Results and Confidence

Grey seal 

The grey seal population abundance and population growth rate (based on no decrease greater that 10% during a 10-year period) parameters exceed the threshold value, indicating that the grey seal population has achieved good status (Results figure 1). Abundance is considerably above the LRL of 10,000. However, growth rate is below the threshold of 7%. As the population is suggested to approach the carrying capacity, grey seals will achieve good status if criteria for this scenario are used, i.e., no decrease greater than 10% during a 10-year period. Data remain inconclusive (i.e. longer time series are required for full statistical evaluation of carrying capacity) though based on expert opinion good status is assigned to the grey seal in this assessment.

 

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Results figure 1. Overall, Baltic grey seals achieved good status with regard to population growth rate and abundance. Abundance is considerably above the limit reference level of 10,000. Grey seals in the Kattegat originate from both the Baltic and the North Sea populations. Thus this area is not included in the assessment. Click here to access interactive maps at the HELCOM Map and Data Service: ringed sealgrey seal and harbour seal.

 

For the grey seal a time series of data from 2003 and onwards is used to estimate the population growth rate and its statistical support. The annual population growth rate during the assessment period 2003-2016 was 5.3%. A Bayesian analysis shows 80% support for a growth rate value of ≥4.8% (Results figure 2). Earlier data from the Swedish monitoring programme indicate that the grey seal population was growing at a rate of about 8% per year from the early 1990s in the Baltic Sea (Stenman et al. 2005; Hårding et al. 2007). Although the growth rate is well below the threshold value for good status (7%), the population growth rate seems to level off, which indicates that the population is approaching the carrying capacity. Under this assumption, the observed growth rate should be evaluated against "rate of decrease less than 10% over a 10-year period". The population has thus achieved good status according to the growth rate parameter, and as more than 30,000 animals have been counted since 2014 the population size is also well above the minimum viable population size (i.e. limit reference level (LRL) of 10,000 individuals).


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Results figure 2. The annual growth rate of Baltic grey seals during the assessment period 2003-2016 was 5.3 % and the Bayesian analysis shows 80% support for a growth rate ≥4.8%. This is well below the threshold value for good status at 7%. However, population growth rate seems to be levelling off, which indicates that the population is approaching the carrying capacity. Under this assumption, the observed growth is evaluated against "rate of decrease less than 10% over a 10-year period".

 

Grey seal counts in the Kattegat amount to approximately 100 animals, of which a majority are found at Læsø, Anholt, Bosserne and Varberg, although single animals are seen all along the Swedish west coast. The grey seals here come both from the Baltic Sea and the Atlantic populations (Fietz et al. 2016), and pupping occurs irregularly on Læsø and Anholt but also other sites (Härkönen et al. 2007).

The confidence of the evaluation for the Baltic grey seal is given by <5% support for a growth rate exceeding the threshold value of 7%. Internationally coordinated survey data up to 2016 are sufficient for the analysis.

 

Ringed seal

Both ringed seal management units are assigned not good status since the population growth rates are below the threshold value for good status (Results figure 3).

 

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Results figure 3. Ringed seals in the two management units (1 - the Bothnian Bay and 2 - the southern unit encompassing the Archipelago Sea, the Gulf of Finland and the Gulf of Riga including Estonian coastal waters) do not achieve good status. Click here to access interactive maps at the HELCOM Map and Data Service: ringed sealgrey seal and harbour seal.

Due to exceptionally mild winters in recent years, the fast ice (ice attached to land) has started to break up during or before the survey time, which has revealed new features in the hauling out behaviour of ringed seals. In ice conditions where a lot of cracks have appeared to the fast ice cover, more ringed seals enter to the ice forming large groups. This phenomenon was not observed during earlier years when the surveys were carried out in more stable "normal" fast ice conditions. The survey results from the mild years revealed that the population size most probably exceeds 20 000 animals in the Bothnian Bay management unit, which is well above the LRL of 10 000 animals. The proportion of the true population which is available for the survey method in different circumstances still remains uncertain.

The ringed seal population in the Bothnian Bay management unit has been increasing at a rate of 4.5% per year since 1988 (Hårding & Härkönen 1999; Karlsson et al. 2008). During 2003-2016 the growth rate was 5.9 % per year, which is still less than the intrinsic capacity and below the threshold value of 7% (Results figure 4; Karlsson et al. 2008). A Bayesian analysis gives 80 % support for a growth rate ≥4.6 % which is well below the threshold value of 7%. The annual growth rate is based on survey results carried out during the ''normal'' ice conditions. Anomalous survey results from very mild winters (2013, 2014 and 2015) are excluded from the trend analysis since they stand out as statistical outliers and since they do not reflect the variation in the true population size, nor sampling bias (Results table 1.), but the exceptional ice-conditions. Further research and relevant quantitative measures for the ice quality are needed to gain a better understanding the haulout behaviour of ringed seals, calibrating the survey results in different ice-conditions and for estimating the true population size. To sum up, the ringed seals in the Bothnian Bay management unit have reached good status for population size but not for growth rate.

 

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Results figure 4. The annual growth rate of ringed seals in the Bothnian Bay during the assessment period of 2003-2016 was 5.9 %, which is below the threshold value of 7 %. For the trend analysis only the comparable data points (circled crosses) were used. The Bayesian analysis shows 80% support for a growth rate ≥4.6 %. Consequently, criteria for good status are not met. Outliers from mild winters (2013-2015) are excluded from trend analysis, see above.

 

Results table 1. Annual ringed seal survey results from the Bothnian Bay showing the number of observed ringed seals on the survey strips, fraction of the area covered with the survey strips and the number of hauling out ringed seals (calculated from the first two variables).

YearObservedSampling fractionHauling out
2003426    13.33203
2004631    13.34744
200544813.33368
200677613.35835
200760213.34526
2008   
200980913.36083
2010174026.66541
201178513.35902
2012324153.26092
2013137513.310338
2014422226.316053
2015344117.2619936
20165026.757437

                                                                                                                                      

In the Southern ringed seal management unit (i.e. Gulf of Riga, Gulf of Finland and Archipelago Sea) improving trends have not been observed and the population sizes only sum up to a small fraction of the LRL (Karlsson et al. 2008; M. Jüssi pers. com; M. Ahola pers comm.). The ringed seal stock in the Gulf of Finland is decreasing, amounting to about 100 animals (M. Verevkin pers. com), and is considered to indicate a very alarming status. Thus, the Southern ringed seal management unit is clearly below good status in both population size and growth rate.

Although it is important for management to obtain better data from the southern part of the population, in terms of evaluation under this indicator it can be confidently stated that the stock in the Southern management unit is very far from good status both with respect to abundance and growth rate, which is why the confidence of the evaluation is high. The Bothnian Bay management unit clearly exceeds the LRL, but the growth rate is still below good status giving a high confidence for the evaluation.

 

Harbour seal

For the harbour seals in the Kattegat and Southern Baltic, good status has been achieved under the abundance criterion of the metapopulation. However, the population growth rate parameters for the Southern Baltic are below the criteria for good status. The Kattegat management unit is evaluated under the assumption that it is approaching carrying capacity for the area. Under this assumption, it achieves good status. (Results figure 5). The population in the Kalmarsund does not achieve good status for either parameter and the status of the Limfjord population is uncertain as studies are limited with regards to the degree of connection to the Wadden Sea population (which is not evaluated here).

 

Population trends harbour seal.png

Results figure 5. Harbour seals occur in three management units, where seals in the Kalmarsund do not achieve good status because of population size well below the LRL and a growth rate below the threshold value of 9%. Harbour seals in the Kattegat and Southern Baltic metapopulation do achieve good status with respect to their combined abundance, but the Southern Baltic unit does not achieve good status because the growth rate does not meet the threshold for that. The Limfjord management unit has not been evaluated separately due to limited information about migration and interbreeding with the Wadden Sea population. Limfjord is part of the Kattegat assessment unit in this indicator assessment, hence the indicated result for the Limfjord population is uncertain. Click here to access interactive maps at the HELCOM Map and Data Service: ringed sealgrey seal and harbour seal.

 

Kalmarsund

The harbour seal population in Kalmarsund is genetically divergent from the adjacent harbour seal populations (Goodman et al. 1998) and experienced a severe bottle-neck in the 1970s when only some 30 seals were counted. Long-term isolation and low numbers have resulted in low genetic variation in this population (Härkönen et al. 2006). The population has increased annually by 9% since 1975 and counted numbers amounted to about 1,000 seals in 2014, see also Härkönen & Isakson (2011).

During the assessment period 2003-2016, the Kalmarsund population has increased on average by 7.9 % per year. A Bayesian analysis of the trend in abundance shows that there is 80% support for a growth rate of ≥6.9%, which is below the threshold value of 9% (Results figure 6). The current population size is well below the LRL of 10,000 individuals, which is why this population does not achieve good status. The confidence of this evaluation is high since it is based on sufficient survey data.


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Results figure 6. The annual growth rate of counted harbour seals in Kalmarsund was 7.9% during the assessment period 2003-2016. According to Bayesian statistics there is 80% support for a growth rate ≥6.9%, which is below the threshold value of 9%. The total number of individuals (approximately 1,000 animals) is well below the LRL of 10,000, which means that this population does not achieve good status.

 

Kattegat, the Danish Straits and the Southern Baltic Sea

Harbour seals in this area experienced a mass mortality caused by a Phocine Distemper Virus (PDV) epidemic in 2002 which is why the growth rate is analyzed over a period starting after this event. We here show that sub-populations of this metapopulation showed different growth rates, but will also summarise results at the metapopulation level. Thus, while the current evaluation is based on abundance at the metapopulation level, the sub-populations are evaluated independently for abundance trends. It should be kept in mind that the situation can differ among subpopulations, which may have different growth and vital rates. Management actions should take special care when dealing with small subpopulations, ensuring that anthropogenic activities do not jeopardize future persistence of such subpopulations.

In the Southern Baltic, the average annual rate of increase during the assessment period 2003-2016 was 6.6% (Results figure 7). According to the Bayesian analysis there is 80% support for a growth rate ≥5.9%, which is below the threshold value of 9%. Thus, this subpopulation does not achieve good status. Confidence of this evaluation is high and based on sufficient survey data.

 

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Results figure 7. The annual growth rate of counted seals in the Southern Baltic harbour seal subpopulation was 6.6% during the assessment period 2003-2016. According to Bayesian statistics there is 80% support for a growth rate ≥5.9%, which is below the threshold value of 9%. The abundance of seals is also well below the set LRL, which means that this subpopulation on its own does not achieve good status.

 

The harbour seal subpopulation in Kattegat and the Northern Great Belt experienced two dramatic mass mortality events due to PDV when more than 50% of the population died in 1988 and about 30% in 2002 (Härkönen et al. 2006). Unusually large numbers also died in 2007, but the reason for this mortality remains unclear (Härkönen et al 2007). In the spring of 2014, some seals appearing to show signs of pneumonia found in Sweden and Denmark, and also on the North Sea coast. Avian influenza H10N7 was isolated from a number of seals (Zohari et al. 2014). Population surveys in August 2014 showed lower numbers at all seal localities, suggesting a total mortality of approximately 10%.

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Results figure 8. The annual growth rate of counted seals in the Kattegat harbour seal subpopulation was 6.3% during the assessment period 2003-2016. There is 80% support for a growth rate ≥5.8%, which is below the threshold value of 9%. The subpopulation is above the LRL and is assumed to be approaching carrying capacity, which is why this subpopulation on its own achieves good status since it meets the criterion "decline not >10% over a 10-year period".

 

The rate of increase between the two PDV epidemics was close to 12% per year as in the adjacent North Sea populations (Table 1). This high annual increase is close to the intrinsic rate of increase in harbour seals (Härkönen et al. 2002).

The annual population growth rate in Kattegat and the Danish Straits was close to 12% per year until 2010, but data suggest that it is levelling off, even if the increased mortality in 2014 is taken into account, which is likely to be caused by density dependence, indicating that the subpopulation is approaching carrying capacity. Independent data on somatic growth support that the population is approaching the carrying capacity of the system (Harding et al in prep.). Additional surveys are needed to establish this with high confidence. However, the increase over the period 2003-2016 at 6.3% is significantly lower than the 12% increase during earlier exponential growth (Results figure 8). Bayesian analysis shows 80% support for a growth rate ≥5.8% for the period 2003-2016, which is below the threshold value of 9%. However, the management unit is regarded as approaching the carrying capacity in this assessment and will thus achieve good status since it meets the criterion "decline not >10% over a 10-year period".

For the Kattegat and Southern Baltic subpopulations combined, the size of the metapopulation is well above the LRL and data on somatic growth indicate that the core of the population in the Kattegat is approaching the carrying capacity. Since no decline has been observed over the past 10 years, the metapopulation meets the criteria for good status. Positive growth rates in subpopulations in the Danish Straits and the Southwestern Baltic indicate that the metapopulation is expanding. The latter subpopulation might be vulnerable to human activities due to their low numbers and hampered growth rates.

The evaluation is based on sufficient data and the confidence is moderate for the Kattegat seals due to uncertainty regarding status relative to carrying capacity and high for the Southern Baltic subpopulation.


Limfjord

The size of the Limfjord harbour seal population appears to have been fluctuating around 1,000 individuals since the early 1990s and appears to have reached its carrying capacity, although an annual increase of 3.2% is suggested by surveys from 2003-2016 (Results figure 9). However, genetic analysis indicates that the seals in the fjord originate from two different populations, (1) the population originally inhabiting the fjord and (2) seals from the Wadden Sea (Olsen et al. 2014). It is not known to what extent the seals from the Wadden Sea use the fjord for other purposes than hauling out and to which extent they interbreed with the native seal population. A proper assessment of the Limfjord harbour seals is contingent on clarification of these issues. Consequently, the status of the Limfjord population is uncertain since immigration may link it to the expanding Wadden Sea population.

The confidence of this evaluation is low since the connectivity to the Wadden Sea population is unclear.

 

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Results figure 9. The annual growth rate of counted harbour seals in the Limfjord was 3.2% during the assessment period 2003-2016, and has been fluctuating around 1,000 seals for 25 years. Immigration from the Wadden Sea link it to the latter population, which is why the status of this population is uncertain.

 

Confidence of the indicator status evaluation

Confidence of the indicator evaluation is considered to be high for all seal species regarding applicable assessment units, except for harbour seals in the Limfjord where the connectivity with the Wadden Sea seals is unknown. Similarly, confidence is not high regarding the evaluation that grey seals and the Kattegat-northern Great Belt harbour seal unit was approaching carrying capacity. A longer series of data, particularly for the period abundances have plateaued, is needed for the confidence to increase. For the other management units, the confidence is generally deemed high as observations are available from all years in most relevant assessment units, with no clear temporal or spatial bias. Annual surveys are carried out for all species and management units except for ringed seals in the Archipelago Sea, Gulf of Finland, Gulf of Riga and Estonian coastal waters. Here new methodology is underway since moulting counts on ice are not feasible under currently regular ice free conditions during moulting time which prevail in most moulting seasons. However, confidence of the indicator evaluation is considered high also for this Southern management unit due to very small number of individuals in these areas.

Monitoring activities are currently carried out at a high spatial and temporal frequency. Historical data on population sizes of seals in all management units are available. The main pressures affecting seals, such as hunting and by-catches, diminishing ice fields and effects of contaminants are well known on a qualitative level, but more work is needed to quantify those pressures. Dedicated studies are needed to quantify by-catches on a regular basis, and it is not known why the nutritive condition of Baltic grey seals shows a negative trend. Furthermore, the low population growth rate of ringed seals is not fully understood.

Survey data is available for harbour seals in the Kattegat since 1979, 1972 in the Kalmarsund, 1990 in Southwestern Baltic, since 1988 for ringed seals in the Bothnian Bay and since 2003 for grey seals in the entire Baltic Sea. For grey seals there are also data from Sweden two decades before this time. Ringed seal data in the southern management unit is scarce. Sufficient data collected in the appropriate moulting periods coupled with well-known population ecology processes, rates the confidence of the indicator evaluation as high. Although data are scarce in the southern management unit of ringed seals, this subpopulation is clearly below good status and hence the evaluation of the populations against the set threshold value is deemed to be reliable.