belt and the Sound) has not achieved the threshold value, and thus indicates not good status (Results figure 9). For the area of occupancy parameter, harbour seals reflect good status in all assessment units.

 

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Results figure 9: Status evaluation outcome for harbour seal distribution in the Baltic Sea.

 

The harbour seal occurs on all suitable haul-out sites in the Kalmarsund, the Kattegat and the Limfjord (Results Figure 9). In the Western Baltic, harbour seals do not currently occur regularly at historical localities south of the island of Fyn or in the Great Belt. Haul-out sites (Results Figure 10) are used for breeding, moulting and resting and thus the distribution of sites reflect both the distribution of breeding sites as well as sites used for other activities. 


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Results figure 10: Haul-out sites of Baltic harbour seals.

 

Harbour seals have been tagged with GSM transmitters to study their movements in the Kattegat and also in the Western Baltic over the period 1995-2015. Seals can travel freely among sites and feeding grounds and the area of occupancy is not diminishing and thus as the threshold value is achieved, good status can be assigned to all populations of harbour seals with regard to area of occupancy.

 

Confidence of the indicator status evaluation

The confidence for both harbour seals and grey seals is considered to be high in all assessment units, as many observations are available from all years in all the relevant assessment units, with no clear temporal or spatial bias. Monitoring activities are currently carried out at a high spatial and temporal frequency. Survey data is available for harbour seals in the Kattegat since 1979, 1972 in the Kalmarsund, 1990 in Western Baltic, and for grey seals data is available since 2000 for in the entire Baltic Sea. For grey seals there are data from Sweden also two decades before this time.

The confidence for the ringed seal assessment is regarded as moderate since surveys are sporadic in their most southerly management area (Archipelago Sea, Gulf of Riga and Estonian Coastal waters). Annual surveys are carried out for all species and management units except for ringed seals in the Gulf of Riga and Estonian coastal waters. However, surveys from ground have been carried out since the beginning of the 1990s. Main pressures such as diminishing ice fields and sand exploitation are well known on a qualitative level, but more work is needed to quantify those pressures. Survey data for ringed seals is available since 1988 in the Bothnian Bay, while ringed seal data in their most southerly management area is scarce. Although data is scarce in their most southerly management area for ringed seals, this subpopulation is clearly not achieving the threshold value and hence the result of the evaluation of the populations against the set threshold values is deemed to be reliable in indicating a not good status.