Bone is remodelled constantly through a balance of bone formation and resorption. This process can be affectedby various factors such as hormones, vitamins, nutrients and environmental factors, which can create an imbalanceresulting in systemic or local bone alteration. The aim of the present study was to analyse the changes in bone mineral density (BMD) over time in skulls of ringed seals (Pusa hispida) from the Baltic and Greenland using museum samples. Overall, 303 skulls (102 Male, 89 Female, 112 unknown) were used for bone investigationsand were divided into three periods according to collection year: before 1958 (n = 167), between 1958 and 1989 (n=40) and after 1994 up to 2019 (n=96). All skulls were examined by dual-energy X-ray absorptiometryto obtain the BMD. Skull BMD of the Baltic seals was positively correlated with the historical polychlorinatedbiphenyls (PCB) contamination having potential effects on the constitution of bones. BMD fluctuated between the three study periods (LM: p-value < 0.001, F-value = 47.5) with the lowest BMD found between 1897 and 1957, in the Gulf of Bothnia, where the highest peak of contaminant concentration was in the second period. BMD levels increased with increasing PCB concentration (LM: p < 0.001). The Greenland population showed significant lower BMD levels in the pollution and post-pollution period than the Baltic population (LM:p < 0.001). It also revealed a higher BMD in males than in females (LM: p=0.03). In conclusion, the variations between 1829 and 2019 in the Baltic Sea and Greenland may to a certain extent reflect normal fluctuations; however, this study revealed several factors affecting BMD, including sex and PCB levels.

The liver plays an important role in the metabolism and elimination of endogenic and exogenic lipid-solublecompounds. Multiple studies have shown that polychlorinated biphenyls (PCB) and dichlorodiphenyl trichloroethane(DDT) lead to morphological changes in liver cells. The aim of the present study was therefore toanalyse liver changes over time in Baltic grey seals (Halichoerus grypus) and to correlate these with historical PCBand DDT contaminations. A total of 191 liver samples were collected between 1981 and 2015 in the Gulf ofBothnia and northern Baltic Proper. Six histological features were evaluated, including portal mononuclear cellinfiltration, random mononuclear cell infiltration, lipid granulomas, hepatocellular fat vacuoles, hepatic stellatecells and mild multifocal bile duct hyperplasia accompanied by portal fibrosis. Three of the six lesions showed asignificant correlation with age. Furthermore, a positive correlation between portal mononuclear cell infiltrationand mild multifocal bile duct hyperplasia was found. Additionally, lipid granulomas were significantly correlatedwith hepatic stellate cells. More importantly, hepatic stellate cells and mild multifocal bile duct hyperplasia werecorrelated with adipose tissue (blubber) concentrations of ƩPCB, measured in a subsample (n = 34) of all individuals.No correlation with lesions and ƩDDT concentrations were found. These results show that age is animportant factor for the development of these liver lesions, but PCBs burden may be an influencing factor. This isin agreement with previous studies of marine mammals in the Baltic Sea as well as in the Arctic. We thereforeconclude that not only age of the animals, but also exposure to PCBs should be taken into account when understandingand evaluating the current health status of Baltic grey seals.

Here we review the state-of-the-art of pathogens in select marine and terrestrial key species of the Baltic Sea, i.e.ringed seal (Pusa hispida), harbour seal (Phoca vitulina), grey seal (Halichoerus grypus), harbour porpoise(Phocoena phocoena), common eider (Somateria mollissima), pink-footed goose (Anser brachyrhynchus) and whitetailedeagle (Haliaeetus albicilla). This review is the first to merge and present available information and baselinedata for the FP7 BONUS BaltHealth project: Baltic Sea multilevel health impacts on key species of anthropogenichazardous substances. Understanding the spread, prevalence and effects of wildlife pathogens is important for theunderstanding of animal and ecosystem health, ecosystem function and services, as well as human exposure tozoonotic diseases. This review summarises the occurrence of parasites, viruses and bacteria over the past sixdecades, including severe outbreaks of Phocine Distemper Virus (PDV), the seroprevalence of Influenza A andthe recent increase in seal parasites. We show that Baltic high trophic key species are exposed to multiplebacterial, viral and parasitic diseases. Parasites, such as C. semerme and P. truncatum present in the colon andliver Baltic grey seals, respectively, and anisakid nematodes require particular monitoring due to their effects onanimal health. In addition, distribution of existing viral and bacterial pathogens, along with the emergence andspread of new pathogens, need to be monitored in order to assess the health status of key Baltic species. Relevantbacteria are Streptococcus spp., Brucella spp., Erysipelothrix rhusiopathiae, Mycoplasma spp. and Leptospira interrogans;relevant viruses are influenza virus, distemper virus, pox virus and herpes virus. This is of special importanceas some of the occurring pathogens are zoonotic and thus also pose a potential risk for human health.Marine mammal handlers, as well as civilians that by chance encounter marine mammals, need to be aware ofthis risk. It is therefore important to continue the monitoring of diseases affecting key Baltic species in order toassess their relationship to population dynamics and their potential threat to humans. These infectious agents arevaluable indicators of host ecology and can act as bioindicators of distribution, migration, diet and behaviour ofmarine mammals and birds, as well as of climate change and changes in food web dynamics. In addition, infectiousdiseases are linked to pollutant exposure, overexploitation, immune suppression and subsequent inflammatorydisease. Ultimately, these diseases affect the health of the entire ecosystem and, consequently,ecosystem function and services. As global warming is continuously increasing, the impact of global change oninfectious disease patterns is important to monitor in Baltic key species in the future.