The status of biodiversity is assessed using several core indicators. Each indicator focuses on one important aspect of this complex issue. In addition to providing an indicator-based evaluation of the population trends and abundance of seals, this indicator will also contribute to the overall biodiversity assessment, along with the other biodiversity core indicators.
The core indicator on population trends and abundance of Baltic 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:
The HELCOM Recommendation 27/28-2 Conservation of seals in the Baltic Sea area outlines the conservation goals of seals agreed on at HELCOM. The recommendation is implemented to reach the BSAP goals. The recommendation conservation goals are used as the basis for defining this indicator's threshold value.
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' and
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' and
Descriptor 8: 'Concentrations of contaminants are at levels not giving rise to pollution effects'
and the following criteria of the Commission Decision (European Commission 2010):
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) and Habitats Directive. The WFD includes status categories for coastal waters as well as environmental and ecological objectives. 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, and member countries are obliged to monitor the status of seal populations .
Being top predators in the Baltic Sea ecosystem, seals are exposed to ecosystem changes in lower trophic levels, but also to variations in climate (length of seasons and ice conditions) and human impacts. These pressures can affect fish stocks, levels of harmful substances 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.
The growth rate of a population is the result of age-specific mortality rates and age-specific fecundity rates. It is therefore a sensitive parameter signalling if mortality or fecundity rates change. Depleted, undisturbed populations are expected to grow by 10% per year (grey and ringed seals) or 12% per year (harbour seals). Significantly lower observed growth rates indicate effects from the environment in form of reduced food availability or impaired health caused by contaminants or diseases. Low growth rates can also be the result of excessive hunting or high levels of by-catches.
All species of marine mammals in the Baltic Sea were severely reduced in the beginning of the 20th century as a result of a coordinated international campaign to exterminate seals. Seal numbers in the Baltic Sea dropped by 80-90% over the period 1920-1945, resulting in extirpation of grey seals in the Kattegat in the 1930s (Heide-Jørgensen & Härkönen 1988) and grey seals and harbour seals along the Polish and German coasts (Harding & Härkönen 1999). Environmental contaminants in the 1960s and 1970s caused infertility in ringed and grey seals, where fertility rates in ringed seals dropped to 17% in the beginning of the 1970s (Helle 1980).
-selective extraction of species, including incidental non-target catches (e.g. by commercial and recreational fishing)
The effects of climate change are a threat to the ringed seal that breeds on sea ice
Fishery and food availability
Contamination by hazardous substance:
- introduction of synthetic compounds
- introduction of non-synthetic substances and compounds
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).
The hunting pressure resulted in extirpation of grey and harbour seals in Germany and Poland in 1912, and grey seals were also extirpated from the Kattegat by the 1930s. Ringed seals declined to about 25,000 seals in the 1940s, whereas grey seals were reduced to about 20,000 (Harding & Härkönen 1999) over the same time period. A similar rate of reduction of harbour seals occurred in the Kalmarsund and the Kattegat (Heide-Jørgensen & Härkönen 1988; Härkönen & Isakson 2011). However, after these heavy reductions, populations appear to have been stable up to the 1960s (Harding & Härkönen 1999).
Then, in the beginning of the 1970s grey seals were observed aborting near full term foetuses, and only 17% of ringed seal females were fertile (Helle 1980). Later investigations showed a linkage to a disease syndrome including reproductive disorder, caused by organochlorine pollution, in both grey seals and ringed seals (Bergman & Olsson 1985). The reduced fertility resulted in population crashes, where numbers of ringed and grey seals dwindled to approximately 3,000 of each species in the beginning of the 1980s (Harding & Härkönen 1999). Increasing numbers of these species were recorded after levels of PCB in biota decreased by the end of the 1980s. Recent samples show that fertility is normal in grey seals, but still impaired in ringed seals (Bäcklin et al. 2011; Bäcklin et al. 2013). The very low numbers of ringed seals in the Gulf of Finland may be caused by impaired female fertility.
Incidental catches of seals in fisheries are known to have substantial effects on the population growth rate in species like the Saimaa and Ladoga ringed seals (Sipilä 2003). The current knowledge on the level of incidental catches of Baltic seal species is limited to a few dedicated studies which suggest that this factor can be substantial. An analysis of reported incidentally caught grey seals showed that approximately 2,000 grey seals are caught annually in the Baltic fisheries (Vanhatalo et al. 2014), but numbers of incidentally caught ringed seals and harbour seals are not known.
Incidentally caught grey seals are significantly leaner compared to hunted seals (Bäcklin et al. 2011), which may suggest that food is a limiting factor for incidentally 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 (also hunted) show a significant decline during the last decade (Bäcklin et al. 2011).
Climate change poses a pressure on species breeding on ice because shorter and warmer winters lead to more restricted areas of suitable ice fields (Meier et al. 2004). This feature alone will severely affect the Baltic ringed seals and the predicted rate of climate warming is likely to cause extirpation of the southern subpopulations (Sundqvist et al. 2012). Grey seals are facultative ice breeders and their breeding success is considerably greater when they breed on ice as compared with land (Jüssi et al. 2008). Consequently, both ringed seals and grey seals are predicted to be negatively affected by a warmer climate. However, effects of climate change should not be included in assessments according to the Habitat Directive.
Most land breeding sites of Baltic seals are protected during critical periods of time, since seals are vulnerable to disturbance during the lactation period. This is especially important for grey seals, where access to undisturbed land breeding sites delimit the expansion of grey seals in the Southern Baltic Sea.