In the exceptionally warm summer of last year, cockles in Zeeland and the Wadden Sea died en masse because they were literally baking in the sun at low tide. This is happening again this year: in some places, 90 percent of the shellfish are dead. NU.nl spoke to a researcher who explains why this is and what the consequences are. Repeating summer heat can once again be a blow to weakened cockles.
The Netherlands is currently experiencing the second official heat wave this summer - and actually the third, because in addition to the official national heat waves of July and August, it was several days well above 30 degrees in June, although the series was then interrupted by cooler days. inbetween. Exceptional statistics, Weerplaza explains: the Netherlands experienced two heat waves in one summer only in 1941, 2006 and 2018.
After this year's new heat records, we would almost forget it, but last summer was also extreme for the Netherlands, with very high temperatures for a long time and record drought.
"In 2018 there was a heat wave for a total of 23 days. We then investigated how many cockles died as a result of the heat," says shellfish researcher Karin Troost of Wageningen Marine Research.
That summer was especially a blow to the cockle stand in Zeeland: "In the Oosterschelde we saw a mortality of 90 percent among the young cockles and even 96 percent among the older cockles." Cockle mortality was less extreme in the Wadden Sea, but two out of three adult cockles also succumbed to the effects of the heat.
Cockles die quickly from 32 degrees
"Cockles are cold-blooded. That means that digestion increases at warmer temperatures, which means that they also need more oxygen, which is often no longer sufficiently present in warm water. The heat can also cause tissue damage: at some point the animals can they no longer have to filter water and they get too little food.
That starts with temperatures above 23 degrees. Cockles die quickly from 32 degrees. This creates a chain reaction that is dependent on specific circumstances. "Last spring we saw that the cockles born in 2018 were very close to each other in some areas, causing them to compete for space and food. In these areas, mortality seems to have begun."
"Due to the high temperatures, dead cockles on the spot quickly rot, causing oxygenlessness and more cockles to die. A cockle that is weakened quickly falls victim to parasites, which often give the final push. So one summer is the others not, and every year there is a different combination of factors. "
In the Wadden Sea, the new generation of cockles dies in large numbers
This year, too, according to Troost, there has been massive cockle mortality as a result of the summer heat, now mainly in the Wadden Sea. "We have no official measurements this year, but estimates, including manual cockle fishermen, range from 50 to 95 percent mortality. The heat wave at the end of July was relatively short, but it has become much hotter than in 2018."
The long-term consequences are still unknown. For example, eider ducks and oystercatchers eat the cockles and are largely dependent on them. "Until now, we have seen the cockle population renewed about every seven years due to a large increase, with many cockles added at once. We also saw this in the summer of 2018, so that despite the large summer mortality, there was also renewal of the population. "
"We may have to wait a few years before there is a new large increase." Karin Troost, shellfish researcher
"But this summer we see that the new generation in the Wadden Sea is dying en masse, while they have probably not yet reproduced. We may have to wait a few years now before there is a new big growth."
The researcher expects that the amount of cockles will decrease if such large deaths are repeated every one or two years, unless the animal is able to adapt to higher temperatures.
Dry sandbars and bright sun
And the current heat wave? "If the temperatures on the mud flats have again reached values of 32 degrees or higher, the chance of further mortality is high, certainly because the still-living cockles will be weakened," says Troost. And that can also happen on days when that temperature in De Bilt is not reached, the KNMI explains in a special message about summer wadden weather - when low tide and bright sun coincide.
The temperature on dry sandbanks can then be considerably higher than in the official weather huts, which measure the shadow temperature. And that is especially at the end of the summer, when the seawater has also heated up considerably.
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