Strange Strandings – The case of a Risso’s dolphin in the southern North Sea | Cetacean Strandings Investigation Programme

A recent stranding gained media attention last week as a Risso’s dolphin (Grampus griseus) washed up on a beach in Norfolk. The Cetacean Strandings Investigation Programme (CSIP) receives around 10 reports of Risso’s dolphins stranding every year, but most of these reports are concentrated in Scotland and the west coast of the UK. This unusual stranding in the southern North Sea meant it was crucial for the CSIP team to retrieve this animal for post-mortem. Post-mortems are essential for us to understand how the animal died, and the possible series of events which may have contributed or occurred leading up to its death.

WARNING: This blog contains photographs of dead stranded cetaceans and post-mortem findings which you may find upsetting

Risso's 329 Goreston CG
Stranded juvenile male Risso’s dolphin on Great Yarmouth beach (Credit: HM Coastguard Gorleston)

Reports came into the CSIP on Saturday 12th May alerting us to the stranding of a large cetacean on the beach at Great Yarmouth. Prior to this, the day before, HM Coastguard Gorleston had received reports of a pod of Risso’s dolphins off the coast of Norfolk and on receipt of photographs of this stranding, CSIP were able to confirm it was a Risso’s dolphin and most likely part of this pod.

On Monday 14th May, a team from the Zoological Society of London, Institute of Zoology (ZSL), a partner organisation of the CSIP, headed east to pick up the animal from the beach. On the way there, another report came into the CSIP hotline, suggesting a second Risso’s dolphin had washed up just south of the first stranding. After some investigation, it was confirmed that the animal was the same as previously reported – it had washed out and come back in on the next tide. The dolphin was retrieved by the ZSL team and taken back to London to await post-mortem.

Why was this stranding unusual?

Risso’s dolphins are a deep water species, found in both temperate and tropical oceans across the globe where they feed almost exclusively on squid and other cephalopods. They are common to UK waters, however usually favour areas such as the North Sea and west coast where deep continental shelves provide nutrient rich upwellings which support high abundance of prey for this species.

As seen on the map below, strandings of Risso’s dolphins follow a similar pattern to their usual distribution in UK waters, with the majority of strandings in Scotland and the west. Records here at the Natural History Museum found the last stranding of a Risso’s dolphin in the southern North Sea region to be back in 1963, demonstrating how the occurrence of this species in Norfolk was highly unusual, and its presence in an abnormal habitat may have contributed to its stranding.

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Map of Risso’s dolphin strandings in the UK between 1990-2018. Red dot shows unusual location of this Norfolk stranding. Credit: CSIP-ZSL (edited by CSIP-NHM)

Findings at post-mortem

Necropsy of the Risso’s dolphin was carried out by the CSIP team at ZSL on Friday 18th May. Post mortem results found this two metre juvenile male to be in very poor nutritional health and extremely thin (see photo A). Squid beaks were found in the cardiac (first) and fundic (second) stomachs of the animal (see photo B), however there was little evidence of recent feeding such as whole or partially digested squid. A segment of rubber glove (see photo C) was also found in the cardiac stomach, however this was not thought to contribute to the death of this dolphin. Finally, lung hypostasis (differential accumulation of blood) suggested the animal live stranded, most likely on its left-hand side (see photo D).

So in summary, the CSIP team at ZSL believe this animal live stranded on the beach at Great Yarmouth due to poor nutritional health causing starvation and hypothermia, most likely a consequence of being in an unusual habitat making feeding harder than usual. The team is just awaiting bacteriology results so that an underlying infection can be ruled out.

A) Lack of dorsal muscle behind head indicating how thin this animal was (Credit: CSIP-ZSL)
A) Lack of dorsal muscle behind head indicating how thin this animal was (Credit: CSIP-ZSL)
B) Squid beaks found in cardiac stomach (Credit: CSIP-ZSL)
B) Squid beaks found in cardiac stomach (Credit: CSIP-ZSL)
C) Fragment of rubber glove in cardiac stomach (Credit: CSIP-ZSL)
C) Fragment of rubber glove found in cardiac stomach (Credit: CSIP-ZSL)
D) Blood accumalation in left lung (hypostasis) suggesting live stranding (Credit: CSIP-ZSL)
D) Blood accumulation in left lung suggesting live stranding (Credit: CSIP-ZSL)

Photographs from the post-mortem of a stranded Risso’s dolphin (SW2018/329) carried out at ZSL London (Credit: CSIP-ZSL)

Get involved

CSIP depends largely on members of the public reporting strandings. If you come across a stranded cetacean (or seal, marine turtle or large-bodied shark) then please call the CSIP hotline on 0800 6520 333 or send an email to strandings@nhm.ac.uk

Kate Swindells is a research assistant at NHM, a partner organisation of the CSIP. She completed her Masters degree in Global Wildlife Health & Conservation at the University of Bristol and also has an undergraduate degree in Marine Biology. She joined the museum in January 2018.