Whale lice (Cyamidae; Amphipoda) are ectoparasitic crustaceans adapted to the marine environment with cetaceans as their host. There are few reports of cyamids occurring in odontocetes from the North Sea, and long-term studies are lacking. Marine mammal health was monitored along the German and Dutch coasts in the past decades, with extensive post mortem investigations conducted. The aim of this study was to analyse archived ectoparasite samples from stranded cetaceans from the North Sea (2010–2019), to determine species, prevalence and impact of ectoparasite infection. Ectoparasites were found on two cetacean species – harbour porpoises (Phocoena phocoena), as the most abundant cetacean species in the North Sea, and on a pilot whale (Globicephala melas), as a rare species here. Prevalence of ectoparasitic crustaceans in cetaceans was low: 7.6% in porpoises stranded in the Netherlands (n = 608) and 1.6% in porpoises stranded in Germany (n = 122). All whale lice infections were found on hosts with skin lesions characterised by ulcerations. Morphological investigations revealed characteristic differences between the cyamid species Isocyamus (I.) delphinii and I. deltobranchium identified. Isocyamus deltobranchium was determined in all infected harbour porpoises. I. delphinii was identified on only the pilot whale. Molecular analyses showed 88% similarity of mDNA COI sequences of I. delphinii with I. deltobranchium supporting them as separate species. Phylogenetic analyses of additional gene loci are required to fully assess the diversity and exchange of whale lice species between geographical regions as well as host specificity. Differing whale lice prevalences in porpoises stranded in the Netherlands and Germany could indicate a difference in severity of skin lesions between these areas. It should be further investigated if more inter- or intraspecific contact, e.g., due to a higher density of porpoises or contact with other cetaceans, or a poorer health status of porpoises in the southern North Sea could explain these differences.

Anthropogenic underwater noise can affect animal behaviour, which in turn is influenced by other intrinsic and extrinsic factors. In this case study, we explored links between behaviour, underwater noise and health on 18 free-ranging harbour seals (Phoca vitulina) in the Elbe estuary and Wadden Sea. Individuals were captured and blood samples were taken to assess the health status through leukograms and molecular biomarkers indicative of stress, sound exposure and immunological status. Seals were fitted with long-term sound and movement tags (DTAGs), recording high-resolution three-dimensional diving behaviour and received sound levels simultaneously. Four behavioural states were identified from the seals' dive data (bottom phase duration, prey capture attempts, bottom phase stroke frequency, post-dive duration, descent velocity) using a Hidden Markov Model, and state transitions were linked to received 2 kHz decidecade levels as vessel noise proxy. State transition probabilities varied, but with increasing noise, bottom resting probability decreased, and transit behaviour likelihood increased. Seals remaining in the Elbe were exposed to vessels regularly and showed higher tolerance to underwater noise exposure than seals exposed to fewer vessels. Immunological health parameters (here white blood cells count) also affected the onset of noise-induced state transitions, highlighting the importance of considering health status in behavioural response studies.

Arthropod ectoparasites of aquatic wildlife often have complex relationships with their host species that have developed over long evolutionary time scales. Specialist parasite occurrence might depend on these hosts’ distributions. Eurasian otter (Lutra lutra) populations are recovering in Northern German federal states, such as Schleswig-Holstein and Lower Saxony. Chewing lice (Lutridia exilis; Ischnocera; Mallophaga) are considered otter-specific yet rare parasites in their known range. In 2022, they were recorded for the first time on nine otters found dead in Northern Germany. All otters originated from the years 2021–2022 and were dissected during population health monitoring programs in 2022. Females (n = 6) were 0–5.5 years old and showed signs of disease in five cases. Males (n = 3), in contrast, were 0–1.6 years old and showed disease in a single case. Individual lice intensity of infection ranged from 1 to 75 specimens per otter. No direct adverse health effects of chewing lice on the otters were noted. Lutridia exilis morphological characteristics were documented and measurements were taken to study specialized adaptations that allow lice to attach to semi-aquatic otters. In addition, morphology was compared between lice from different geographical regions and specimens from previous reports. A region of the COI mDNA was amplified to molecularly characterize L. exilis for the first time and detect genetic differences between otter lice populations in Germany. It is believed that specialist parasites reduce in numbers even before their host populations decline. Recovering otter populations in Northern Germany could be an example of a reverse effect, where the comeback of a host species results in the return of a specialist parasite, which reflects an ultimate boost in overall species biodiversity.

The Common Eider (Somateria mollissima) inhabits the entire northern hemisphere. In northern Europe, the flyway population reaches from the southern Wadden Sea to the northern Baltic coast. The European population is classified as endangered due to declines in Common Eider numbers across Europe since 1990. In this study, we assessed 121 carcasses of Common Eiders, captured incidentally in gillnets in the Western Baltic between 2017 and 2019. The most common findings were parasitic infections of the intestine by acanthocephalans in 95 animals, which correlated with enteritis in 50% of the cases. Parasites were identified as Profilicollis botulus in 25 selected animals. Additionally, oesophageal pustules, erosions, and ulcerations, presumably of traumatic origin, were frequently observed. Nephritis and hepatitis were frequent, but could not be attributed to specific causes. Lung oedema, fractures and subcutaneous haemorrhages likely resulted from entangling and drowning. Two Common Eiders had mycobacterial infections and in one of these, Mycobacterium avium subspecies (ssp.) avium was identified. This study gives an overview of morphological changes and infectious diseases from one location of the European flyway population. It contributes to future health studies on Common Eiders in the Baltic and Wadden Seas by providing baseline information to compare with other areas or circumstances.

Marine top predators such as ringed seals biomagnify environmental contaminants; and with the increasing human activities in the Arctic, ringed seals are exposed to biologically significant concentrations of trace elements resulting in reproductive impairment, immunosuppression, and neurological damages. Little is known about the molecular effects of heavy metals on these vulnerable apex predators suffering from a rapidly changing Arctic with significant loss of sea-ice. In the present study, concentrations of cadmium (Cd), mercury (Hg) and selenium (Se) were measured in liver of sixteen Greenlandic ringed seals (nine adults and seven subadults) together with molecular biomarkers involved in bio-transformation, oxidative stress, endocrine disruption and immune activity in blood and blubber. The concentrations of trace elements increased in the following order: Hg > Se > Cd with levels of mercury and selenium being highest in adults. Aryl hydrocarbon receptor nuclear translocator (ARNT), peroxisome proliferator activated receptor alpha (PPARα, estrogen receptor alpha (ESR1), thyroid hormone receptor alpha (TRα) and interleukin - 2 (IL-2) mRNA transcript levels were highest in blubber, while heat shock protein 70 (HSP70) and interleukin - 10 (IL-10) were significantly higher in blood. There were no significant correlations between the concentrations of trace elements and mRNA transcript levels suggesting that stressors other than the trace elements investigated are responsible for the changes in gene expression levels. Since Hg seems to increase in Greenlandic ringed seals, there is a need to re-enforce health monitoring of this ringed seal population.

The Eurasian otter ( Lutra lutra ) is a semiaquatic, freshwater top predator of conservation concern due to its significant global population decline in the last century. Following its recovery and recolonization in parts of Europe, the population faces ongoing threats, including habitat loss and fragmentation due to human activities, climate change, prey depletion, and exposure to emerging contaminants. Accurately identifying its diet is crucial for understanding ecological interactions and proposing effective management strategies. DNA metabarcoding based on high‐throughput sequencing is a reliable diet analysis tool with better resolution than traditional methods. This study, for the first time, applied two 16S rRNA primers targeting different prey classes on the dietary spectrum of two Eurasian otters populations returning to Northern Germany (Schleswig‐Holstein and Lower Saxony). DNA fragments of prey items from the gut contents of 156 otters collected between 2005 and 2022 were sequenced. Fifty‐five operational taxonomic units (OTUs) were identified in the dietary remains, including freshwater and marine fish, anurans, caudates, and water birds. Otters in the study area are opportunistic predators with wide dietary plasticity, feeding on the most abundant species. Nine‐spined sticklebacks, common roach, and common frogs were important prey items, and marine fish species were detected in the diet of some of the investigated otters. The otters' diet differed between Lower Saxony and Schleswig‐Holstein, between sexes, and a slight difference was observed between adults and subadults. This study provides new insights into the diet composition of otters and their ability to exploit a variety of resources in their habitat. This information is vital for understanding their chances of survival in an environment that is continuously changing due to human impacts and for developing effective management tools.