Extreme cockle mortality on tidal flats of Dutch coastal waters

Heatwave most likely cause of death

Cockles by nature have a high mortality rate. But what happened in July and the beginning of August this year is exceptional and has not been observed since 1990. In that year, Wageningen Marine Research, commissioned by the Dutch Ministry of Agriculture, Nature and Food Quality, started an annual inventory of cockle stocks in Dutch coastal waters. The most obvious cause of death seems to be the unusually long-lasting heat wave in 2018, although other causes cannot yet be ruled out. WMR researchers conclude this from a combination of factors. For example, mortality occurred in all saline coastal waters where cockles occur on tidal flats. This indicates that the mortality was caused by conditions that were present simultaneously in both the Wadden Sea and the Zeeland waters. "If a disease or environmental pollution had been the culprit, the mortality would probably be more localised," according to WMR researcher Karin Troost. "In addition, the deaths occurred at the end of the unusually long heatwave. On 27 July, a maximum temperature of 36.8°C was recorded in Vlissingen."

A 2015 Portuguese study found that 100% of cockles died if exposed to a temperature of 35°C for six hours. It is possible that the cockles in the Eastern Scheldt were exposed to such high temperatures during the hours around low tide. This is because the upper sediment layer and stagnant water on the plate can heat up quickly then. On 2 August, fisheries official Harry Heidekamp measured a maximum temperature of 30.9°C in the layer of seawater that was still standing on a plate in the Eastern Scheldt at low tide, while that day in Vlissingen a maximum air temperature of (only) 25.6°C was measured (www.knmi.nl).

In consultation with the Dutch Food and Consumer Product Safety Authority, samples were taken from the Eastern Scheldt because a disease or infection could not be ruled out as the cause. These samples have been submitted to the Laboratory for Fish, Crustacean and Shellfish Diseases of Wageningen Bioveterinary Research in Lelystad for analysis. The results are expected in early September.

Bad news for oystercatchers and cockle fishers

The extremely high cockle mortality is bad news for birds and fishermen. Oystercatchers need enough shellfish as a food source to get through the winter. Cockles are an important part of their diet. For small-scale cockle fishermen in the Wadden Sea, the fishery was already getting tighter this year. Because the stock estimate for the autumn of 2018 is below the threshold value for a 'cockle-rich' year for the first time since 2011. This means that restrictive measures have to be taken. For example, the number of vessels allowed in the so-called 'draw areas' will be reduced from three to two. Due to the extreme summer mortality, there are even fewer cockles to fish.

Summer mortality precisely in a year with a lot of cockle spawn

"The summer mortality rate occurs in a year when a lot of cockle spawn, i.e. cockles born in 2018, has been seen in the Wadden Sea and Eastern Scheldt," says Troost. In the Wadden Sea, the last major breeding event was in 2011. The expectation that a new large breeding event would occur around 2018 - because this has happened about every 6-8 years since the first measuring moment in 1990 - seemed to come true this summer. The fisheries officials, the Wadden Unit of the Ministry of Agriculture, Nature and Food Quality and the cockle fishermen saw new broods over large areas. Although the new brood seems to survive quite well compared to the older cockles, it is impossible to estimate how much of it will actually remain. Should this brood not survive well either, it might take another seven years before a new large breeding event takes place and cockle stocks can recover.

Resampling and stock assessments planned

In the coming weeks, WMR, fishery officials from the South West Netherlands region and the Wadden Unit will re-sample the Oosterschelde and Wadden Sea. This will show how many cockles have died and whether certain age categories have been hit harder than others. WMR will also estimate the stock size in autumn 2018. This is to know how many cockles will still be present then as food for birds and people.

Latest news update 14 December 2018 - Revised cockle stock estimate Wadden Sea and Eastern Scheldt

In the meantime, the resampling has shown that the observed cockle mortality was indeed extremely high. In the Wadden Sea, more than 60% of the cockles older than one year died during the summer. No increased mortality was observed among 1-year-old cockles, possibly because they entered the heat wave in a higher condition. This is assumed because one-year-old cockles do not yet participate fully in spawning, while older cockles are usually exhausted. In the Eastern Scheldt, more than 90% of the 1-year old and older cockles died. The cause of the differences between the Eastern Scheldt and the Wadden Sea has not been investigated. In both areas, a lot of cockle spawn was found, which in the Wadden Sea had already grown strongly in some places. This gives perspective for the coming years.