Science - August 22, 2019

Massive die-off of cockles in the Wadden Sea

Tessa Louwerens

The problems in the Eastern Scheldt appear to be less severe than anticipated, but in the Wadden Sea massive cockle mortality has been observed for the second year in a row: an estimated 80 percent of the population has died in some areas. ‘The warm summer probably played a role,’ says Karin Troost, marine ecology researcher at Wageningen Marine Research (WMR).

Climate signs
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How severe is the problem?
‘At WMR, every spring we make an inventory of cockle populations and estimate how many cockles there will be in September. The Ministry of Agriculture, Nature and Food Quality then sets a quota for the cockle fishery based on this estimate, which is based on certain percentages of mortality and growth. Normally, in the summer months the average cockle mortality is about thirty percent. In some years the mortality is a little higher and in others it is a little lower. As long as it stays within certain margins, there is no reason for alarm. In the Eastern Scheldt, the situation now seems to be better than expected, but the Wadden Sea has experienced much higher cockle mortality this year.’

To what extent is this related to climate change?
‘During last year's heatwave there was extreme cockle mortality on the exposed cockle beds, particularly in the Eastern Scheldt. I have the impression that the mortality is less severe this year. The warm summer has probably played a role this year as well, possibly in combination with other factors. For example, during the spring inventory in the Wadden Sea, we saw that the cockles were packed very densely, sometimes as many as thousands per square meter.
Through mutual competition they literally grow each other out of the sediment they are embedded in, and once on the surface they weaken quickly. They also compete for food. That is a natural process that you have to take into account. If they are very densely packed, even a normal mortality rate of about thirty percent results in large numbers of dead cockles. That seems rather serious, but what matters is the number of living cockles that are still embedded in the sediment.’ 

What are the consequences?
‘Cockle mortality has consequences for fishing, but also for nature. For example, cockles are an important food source for oystercatchers. Last year, the Dutch province of Friesland changed the quota following the massive mortality due to fears that the oystercatchers would otherwise not have enough food. This year we began with a high cockle population in the spring, so it is more likely that there will still be enough cockles despite the high mortality rates. In addition, this year’s stock of mussels is large, a species that is also on the oystercatcher’s menu. The cockle fishery is very concerned due to the severe mortality last year. For the fishery to be profitable, the question is now whether there will be enough locations left next season with sufficient cockle densities and cockles of the right size.’

How can we best deal with this?
‘Due to climate change, heat waves are more frequent. This spring we therefore initiated a study to determine whether our calculations for estimating stocks are still accurate, or whether we need to adapt them. In any case, we have measurements of the effects of this year’s heat wave. Before we can adapt the calculations, however, we need to collect data for several more years.’