Pathological findings in stranded harbor porpoises (Phocoena phocoena) with special focus on anthropogenic causes

Abstract

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.

Using marine mammal necropsy data in animal health surveillance: the case of the harbor porpoise in the Southern North Sea

“…We recently reported pathological findings of harbor porpoises and related these to anthropogenic and natural cause-of-death categories (IJsseldijk et al, 2022). In the present study, these stranding, necropsy, dietary and marine debris data were analyzed in more depth, to determine the classification of, and correlation between, 31 possibly relevant variables and to determine their relative importance for accurately predicting cause-of-death categories.…”

“…The individuals in this cluster had absence of disease, parasites and solid stomach content. They fell within the cause-of-death category 'perinatal death', which logically only includes neonates, and also the category 'starvation', which appears a common mortality cause for neonates (Kirkwood et al, 1997;Siebert et al, 2001;IJsseldijk et al, 2022). It is thus not unexpected that the neonates can rather easily be distinguished in the data based on their pathological profiles.…”

“…The primary aim of the post-mortem examinations was to determine the animals' most likely causes of death and their health status prior to death. Each case was assigned to one of ten cause-of-death categories reflecting the most likely cause of death or stranding: accidental bycatch in fisheries (referred to as bycatch), acute grey seal (Halichoerus grypus) predation, previously induced but still active bite lesions from a grey seal attack, infectious disease, emaciation, starvation, perinatal death, blunt force and sharp trauma, other and unknown (for more details see: IJsseldijk et al, 2022). Here, the necropsy reports, including photographs, gross-and histopathological descriptions and information on diet and marine debris ingestion, of these 612 cases were reassessed and scored on 31 points (referred to further as variables).…”

“…The primary species found stranded in the Netherlands is the harbor porpoise (Phocoena phocoena) (IJsseldijk et al, 2020). Pathological findings related to anthropogenic and natural cause-of-death categories in 612 harbor porpoises that stranded between 2008 and 2019 on Dutch shores were recently published (IJsseldijk et al, 2022). In this follow-up study, we applied supervised and unsupervised machine learning techniques to further explore and assess the postmortem data and to investigate the usefulness of such methods to aid in surveillance and to further unlock information that is contained in these data.…”

Fatal aspergillosis and evidence of unrelated hearing loss in a harbor porpoise (Phocoena phocoena) from the German Baltic Sea

“…In part, this is due to the difficulty to provide a large number of individuals whose ears can be fixed rapidly after their death. To date, 3 out of 21 (14.3%) harbor porpoise individuals that had the beginning of the apex in a good preservation status for diagnosis, showed evidences of lowfrequency hearing loss (Morell et al, 2015;Morell et al, 2021;Morell et al, 2022a;Ijsseldijk et al, 2022). However, this prevalence may be underestimated since not all 21 individuals had a complete information of the apex.…”