Following the stranding of a number of sperm whales on the English coast last weekend, scientists from the Zoological Society of London (ZSL) and the Museum visited Lincolnshire from Monday 25 January to conduct autopsies on the dead leviathans.
I interviewed Rebecca Lyal, Cetacean Stranding Support Officer of the Cetacean Strandings Investigation Programme (CSIP), to find out what the autopsies have revealed thus far about the cause of death.
Rebecca, how do you go about assessing the cause of death of whales?
That can be done by looking at the recent movement of the whales, where they have come from and what their behaviour has been and then, once we’d deduced that they didn’t strand because they had got caught in a net, or had any wounds that may have made them unable to swim, we can start looking a bit deeper. This is when we started to take samples of the skin, the blubber, reading the blubber thickness, and then muscles and blood.
[Warning: readers may find the images that follow in this post upsetting.]
Sperm whales are huge animals, taking samples must be really challenging?
From the first whale we looked at on the Monday morning, we removed quite a large section of the abdomen. We used a winch from a car to remove the blubber. You basically have to flense, or separate, the muscle from the blubber layer using big knives that look like ice hockey sticks. But the knives get blunt really quickly so you have to keep sharpening them.
After that we could get to the gut, but as this whale had been dead for about 24 hours or so, it had already started to decompose and within the body cavity pressure had already started to build up.
The blubber was essentially insulating the animal, leading to it ‘cooking’ on the inside and heating up the air. So when we started to make incisions, it sounded like an enormous release of gas, which some in the media branded as an explosion. It’s been sensationalised a little bit, but it was, I suppose, exploding out of it and it did look bizarre.
With this release of gas came a lot of the guts, so the intestines came flying out, which was actually very helpful because then we could more easily get to them. After about 20 minutes the stomach came out too, which was also very useful for us because that was initially deep inside the whale and very difficult to get to.
What can these samples tell you?
We could look inside the stomach and see what it had been feeding on recently which helps with our investigation into the cause of death and the life history of the animal. In this first whale, which was a juvenile of about 12.5 metres, we found just a couple of squid beaks in the stomach, which suggests that they couldn’t find many squid around. They don’t really like the little north east squid – they need giant squid because sperm whales require about a tonne of food a day to maintain themselves.
We also measured the blubber thickness which is an indication of how well they have been feeding, and this is suggestive of their overall health. In addition to all that, we also take tooth samples and some people may have seen pictures of the scientists sawing off the jaw of the sperm whales.
Their teeth are like tree rings, when you cut them in half, you can see it grows in layers and each layer reflects one feeding season. The thickness of each growth layer reflects how well it has been feeding in that season, so a thicker layer means a more successful feeding season than a thinner layer, providing a snapshot of their life history.
So from what you and the team saw of the whales over the week, what have you concluded about their deaths?
The pod of 17 sperm whales [n.b. since I interviewed Rebecca, another 8 whales have stranded in Germany yesterday, meaning at least 25 whales have stranded in the North Sea in the past month] were first seen stranding along the coast of Germany, where six stranded, then a further six got caught up in the Netherlands and the rest came over and beached in Norfolk, where one of them died in Hunstanton. The last four died in Skegness.
They’re all juvenile males. We only really get male sperm whale strandings in England, because all the females stay at the equator, and the males migrate down to see them, breed and then come back up again. And these young males hang around in batches of pods.
The young ones may not have got the hang of reading the bathymetry, the landscape of the seabed, with their sonar, so they may have been foraging, chasing squid and then not realising that they were actually entering quite shallow water.
Sperm whales are deep sea mammals and are coming in from about 1,000m, when they hit the continental shelf this leaves them with around 200m of depth, and then the North Sea peaks to about an average of 50m, so the deepest point is about 70m and the shape of it is like a giant funnel. In shallow waters the whales can’t make proper use of their sonar, it’s like they’ve been blinded, which can lead to them beaching.
It’s fascinating to get an insight into what happened in this historic yet very sad event, and you can find out more about Rebecca’s role in another interview here.
The scientists from ZSL are still to analyse the blubber, muscle and blood samples that they collected from the sperm whales, but will share their findings through the CSIP facebook page and through their quarterly reports.