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Marine Top Predators
The Department of Contaminant Research at the Swedish Museum of Natural History is responsible for implementing this monitoring (where the white-tailed sea eagle is concerned, in collaboration with the Swedish Society for Nature Conservation). The aim is to study population trends and, when possible, the rate at which these species reproduce. Furthermore, the health of seals is studied by autopsy on any animal found dead.
White-tailed sea eagle
The white-tailed sea eagles and the seals were the first species to show, very clearly, the injurious effects of contaminants in marine environments. An impaired ability to reproduce was noted in sea eagles in the early 1960s, and in seals in the middle of the 1970s. Retrospective studies have revealed that reproduction in both species was affected ten years earlier. With the knowledge we have today and with access to extensive reference material, the chances of detecting future abnormalities in these species are good.
Long series of variables
Both white-tailed sea eagles and the seals have now been under intense study for many years, so there is now a long series of variables for sea eagles and seals in the Baltic dating back to the mid-1960s and the 1970s respectively. Their population status was critical at that time. Sea eagles showed an extremely low rate of reproduction, only one-fifth of the normal for the period 1965 - 1985.
The situation has improved since the 1980s. Both species have been able to reproduce more successfully, and the populations of both grey seals and white-tailed sea eagles have increased notably. Sea eagles reached an almost normal level of reproduction in 1995 and 1996. Today, the Swedish population of white-tailed sea eagles is at least 240 pairs (counted) on the Baltic coast, 70 pairs in the south and central Swedish lakes and 53 pairs in Lapland; i.e., a total of at least 363 pairs. The estimated number is somewhat higher, around 400 pairs.
Grey seals, Harbour seals and Ringed seals
The grey seal population status is not entirely positive. The increase in population in the Baltic Proper is very weak and the counted population, comprising the four southernmost colonies (Skåne - Småland - Gotland), does not exceed 200 individuals.
A probable reason for the poor development of the population in the southern part of the Baltic is the unintentional death of seals, mostly yearlings, caught in fishing gear. An Estonian report states that approximately 250 grey seal yearlings died in this way in 1994 alone. The number of grey seals along the Swedish coast, counted from the air and from boats, currently amounts to 2,800 head. This is approximately half the total number of grey seals counted in the Baltic.
In 1988 and in 2002, the harbour seal population along the Swedish west coast was afflicted with the Phocine Distemper Virus (PDV). This epizootic disease in both cases reduced stocks significantly in the Skagerack, Kattegatt and Öresund. After the epizooty of 1988, the population in the Skagerack - Kattegatt region subsequently increased from approximately 5,000 animals (1988) to approximately 19,000 in 2002 (counted from the air). During the later epizooty about 10,000 animals died. The stock has now (2005) regained to about 9,500 animals. The small population of harbour seals in the southern Baltic is currently estimated at barely 900 animals, of which approximately 600 live in an isolated subpopulation in Kalmarsund.
A new article has been published in the scientific journal Diseases of Aquatic Organisms, Vol 56, No 2 (2006): Tero Härkönen et al. "A review of the 1988 and 2002 phocine distemper virus epidemics in European harbour seals".
A census has been kept of the population of ringed seals in the Bothnian Bay since 1988. The result indicates an increase from 2,000 counted animals in 1988 to approximately 4,100 in 2005. The status of the ringed seal population in Estonia and Latvia and in the Gulf of Finland is more precarious. In 1991, half the adult population in the Gulf of Finland died for unknown reasons. Recent counts indicate that the population at present amounts to only 150-200 animals in the Gulf of Finland. In the Gulf of Riga a drastic decrease is noted since 1995, as the count have diminished from 1,500 animals to about 800 (2003).