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Wadden area may become bottleneck for knot

21 April 2010

The Dutch Wadden Sea, due to a decline in food resources, may become a bottleneck in the annual cycle of the knot. This has been revealed by PhD research by Casper Kraan. He studied two subspecies of the knot, the Calidris canutus islandica and the Calidris canutus canutus. ‘It appears that neither species is able to fatten up properly in the Wadden area any more’, says Kraan. He conducted his research at the Royal Netherlands Institute for Sea Research (NIOZ) and will be awarded a PhD by the University of Groningen on 26 April 2010.

The islandica breeds on the Canadian and Greenland tundra and then flies to the Wadden area to overwinter. The canutus breeds in Siberia and uses the Wadden Sea to fatten up for the last stage to western Africa. Kraan: ‘Although the two subspecies have different strategies, they have the same problem – declining food resources.’

Lack of diversity

Kraan investigated what species there were in the tidal mudflats. ‘The Wadden Sea has changed enormously since the 1950s’, according to Kraan. ‘The number of shellfish and crustaceans has declined while the number of worms has remained the same. Many of the animals that gave extra structure to the mudflats have significantly declined in numbers. That’s true, for example, for mussels and cockles. This reduction in diversity of the tidal flat system has a negative effect on the knot population. Between 1996 and 2005, wintering knots lost 55% of their foraging area. The decline in the number of knots at 42% is pretty much parallel.’


The Macoma balthica, a clam that is the most important food source for knots, was at the centre of Kraan’s research. ‘This used to be a very common shellfish in the Wadden area, but now it’s unusual to find these clams in a soil sample. One of the possible causes is the mechanical cockle fishing industry that was permitted up until 2005, which resulted in a decline in the quality of shellfish. But there are also other damaging influences, such as the recent mild winters,’ emphasizes Kraan.

Predictive model

Kraan developed a mathematical model to explain the relationship between animal species (in this case the clam) and environmental variables. ‘The existing mathematical approaches were not able to explain the so-called spatial auto-correlation. Auto-correlation is the idea that a group of clams in a certain place is not independent of another group of clams a little way away. Now there is a new method that can better predict the relationship between the species and its habitat, not only clams, but also, for example, seals or vegetation. We can now clearly show which environment suits a certain species and which does not.’

Unexpected turn

A new mathematical model was not exactly Kraan’s aim when he started his research. ‘I started with the idea of taking nice samples on the mudflats and doing fun things with them’, says the PhD student. ‘At a certain moment I had a fantastic data set, but then I discovered that the existing statistical techniques were not able to cope. That’s how I ended up in difficult mathematics territory. My maths teacher from secondary school thinks it’s hilarious – I was never particularly good at maths. But once you get the hang of it, maths is great fun. With hindsight I’m really pleased things worked out like this.’

Curriculum Vitae

Casper Kraan (Doetinchem, 1977) studied biology at the University of Groningen. His PhD research was performed at and funded by NIOZ, the Royal Netherlands Institute for Sea Research on Texel. His thesis is entitled Spatial ecology of intertidal macrobenthic fauna in a changing Wadden Sea. Kraan will be awarded a PhD by the Faculty of Mathematics and Natural Sciences of the University of Groningen. His supervisors are Prof. T. Piersma (University of Groningen and NIOZ) and Prof. Jaap van der Meer (VU Amsterdam and NIOZ).

Note for the press

Contact: Casper Kraan, tel. 0222-369 592, e-mail: or Jan Boon (spokesman for NIOZ), tel. 0222-369 466, e-mail:,

Last modified:15 September 2017 3.29 p.m.

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