Gross pathology and liver mercury concentrations in harbour porpoises, harbour seals and grey seals in Denmark, Northern Europe

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Here we report the first investigation of gross pathology and mercury (Hg) in liver tissue from harbour porpoises, harbour seals and grey seals from Denmark, Northern Europe. Mercury concentrations ranged between 0.2 and 248 μg/g wet weight (ww) with highest concentrations found in grey seals and subadult harbour seals from the Baltic Sea, with no relationship to body condition. Necropsy findings across all three species decreasing in the following order: pneumonia (n = 60) > respiratory parasitism (n = 56) > wounds (n = 18) > GI-parasites (stomach nematodes and/or parasitic colitis) (n = 16) > ectoparasites (skin lice) (n = 12) > hepatic parasites/lesions (n = 8) > focal alopecia (n = 5) > nephropathy (n = 4) > middle ear complex parasites (n = 3) > nasal parasites (n = 2). Heart and/or lung worms were significantly highest in subadult harbour porpoises, GI parasites and nephropathy significantly lowest in subadult harbour seals and focal alopecia significantly highest in subadult harbour seals. Most cases of pneumonia were associated with respiratory parasites (68 %), while nine cases of wounds led to signs of septicaemia. Significant positive relationships were observed between Hg and the presence of respiratory parasites in subadult harbour porpoises and between Hg and the presence of focal alopecia, nephropathy, and gastrointestinal parasites in subadult harbour seals. Levels of Hg were in the categories for low risk (16–64 μg/g ww) in 18 %, moderate risk (64–83 μg/g ww) in 3 %, high risk (83–123 μg/g ww) in 2 % and severe risk (>123 μg/g ww) in 3 % of all individuals for health effects in marine mammals. In conclusion, using marine mammals as integrative sentinel species for Danish North Sea, Inner Danish Waters and the Baltic Sea ecosystems provides monitoring of ocean health in terms of multiple stressors such as anthropogenic contaminants and infectious diseases all being important in the context of global change.