CSI of the Sea: post-mortem findings

Rob Deaville

The CSI of the Sea event took place at ZSL on 10th January and gave viewers the opportunity to watch a harbour porpoise post-mortem in real time via a video-link.

Our strandings expert, Rob Deaville, gives an update on what the team learnt from the post-mortem examination and follow up analyses.  

The CSIP team performing a necropsy on a porpoise
The CSIP team performing a necropsy on a porpoise

The Cetacean Strandings Investigation Programme (CSIP) was launched in 1990 to coordinate the investigation of all cetaceans (whales, dolphins and porpoises), marine turtles and basking sharks that strand around the UK coastline. On average around 600 cetaceans strand around the UK every year and the CSIP team carry out post-mortems on a proportion of these to investigate the causes of death, so we can learn more about the threats these species face in UK waters. In the recent CSI of the Sea event, a live audience was able to witness an investigation of a harbour porpoise, which is the most common species to strand in the UK, with around 300 strandings each year.

The harbour porpoise (Phocoena phocoena) was a 152cm long adult female that had live stranded shortly before dying on the beach at Caernarfon in Gwynedd, Wales in August last year. The body of the porpoise was recovered and transported to the CSIP Welsh strandings coordinator by a colleague, Nia Haf Jones, after which it was stored frozen until the post-mortem examination took place. 

The porpoise was judged to be in moderate nutritional condition and there was no evidence of recent feeding in the gastrointestinal tract. There were moderate parasite burdens within the airways and blood vessels of the lungs and a moderate to heavy burden of fluke parasites in the wall of the second stomach. Light burdens of parasites were also found in the liver and the spaces around the ear bones. 

Whilst we were completing the examination following the close of the live event, we found a large tracking abscess within the left musculature on the back of the porpoise. The discovery of this lesion suggested that the animal might have had a severe underlying infectious process. 

Subsequent bacteriological investigation found a species of the bacteria Streptococcus in pure culture at every site in the body, including the brain. Ongoing work is continuing to type the bacterium down to species level, but we consider the infection to be the cause of the abscess in the musculature and also the likely proximal cause of the animal’s live stranding and death. 

The cause of death is therefore currently given as “generalised bacterial infection (Streptococcus sp.)”. 

CSI porpoise post-mortem
The porpoise necropsy was streamed live from ZSL

Infectious disease related mortality is the most common cause of mortality in the 2,095 harbour porpoises we have examined at post-mortem, since the project began in 1990 (n=495, data 1990-2015). 

Research led by ZSL and colleagues at the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) has demonstrated a correlation between levels of a contaminant of particular concern (polychlorinated biphenyls or PCBs) and infectious disease mortality in UK stranded harbour porpoises. And research published last year indicated that PCBs are found at globally significant levels in the blubber of some European dolphins, such as killer whales and bottlenose dolphins, and may potentially lead to the localised extirpation of some populations in Europe. 

Other common causes of death established during the 26 year length of the project include interspecific aggression (primarily bottlenose dolphin attacks, with a small number of recent cases of grey seal predation), by-catch (the incidental entanglement in fishing gear) and starvation (both neonatal and juvenile/adult animals).

I hope that those who watched the event, both in the ZSL auditorium and online found it an interesting experience. Although strandings can be sad events, examinations like this one enable us to learn more about the threats these species may face. 

Beyond this main driver of the project though, the wide range of samples and data we collect during post-mortems have also helped improve our understanding of the lives of these marine species. 

Moreover, the research carried out by the CSIP over 26 years of continuous funding by Defra and the Devolved Governments in Scotland and Wales has also lead, directly and indirectly, to various policy decisions, which we hope will help improve the long term conservation status of cetaceans found in and around UK waters.  

Find out more about CSIP.  

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