Abstract

Fishery bycatch is one of the leading anthropogenic causes of mortality affecting cetaceans around the world. Proper documentation of bycatch incidents is essential, yet often challenging due to the difficulties in differentiating bycatch-related mortality during the postmortem investigation of stranded cetaceans. Peracute underwater entrapment (PUE) refers to the acute and agonal process during which bycaught cetaceans become entangled with fishing gear and eventually die of forced submersion and drowning.¹ To date, the only widely accepted and reliable pathological indication of PUE is the presence of fresh ligature marks or injuries indicating acute entanglement.^1-5 However, this is not always present in bycaught cetaceans, while other pathological features such as pulmonary changes are often non-specific and only provide circumstantial evidence for the diagnosis of bycatch-related mortality in cetaceans.^1-5

In humans, postmortem computed tomography (PMCT) has been used to study the pathological features of drowning victims.⁶ As part of the virtopsy technique, Kot and his team have pioneered the use of PMCT in the postmortem assessment of stranded cetaceans.⁷ Since 2017, PMCT has routinely been used to investigate natural and anthropogenic causes of death in stranded cetaceans from Hong Kong and adjacent waters, and have preliminarily been demonstrated to be useful in assessing fishery-related mortality.⁸ To advance the application of PMCT, this study aimed to characterise and diagnose PUE or bycatch-related features in stranded cetaceans using PMCT. Similar studies have been conducted with conventional necropsy,^1-5 but to the best of our knowledge have not been attempted with virtopsy.

We retrospectively analysed the PMCT scans of 54 known-bycaught narrow-ridged finless porpoises (Neophocaena asiaeorientalis), with reference to a list of PUE-related features that were established based on previous necropsy-based studies.^1-5 The PMCT findings were correlated and verified with the necropsy reports of these finless porpoises. Using relevant case studies and the data originated from these bycaught finless porpoises, we demonstrated that PMCT is effective in the visualisation and documentation of PUE-related features, especially for skeletal fractures and respiratory abnormalities such as froth or fluid deposition in the airways and pulmonary changes, as previously suggested.⁹ Therefore, PMCT is a powerful tool that can be used prior to and in conjunction with conventional necropsy during the postmortem investigation of stranded cetaceans, particularly to improve the ability to diagnose PUE and bycatch-related mortality. Further directions of this study include establishing a set of diagnostic criteria¹ that integrate findings from both virtopsy and necropsy to facilitate better assessment of bycatch-related mortality in stranded cetaceans in Hong Kong and worldwide.

Acknowledgements

We would like to thank the Agriculture, Fisheries and Conservation Department of the HKSAR Government for the continuous support in the cetacean virtopsy project, as well as the Institute of Hydrobiology, Chinese Academy of Science for providing the bycaught finless porpoise specimens involved in this study. Sincere appreciation is extended to veterinarians, staff, students, and volunteers from the Aquatic Animal Virtopsy Lab, City University of Hong Kong and the Ocean Park Hong Kong, especially for those who assisted in the acquisition of PMCT and necropsy data from the bycaught finless porpoises. This project is financially supported by the Hong Kong Research Grants Council (Grant No. UGC/FDS17/M07/14), the Marine Ecology Enhancement Fund (Grant No. MEEF2017014, MEEF2017014A, MEEF2019010, and MEEF2019010A) of the Marine Ecology & Fisheries Enhancement Funds Trustee Limited. Any opinions, findings, conclusions, or recommendations expressed in this proceedings do not necessarily reflect the views of the Marine Ecology Enhancement Fund or the Trustee.

Literature Cited

1.  Moore MJ, van der Hoop J, Barco SG, Costidis AM, Gulland FM, Jepson PD, Moore KT, Raverty S, McLellan WA. 2013. Criteria and case definitions for serious injury and death of pinnipeds and cetaceans caused by anthropogenic trauma. Dis Aquat Org 103:229–264.

2.  Kuiken T. 1994. Review of the criteria for the diagnosis of by-catch in cetaceans. Diagnosis of By-Catch in Cetaceans: Proceedings of the Second European Cetacean Society Workshop on Cetacean Pathology, Montpellier, France; Pp. 38–43.

3.  Bernaldo de Quirós Y, Hartwick M, Rotstein DS, Garner MM, Bogomolni A, Greer W, Niemeyer ME, Early G, Wenzel F, Moore M. 2018. Discrimination between bycatch and other causes of cetacean and pinniped stranding. Dis Aquat Org 127:83–95.

4.  Epple AL, Daniel JT, Barco SG, Rotstein DS, Costidis AM. 2020. Novel necropsy findings linked to peracute underwater entrapment in bottlenose dolphins (Tursiops truncatus). Front Mar Sci 7:503.

5.  IJsseldijk LL, Scheidat M, Siemensma ML, Couperus B, Leopold MF, Morell M, Gröne A, Kik MJL. 2021. Challenges in the assessment of bycatch: postmortem findings in harbor porpoises (Phocoena phocoena) retrieved from gillnets. Vet Pathol 58(2):405415.

6.  Plaetsen SV, De Letter E, Piette M, Van Parys G, Casselman JW, Verstraete K. 2015. Post-mortem evaluation of drowning with whole body CT. Forensic Sci Int 249:35–41.

7.  Tsui HCL, Kot BCW, Chung TYT, Chan DKP. 2020. Virtopsy as a revolutionary tool for cetacean stranding programs: implementation and management. Front Mar Sci 7:542015.

8.  Kot BCW, Tsui HCL, Chung TYT, Ho HHN, Robles Malagamba MJ, Kwok JYC, Leung EKC, Ho GYH, Kwan ASY, Yeong JWY. 2021. Virtopsy investigation of stranded cetaceans in Hong Kong waters (2017–2020) (SC/68C/SM/09 Rev1). Paper submitted to the International Whaling Commission, May 2021.

9.  Dennison S, Saviano P. 2018. Diagnostic Imaging. In: Gulland FMD, Dierauf LA, Whitman KL, editors. CRC Handbook of Marine Mammal Medicine (3rd Edition). Boca Raton (FL): CRC Press. p 537–551.