Whales are some of the largest animals that have ever existed and even moderately sized whales are likely the largest animals that any of us will ever experience. So when a whale washes up on the coast it is an event that draws people, engages their curiosity and inspires awe in a way that few other experiences can.
Opportunities to see this spectacle have been remarkably frequent in the last few months, with 7 strandings of sperm whales in the UK and ~30 in total around the European coasts of the North Sea. This has left people asking:“why is this happening?"
Opportunities to see this spectacle have been remarkably frequent in the last few months, with 7 strandings of sperm whales in the UK and ~30 in total around the European coasts of the North Sea. This has left people asking:“why is this happening?"
Stranded sperm whale on the Kent coast in 2012, where I assisted with the dissection (blue cap).
The short answer to this question is: “we don’t know”. And I’m afraid that it is as brutally simple as that. It is not an answer that we are used to getting in this age of all-knowing science, but it is surely the case here.
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What we do know is that sperm whale strandings have been a common event in the North Sea for over 350 years. In fact there was a peak in strandings during the 18th century, so I‘m a little sceptical about pointing the finger at navy sonar or wind farms. Instead, it seems that the shallow North Sea may act as a trap for this species that is more at home in deep ocean habitats.
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Sperm whale strandings (dots) across the North Sea (Pierce et al. 2007)
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Male sperm whales in the North Atlantic migrate southwards in the winter from the arctic waters (females mostly hang out in the tropics), normally heading around the northern tip of Scotland. Apparently, some take a wrong turn down into the North Sea and become trapped in this shallow basin.
The lack of any sightings in the English Channel suggests that they never make it back out to deep water this way. Instead they become funnelled into ever shallower water, possibly getting disorientated and malnourished because their normal squid diet are deep water species. Eventually they get stuck on shallow beaches and once this happens there is little hope for survival.
But what makes some whales take the wrong turn into the North Sea in the first place? There are certainly peaks in the number of strandings over time, which has led some scientists to try and correlate these peaks with external factors.
The lack of any sightings in the English Channel suggests that they never make it back out to deep water this way. Instead they become funnelled into ever shallower water, possibly getting disorientated and malnourished because their normal squid diet are deep water species. Eventually they get stuck on shallow beaches and once this happens there is little hope for survival.
But what makes some whales take the wrong turn into the North Sea in the first place? There are certainly peaks in the number of strandings over time, which has led some scientists to try and correlate these peaks with external factors.
Numbers of reported sperm whale strandings over time. (Pierce et al. 2007)
A recent study found that the best climatic variable that could explain the frequency of strandings was the annual temperature anomaly, which is the degree to which a given year’s temperatures departs from the long term average. In other words if a year is particularly warmer than average, then there were more strandings. Still, this could only explain ~8% of the variation in the strandings data; hardly useful for making future predictions.
Above: Trends in annual temperature anomalies (Pierce et al. 2007). Right: Global sea surface temperature anomaly for January 2016.
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Regions off of Iceland and Norway were much warmer than usual this January, but there is no way that we could say for sure that this was the reason so may sperm whales headed into the North Sea, given the weakness of the link between temperature anomalies and strandings.
To conclude: post mortem examinations may tell why any given whale may have died (i.e. the proximate causes: starvation, sickness injury etc.), but the ultimate cause of why so many whales have ended up where they should not be remains a mystery. What we need are better long term records, and the Cetacean Standings Investigation Programme aims to do just that. Long may it continue.
Click below to see my interview with Sky News about the recent strandings:
Further reading
G.J. Pierce, M.B. Santos, C. Smeenk, A. Saveliev, A.F. Zuur (2007) Historical trends in the incidence of strandings of sperm whales (Physeter macrocephalus) on North Sea coasts: An association with positive temperature anomalies. Fisheries Research, 87: 219-228. doi:10.1016/j.fishres.2007.06.001
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