WARMEST JUNE EVER RECORDED IN WADDEN SEA; CONCERNS OVER COCKLE MORTALITY

Warmest June ever recorded in Wadden Sea; concerns over cockle mortality. In the past six summers, there were as many as five with high mortality among cockles in the Wadden Sea in the Netherlands. That mortality always occurred several weeks after above-average water temperatures were recorded.

“I therefore hold my breath for the shellfish, after the record temperatures of mid-June,” said researcher Katja Philippart.

Dutch research institute NIOZ has been measuring the temperature of Wadden Sea water since 1861. As of 2001, this has even been done every ten minutes, allowing a very reliable daily average to be determined, regardless of high or low tide. After this year’s relatively warm winter and an initially chilly spring, temperatures suddenly shot up in mid-June. June turned out to be the warmest seawater month in the Wadden Sea in 160 years. The week of June 19th was also warmest ever recorded for that week: over 21 degrees!

Between 2018 and 2023, only 2020 was an average year for cockles. All other summers saw high mortality, particularly in the second half of August. “That looks pretty dramatic,” says ‘hand cockle fisherman’ André Seinen of the Harlingen-based company Meromar. “Cockles that die, crawl to the surface of the mudflats. Eventually, you see the contents of the shells  floating around, because unlike a clam, for example, a cockle lets go of its shell when it dies. All that remains are empty shells on the mudflats.”

Zhengquan Zhou, a PhD-candidate at NIOZ and Utrecht University, recently published the results of experiments with cockles he exposed to high temperatures in captivity. He showed that the animals suffered increased mortality only from four weeks after an experimental increase in temperature. Like Philippart, Seinen can understand those results. “A cold-blooded animal like the cockle needs more food at higher temperatures. The first few weeks such an animal may still be able to draw on its reserves, but over time those reserves are depleted, after which the cockle dies.”

Philippart emphasizes that temperature is probably not the whole story. “In hot and dry summers, the supply of freshwater from the IJsselmeer is also affected. The many algae in that water are also food for shellfish. So if less water is sluiced to the Wadden Sea due to drought, that is an extra problem for shellfish like the cockle, on top of the stress they have to endure due to the heat. And with easterly winds, the mudflats are also dry longer, giving the cockles less time to eat.”

Both Seinen and Philippart are keeping a close eye on the cockle situation in the coming weeks, following June’s record high temperatures. “In any case, all professionals are on edge, because of the high temperatures now being measured in the seas and oceans around the world,” Philippart said. “We are currently seeing the concrete consequences of the changing climate play out before our eyes.”

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