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In August 2021, a large-scale mortality event affected harbor porpoises (Phocoena phocoena) in the Netherlands. Pathology and ancillary testing of 22 animals indicated that the most likely cause of death was Erysipelothrix rhusiopathiae infection. This zoonotic agent poses a health hazard for cetaceans and possibly for persons handling cetacean carcasses.

In August 2021, a large-scale mortality event affected harbor porpoises (Phocoena phocoena) in the Netherlands. Pathology and ancillary testing of 22 animals indicated that the most likely cause of death was Erysipelothrix rhusiopathiae infection. This zoonotic agent poses a health hazard for cetaceans and possibly for persons handling cetacean carcasses.

In August 2021, a large-scale mortality event affected harbor porpoises (Phocoena phocoena) in the Netherlands. Pathology and ancillary testing of 22 animals indicated that the most likely cause of death was Erysipelothrix rhusiopathiae infection. This zoonotic agent poses a health hazard for cetaceans and possibly for persons handling cetacean carcasses.

In het broedseizoen van 2022 werden veel wilde vogels in Noordwest Europa getroffen door een zeer besmettelijke, dodelijke variant van het vogelgriepvirus. In negen van de tien Nederlandse kolonies van de grote stern stierven grote aantallen broedvogels (tientallen procenten van alle aanwezige broedvogels) en vervolgens ook de kuikens. In verschillende kolonies, binnen Nederland en in het buitenland, werd hierop verschillend gereageerd door beheerders en onderzoekers, wat achteraf de mogelijkheid gaf om na te gaan hoe het beste om te gaan is met een dergelijke uitbraak. In dit rapport wordt nagegaan, wat de impact is geweest van deze uitbraak en massasterfte en wat het handelingsperspectief is bij volgende uitbraken. Gebleken is, dat het structureel weghalen van dode vogels (broedvogels en kuikens) bij een besmetting in een kolonie, kan helpen om het sterftepercentage te verkleinen. Geadviseerd wordt daarom, om bij iedere volgende uitbraak, zo snel mogelijk te starten met het weghalen van dode vogels uit getroffen kolonies. De infectiegolf kent een aanloopfase van één tot drie weken, waarin er dagelijks nog weinig slachtoffers vallen in de kolonie. Het is zaak om een uitbraak zo vroeg mogelijk te detecteren en daar dan meteen op in te spelen: dit biedt de beste kans om de infectie nog te beteugelen. Voorwaarde voor een snelle en adequate respons is een goede surveillance en een draaiboek, inclusief materiaal om veilig te kunnen werken, dat vooraf beschikbaar is. Andere mogelijkheden om infectiegolven te beteugelen lijken vooralsnog beperkt. Het is aan te raden om kolonies, na een infectiegolf als het broedseizoen voorbij is, te ontdoen van achtergebleven karkassen. Dit om een eventuele persistentie van virus in de bodem, of het doorgeven van het virus aan andere vogels die het gebied in de seizoenen na het broedseizoen gebruiken, zo klein mogelijk te maken. Of, en zo ja hoe lang, het virus in (de omgeving van) dode vogels levensvatbaar aanwezig blijft in kolonies

Rapid changes of marine ecosystems resulting from human activities and climate change, and the subsequent reported rise of infectious diseases in marine mammals, highlight the urgency for timely detection of unusual health events negatively affecting populations. Studies reporting pathological findings in the commonly stranded harbor porpoise (Phocoena phocoena) on North Atlantic coastlines are essential to describe new and emerging causes of mortality. However, such studies often cannot be used as long-term health surveillance tools due to analytical limitations. We tested 31 variables gained from stranding-, necropsy-, dietary- and marine debris data from 405 harbor porpoises using applied supervised and unsupervised machine learning techniques to explore and analyze this large dataset. We classified and cross-correlated the variables and characterized the importance of the different variables for accurately predicting cause-of-death categories, to allow trend assessment for good conservation decision. The variable ‘age class’ seemed most influential in determining cause-of-death categories, and it became apparent that juveniles died more often due to acute causes, including bycatch, grey-seal-predation and other trauma, while adults of infectious diseases. Neonates were found in summer, and mostly without prey in their stomach and more often stranded alive. The variables assigned as part of the external examination of carcasses, such as imprints from nets and lesions induced by predators, as well as nutritional condition were most important for predicting cause-of-death categories, with a model prediction accuracy of 75%. Future porpoise monitoring, and in particular the assessment of temporal trends, should predominantly focus on influential variables as determined in this study. Pathogen- and contaminant assessment data was not available for all cases, but would be an important step to further complete the dataset. This could be vital for drawing population-inf

We collected data on mass mortality in Sandwich terns (Thalasseus sandvicensis) during the 2022 breeding season in the Netherlands. Mortality was associated with at least 2 variants of highly pathogenic avian influenza A(H5N1) virus clade 2.3.4.4b. We report on carcass removal efforts relative to survival in colonies. Mitigation strategies urgently require structured research.

Humans impact natural systems at an unprecedented rate. The North Sea is one of the regions in the world with the highest levels of anthropogenic activity. Here, the harbor porpoise (Phocoena phocoena) is an abundant species and is often regarded as an ecosystem sentinel. A post-mortem surveillance program was established in the Netherlands aimed at increasing knowledge of the effects of human activities on harbor porpoises. In this study, we describe the pathological findings related to anthropogenic and natural causes of death categories in 612 harbor porpoises that stranded between 2008 and 2019, and assess their relations to age, sex, season, and location. The largest anthropogenic category was bycatch (17%), with mainly juveniles affected and peak periods in March and September–October. Other, infrequently diagnosed anthropogenic causes of death were trauma (4%), largely most likely due to ship collisions, and marine debris ingestion and entanglement (0.3%). The risk of dying from anthropogenic causes was highest for juveniles. Lesions compatible with noise-induced hearing loss were investigated in carcasses which were fresh enough to do so (n = 50), with lesions apparent in two porpoises. Non-direct human-induced threats included infectious diseases, which were by far the largest cause of death category (32%), and affected mainly adults. Also, gray seal (Halichoerus grypus) attacks were a frequently assigned cause of death category (24%). There were more acute predation cases in the earlier study years, while porpoises with lesions that suggested escape from gray seal attacks were diagnosed more recently, which could suggest that porpoises adapted to this threat. Our study contributes to understanding porpoise health in response to persisting, new, emerging, and cumulative threats. Building up such knowledge is crucial for conservation management of this protected species.

Bycatch is considered one of the most significant threats affecting cetaceans worldwide. In the North Sea, bottom-set gillnets are a specific risk for harbor porpoises (Phocoena phocoena). Methods to estimate bycatch rates include on-board observers, remote electronic monitoring, and fishermen voluntarily reporting; none of these are systematically conducted. Additionally, necropsies of stranded animals can provide insights into bycatch occurrence and health status of individuals. There are, however, uncertainties when it comes to the assessment of bycatch in stranded animals, mainly due to the lack of diagnostic tools specific for underwater entrapment. We conducted a literature review to establish criteria that aid in the assessment of bycatch in small cetaceans, and we tested which of these criteria applied to harbor porpoises retrieved from gillnets in the Netherlands (n = 12). Twenty-five criteria were gathered from literature. Of these, “superficial incisions,” “encircling imprints,” and “recent ingestion of prey” were observed in the vast majority of our confirmed bycatch cases. Criteria like “pulmonary edema,” “pulmonary emphysema,” and “organ congestion” were also frequently observed, although considered unspecific as an indicator of bycatch. Notably, previously mentioned criteria as “favorable health status,” “absence of disease,” or “good nutritional condition” did not apply to the majority of our bycaught porpoises. This may reflect an overall reduced fitness of harbor porpoises inhabiting the southern North Sea or a higher chance of a debilitated porpoise being bycaught, and could result in an underestimation of bycatch rates when assessing stranded animals.

Bulk carbon and nitrogen stable isotope values from baleen from stranded and bowcaught whales in the North Sea. Baleen from 5 animals were sampled incrementally and used to examine growth interval, migration pattern, metabolism and trophic position along with bulk isotope data for carbon and nitrogen provided in another data set., Bulk carbon and nitrogen stable isotope values from baleen from stranded and bowcaught whales in the North Sea. Baleen from 5 animals were sampled incrementally and used to examine growth interval, migration pattern, metabolism and trophic position along with bulk isotope data for carbon and nitrogen provided in another data set. Amino acid nitrogen data set supporting the manuscript: Reconstructing the diet, trophic level, and migration pattern of Mysticete whales based on baleen isotopic composition.

Bulk carbon and nitrogen stable isotope values from baleen from stranded and bowcaught whales in the North Sea. Baleen from 5 animals were sampled incrementally and used to examine growth interval, migration pattern, metabolism and trophic position along with compound specific amino acid analysis provided in another data set., Bulk carbon and nitrogen stable isotope values from baleen from stranded and bowcaught whales in the North Sea. Baleen from 5 animals were sampled incrementally and used to examine growth interval, migration pattern, metabolism and trophic position along with compound specific amino acid analysis provided in another data set. Bulk carbon and nitrogen data set supporting the manuscript: "Reconstructing the diet, trophic level, and migration pattern of Mysticete whales based on baleen isotopic composition."

Quantifying consumption and prey choice for marine predator species is key to understanding their interaction with prey species, fisheries, and the ecosystem as a whole. However, parameterizing a functional response for large predators can be challenging because of the difficulty in obtaining the required data on predator diet and on the availability of multiple prey species. This study modeled a multi-species functional response (MSFR) to describe the relationship between consumption by harbour porpoises (Phocoena phocoena) and the availability of multiple prey species in the southern North Sea. Bayesian methodology was employed to estimate MSFR parameters and to incorporate uncertainties in diet and prey availability estimates. Prey consumption was estimated from stomach content data from stranded harbour porpoises. Prey availability to harbour porpoises was estimated based on the spatial overlap between prey distributions, estimated from fish survey data, and porpoise foraging range in the days prior to stranding predicted from telemetry data. Results indicated a preference for sandeels in the study area. Prey switching behavior (change in preference dependent on prey abundance) was confirmed by the favored type III functional response model. Variation in the size of the foraging range (estimated area where harbour porpoises could have foraged prior to stranding) did not alter the overall pattern of the results or conclusions. Integrating datasets on prey consumption from strandings, predator foraging distribution using telemetry, and prey availability from fish surveys into the modeling approach provides a methodological framework that may be appropriate for fitting MSFRs for other predators.

Bycatch is considered one of the most significant threats affecting cetaceans worldwide. In the North Sea, bottom-set gillnets are a specific risk for harbor porpoises (Phocoena phocoena). Methods to estimate bycatch rates include on-board observers, remote electronic monitoring, and fishermen voluntarily reporting; none of these are systematically conducted. Additionally, necropsies of stranded animals can provide insights into bycatch occurrence and health status of individuals. There are, however, uncertainties when it comes to the assessment of bycatch in stranded animals, mainly due to the lack of diagnostic tools specific for underwater entrapment. We conducted a literature review to establish criteria that aid in the assessment of bycatch in small cetaceans, and we tested which of these criteria applied to harbor porpoises retrieved from gillnets in the Netherlands (n = 12). Twenty-five criteria were gathered from literature. Of these, “superficial incisions,” “encircling imprints,” and “recent ingestion of prey” were observed in the vast majority of our confirmed bycatch cases. Criteria like “pulmonary edema,” “pulmonary emphysema,” and “organ congestion” were also frequently observed, although considered unspecific as an indicator of bycatch. Notably, previously mentioned criteria as “favorable health status,” “absence of disease,” or “good nutritional condition” did not apply to the majority of our bycaught porpoises. This may reflect an overall reduced fitness of harbor porpoises inhabiting the southern North Sea or a higher chance of a debilitated porpoise being bycaught, and could result in an underestimation of bycatch rates when assessing stranded animals.

The Lesser Black-backed Gull, Larus fuscus Linnaeus, 1758, is a surface feeder with a broad prey spectrum that forages in the North Sea on subsurface pelagic fishes and crabs swimming within 0.6 m of the sea surface. In July 2006 and in July/August 2009, we surveyed the Frisian Front area in the southern North Sea (53.4°N–54.3°N, 4.0°E–5.5°E) to determine the prey base for Lesser Black-backed Gulls. In July/August 2009, we conducted bird counts in an integrated ecosystem survey by simultaneously sampling subsurface pelagic prey by a combination of fishing and echosounding. Lesser Black-backed Gulls make use of facilitated foraging to improve the catchability of their prey. The two most important aids for facilitated foraging by Lesser Black-backed Gull in the Frisian Front were (1) multispecies feeding associations with Common Murre, Uria aalge (Pontoppidan, 1763), and (2) fisheries discarding bycatch and offal. We found that in the top meter of the water column, the natural prey base is large enough to provide for self-foraging; however, Lesser Black-backed Gulls were found to rely mainly on facilitated foraging.

Between the 8th January and the 25th February 2016, the largest sperm whale Physeter macrocephalus mortality event ever recorded in the North Sea occurred with 30 sperm whales stranding in five countries within six weeks. All sperm whales were immature males. Groups were stratified by size, with the smaller animals stranding in the Netherlands, and the largest in England. The majority (n = 27) of the stranded animals were necropsied and/or sampled, allowing for an international and comprehensive investigation into this mortality event. The animals were in fair to good nutritional condition and, aside from the pathologies caused by stranding, did not exhibit significant evidence of disease or trauma. Infectious agents were found, including various parasite species, several bacterial and fungal pathogens and a novel alphaherpesvirus. In nine of the sperm whales a variety of marine litter was found. However, none of these findings were considered to have been the primary cause of the stranding event. Potential anthropogenic and environmental factors that may have caused the sperm whales to enter the North Sea were assessed. Once sperm whales enter the North Sea and head south, the water becomes progressively shallower (<40 m), making this region a global hotspot for sperm whale strandings. We conclude that the reasons for sperm whales to enter the southern North Sea are the result of complex interactions of extrinsic environmental factors. As such, these large mortality events seldom have a single ultimate cause and it is only through multidisciplinary, collaborative approaches that potentially multifactorial large-scale stranding events can be effectively investigated.

Two marine ornithologists spent two days watching seabird behaviour from a fixed platform at the periphery of offshore wind farm Luchterduinen, The Netherlands, in January 2018. The aim of this study was to assess whether meaningful observations could be made from a non-moving platform, that was part of the wind farm scenery, i.e., one of the turbine foundations. On each observation day, a turbine was selected that was located at the wind farm perimeter, that offered views both of the interior of the wind farm and to waters just outside the wind farm. Earlier studies of seabirds in offshore wind farms have shown that many species tend to avoid wind farms, but also that some individuals, of most species, do enter. However, as these studies are typically conducted from moving platforms (ships or aircraft), it is not known how birds behave within a wind farm perimeter. Birds that find themselves between moving turbines might be intimidated. This might impair their normal feeding behaviour at sea, if the birds would be overly watchful, or mainly seeking to exit the wind farm. On the other hand, birds may specifically move into a wind farm, if they can deal with the fact that turbines are present, and if feeding conditions within the wind farm are good. Such birds would be expected to show feeding behaviour, such as diving.Seabirds may also be habituating to the presence of wind farms in their environment. In the airspace below the rotors, at the sea’s surface and under water, there is no danger to seabirds from collision. Seabirds can thus safely enter and feed in offshore wind farms, and may, over the years, have learnt to exploit this new habitat. Therefore, even though earlier studies have shown displacement of seabirds away from offshore wind farms, this may no longer be the status quo as seabirds may be adapting to the new situation: a marine environment with offshore wind farms.Two auk species, the common guillemot and the razorbill, were seen to move through the