NORTH HOLLAND – Since 2005, many more dead porpoises have washed up on Dutch beaches than in the period between 1990 and 2005. Remarkably, this increase did not apply to countries such as Germany and Denmark, which border the northern part of the North Sea. This is the conclusion reached by researchers at Utrecht University in an international study.
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Of the more than 16,000 porpoises that washed ashore in countries on the North Sea between 1990 and 2017, as many as 6,000 washed up in the Netherlands. And indeed, the data show that between 1990 and 2005 much less (approximately 1,000) washed ashore than in the period 2005 – 2017 (approximately 5,000).
Last year, the same Utrecht scientists concluded that a recent increase in the number of stranded porpoises could be attributed to gray seals. Some porpoises die immediately after a seal bite, but porpoises that initially survive an attack are also at risk. In the event of a bite, the seal can transmit a bacteria that slowly weakens the porpoise and which eventually causes it to die.
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“For this research we go a little further back in time,” explains researcher Lonneke IJsseldijk. “The research shows that there was a break in the trend in 2005. From that specific year on, there seem to have been many more beachings in the southern North Sea, but we do not yet know what happened that year and could have caused the break in the trend.”
That is also difficult to find out retroactively, she says. “Because at that time there was no investigation into the cause of death in the Netherlands. It started in 2006, following the sudden increase in the number of strandings.” She suspects that around 2005 seals hardly played a role in the high porpoise mortality. “Because there weren’t that many seals in the southern North Sea at the time.”
Harbor porpoises migration
The researchers now attribute the increase to migration of the population from the northern part of the North Sea to the southern part of the North Sea. “Because the same tendency is seen in living animals at sea”, says IJsseldijk. Moreover, animals that strand in the northern part of the North Sea are often younger than their counterparts who do not survive in the southern part (off the Dutch and Belgian coast). “This probably reflects how the animals spread across the North Sea.”
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Harbor porpoises that wash up on Dutch beaches are often ‘juvenile animals from a weaker part of the population’, IJsseldijk emphasizes. Often it concerns emaciated explants, as in August last year.
Rijkswaterstaat client
But what difference does it make where in the North Sea the weak part of the porpoise population is located, you might ask? “Knowledge about where weak populations are located in the North Sea is important to gain approval for activities at sea.” She refers, among other things, to the construction of wind farms. “Rijkswaterstaat commissioned this study.”
There is no lack of ambition to conduct follow-up research, she emphasizes. “At the moment it is still limited to what, where and when, but hopefully we can also include the causes of death in the future. Unfortunately, it has not yet been investigated which part of the washed up porpoises on English, German and Danish beaches have been killed by seals. ”
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