Abstract

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.

Highlights

  • Cetacean strandings occur across all the world’s oceans and have been recorded throughout history [1]

  • Through a comprehensive pathological investigation, comprising gross post mortem assessment and a range of ancillary diagnostic tests, we investigated the hypothesis that this stranding event could be explained by a compromise in the health status of one or more of the stranded animals

  • In order to elucidate potential reasons for the largest sperm whale stranding event that has ever been recorded in the North Sea we conducted a systematic pathological examination of the majority of the stranded whales, leading to one of the most extensively investigated sperm whale mortality events that has ever taken place

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Summary

Introduction

Cetacean strandings occur across all the world’s oceans and have been recorded throughout history [1]. Stranding events can consist of an individual live or dead cetacean and less frequently of multiple live and/or dead animals [2]. One of the species known to be involved in such mass stranding events is the sperm whale Physeter macrocephalus. The sperm whale is a widely distributed species, inhabiting deeper waters from the equator to the polar regions. Commercial whaling from the 1800’s to the 1960’s significantly reduced populations, with a global pre-whaling population estimate of 1,100,000 reduced by approximately 67% [3]. Since the moratorium on commercial whaling in the 1980’s, the current global population size is estimated at around 360,000 individuals (coefficient of variation = 0.36) with no evidence of a notable increase or reduction [3,4]. Sperm whales show distinct spatial population segregation, with females and calves normally resident year-round near breeding areas around the equator, and males migrating between the breeding areas and high-latitude feeding grounds [4,5,6,7]

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