Assessing seal carcasses potentially subjected to grey seal predation

Abbo Van Neer,Tina Kesselring,Ursula Siebert,Giulia Roncon,Eva Ludes-Wehrmeister,Miguel L Grilo,Stephanie Gross

doi:10.1038/s41598-020-80737-9

Abstract

In order to conduct an objective evaluation of potential ecological effects of grey seal predation on marine mammals, it is essential to establish a broad knowledge base helping in the thorough identification of such cases during post-mortem examination. The aim of this work is to report and discuss outcomes resulting from a retrospective evaluation of harbour (Phoca vitulina) and grey seal (Halichoerus grypus) stranding and necropsy data (n = 3274). In addition, the results are compared to a recent case of definite grey seal predation from Germany as well as reports from other countries. Carcasses potentially subjected to grey seal predation show severe lacerations with a circular pattern leaving a smooth, linear and cut-like wound margin. Large parts of skin and underlying tissue are detached from the body and loss of blubber is common. Occurrence frequencies of encountered lesions are presented and a list of parameters to be used for the assessment of similar cases as well as a complementary decision tree are suggested. With the proposed parameters, categories and tools, a baseline can be built in order to facilitate the standardised recognition of predation cases during post-mortem examinations of seals between groups working with populations across several geographic ranges.

Highlights

In order to conduct an objective evaluation of potential ecological effects of grey seal predation on marine mammals, it is essential to establish a broad knowledge base helping in the thorough identification of such cases during post-mortem examination
In the German Wadden Sea the average increase of adult grey seals counted during moult surveys is 12% between the years 2006 and 2018, harbour seals (Phoca vitulina) are still in the majority in this location[3,4,5,6]
When in 2012 and 2013 the first reports of suspected grey seal predation on harbour porpoises were published, it slowly became clear that the prey spectrum utilised by grey seals is more diverse than previously a nticipated[11,12]

Summary

Introduction

In order to conduct an objective evaluation of potential ecological effects of grey seal predation on marine mammals, it is essential to establish a broad knowledge base helping in the thorough identification of such cases during post-mortem examination. The aim of this work is to report and discuss outcomes resulting from a retrospective evaluation of harbour (Phoca vitulina) and grey seal (Halichoerus grypus) stranding and necropsy data (n = 3274). When in 2012 and 2013 the first reports of suspected grey seal predation on harbour porpoises were published, it slowly became clear that the prey spectrum utilised by grey seals is more diverse than previously a nticipated[11,12]. Brownlow et al.[17] described cases from Scotland of juvenile grey seals that were observed to have been preyed on by a grey seal bull Their gross pathological examination revealed traumatic lesions with detachment of skin and blubber, fractures and canine puncture wounds on the skull. Results are discussed in comparison to a definite case of predation observed on Helgoland (Germany), as well as to cases reported in the literature and cases with other origins of trauma

Objectives

Methods

Results

Discussion

Conclusion