Since last Wednesday, about twenty dead porpoises have washed ashore on the Wadden Islands. Porpoises beach every day on the Dutch coast, but so many at once is unusual.
“It is striking that the animals that wash ashore at the same time are all in the same state of decomposition,” says porpoise researcher Lonneke IJsseldijk of Utrecht University. “The animals that ended up on the shore yesterday already looked a lot worse than the animals that ended up on the beach on Wednesday.” So it seems that all the animals died at about the same time.
It is not yet known how many animals are involved, but IJsseldijk suspects that it concerns a few dozen. “On Saturday there were twenty on Ameland alone. Then it gets tough.”
There are plenty of harbor porpoises in the Dutch part of the North Sea, estimates are based on about 85,000. Every day some of those whales strand on the Dutch coast. Last year there were 431. But so many at once is unusual. IJsseldijk: “This year has been a quiet year so far with approximately 300 animals washed ashore, but it may well be a busy year after all.”
Also read: Porpoises are increasingly washing ashore along the North Sea
Volunteers from the Dutch Strandings Network collect the washed up animals, bury them or bring them to Utrecht, where IJsseldijk is investigating the cause of death. She examines between fifty and a hundred animals a year.
The biologist has twenty-five animals from the mass stranding in the freezer to examine and expects it to stay that way. Animals that still wash ashore have already decomposed too far to properly determine the cause of death. “In addition, it is logistically impossible to bring all the animals here and examine them. They are animals of an average of fifty kilos. We don’t have that much storage capacity in the freezers here. But I think this one subset is representative of the whole group.”
Explosion of virus
IJsseldijk will start the pathological examination on Wednesday, but was already so curious that she wanted to see a whale. “It is now lying in front of me on the cutting table. He is not fresh.” She can’t say much about the cause of death, but it is striking that they are all adult animals.
“In 2011, we also had a mass stranding. They were all young, starving animals. If adult animals die at the same time, it could be due to explosions at sea or a virus infection, for example.” The Defense Ministry is currently investigating whether there have been military exercises or bombs detonated in recent weeks.
IJsseldijk first looks at the outside of all animals. She looks at the weight and whether there are noticeable abscesses or parasites. Then she opens the animal and examines all the organs one by one. “If there is food in the stomach, the animal has recently caught fish and death is likely to have come suddenly. If the animal was already weakened by illness, there is often insufficient food in the stomach.” After IJsseldijk’s research, all stomachs go to Wageningen, where researchers find out what an animal has eaten.
Using PCR tests, IJsseldijk can find out whether the harbor porpoises are infected with certain viruses. “For example, I want to look at the morbillivirus, a virus that has previously caused large-scale deaths among dolphins.”
Floating models would allow researchers to find out where the animals died. Given the state of decomposition, IJsseldijk thinks that the first harbor porpoises that washed ashore were dead for three days. “The animals probably come from the North since the strandings started on the islands, and since yesterday have also reached the Groningen coast.” The biologist is in close contact with Danish and German researchers to find out whether whales also stranded there. “So far they haven’t seen anything remarkable.”
According to IJsseldijk, the end is not yet in sight. “Maybe we haven’t even peaked yet. If this continues for a few more days, it could become a problem for the porpoise population here.”
A version of this article also appeared in NRC in the morning of August 31, 2021