Counting shellfish for fishermen and birds

In the summer of 2024, marine ecologist Karin Troost received many enthusiastic messages from the Wadden Sea: '"The whole Wadden is full of mussels", they told me, and "You can walk across the mussels to Ameland". When we went to see the first radar images in September, we were hugely impressed. We had never seen so many new mussel beds. We were curious whether they would survive the winter...'

Troost and her fellow researchers at WMR map the number of shellfish every year. They mainly look for five species: the cockle, mussel, Atlantic jackknife clam, cut through shell and Pacific oyster. Inventories have been taking place since 1990, giving researchers a good idea of how shellfish stocks evolve over time. Most of WMR's inventories are part of the Statutory Research Tasks (WOT) for the fishing industry. 

All the data that Wageningen researchers retrieve are delivered to government agencies such as the LVVN, Rijkswaterstaat and the coastal provinces. “What makes shellfish monitoring so interesting is that it is comprehensive,” says WOT programme leader Sieto Verver. “It is a combination of data collection - with your feet in the clay, or even up to your groin - and an advisory function that forms the basis for Dutch policy.” 

Karin Troost takes to the water every year to learn more about marine life. She starts as early as March, in the Wadden Sea, Eastern Scheldt and Western Scheldt estuaries, Grevelingenmeer and Veerse Meer. “We go out on ships from the Governmental Shipping Company. And in the Wadden Sea, we rent the Anna Elizabeth, a former cockle cutter. In the spring, we then spend weeks gathering information.” Troost's research is bound to tight deadlines so that the government can determine in time how much shellfish fishing is allowed.

WMR's monitoring study consists of several parts. The survey starts by surveying the shellfish beds on the tidal flats. “There are mussel beds and oyster beds on the tidal flats. We map these by simply walking around them with a GPS receiver. You sometimes end up in very soft places, in which you can sink way down. It looks like custard sometimes,” laughs Troost. 

Based on these inventories, researchers can determine the number of hectares of mussel and oyster beds. “We use all kinds of tools to do this. We used to use aircraft to scout mussel beds before going into the field, but now we can do it with satellites. That means that cloudy conditions are no problem and you don't need daylight.” 

Most of the monitoring consists of estimating the number of shellfish by taking samples. For this, the researchers first determine the most promising places to find shellfish species. They then take quite large samples from the bed in those places. Troost: “With other benthic animals, a small sample from the bed is enough, but shellfish are relatively large animals that live in clusters. You then need a large area to estimate their numbers properly. Otherwise, there's a chance you won't find shellfish at all in your sample.”

For sampling, WMR researchers have several methods at their disposal. One method uses a kind of hoover that sucks up a long strip 10 centimetres by 150 metres, to a depth of 7 centimetres in the bed. This requires large vessels, but there are also smaller boats for research in shallow water, Troost explains: “Then we use a long stick with a grabber attached, which you put on the bed. That digs itself in and takes a bite out of the bed. You then pull it up."

Samples should always be sieved to remove sand and silt, leaving only the animals. Besides all the shellfish species, crabs and starfish are also counted, because these predators feed on shellfish. Animals are also sometimes collected and counted for other projects, if the sampling method is appropriate. One example is the counting of sand eels in the coastal zone of the North Sea. That is cost efficient, says Verver: “Why send a separate ship for that, with researchers and crew? You're there anyway, so you can do those counts right away.” 

In the onboard laboratory, all the species are sorted by species, age or body size and then counted, measured and weighed. Then everything is returned to the water. Based on the sample, researchers can then calculate the number of shellfish per square metre, and based on that, the total amount per area.

This method is fast and effective, says Troost: “Because we can measure everything on board, we don't need expensive time in an on-shore lab. And we can also produce our results very quickly as a result. For cockle fishing in the Wadden Sea, for example, we carry out the fieldwork in the spring, until sometime in June. By 1 July, we are already delivering our first provisional estimate.” 

Several fish quotas are set based on WMR's fishing quotas. These quotas determine how much can be caught for each species and area. “Very different arrangements have been made for each fishery and for all waters,” says Troost. “For cockle fishing in the Wadden Sea, for example, fishermen, government and nature organisations are in a management covenant agreeing on how many cockles can be fished. As soon as we provide our estimate of how much is there, they know how much of that they can catch.” In autumn, WMR conducts an additional count. If it shows that an extremely high number of cockles died during the summer, the quota could still be adjusted. 

In the case of cockle fishing, there is another factor at play: cockles are also on the menu of the oystercatcher. There must be enough food left to sustain the population of this bird. On cockle banks with more than 50 cockles per square metre - which oystercatchers like to visit - fishermen are therefore not allowed to fish more than 2.5% of the stock. 

WMR also regularly plays an advisory role, in which shellfish monitoring is an important part. Troost: “When the mussel stock counts are done, there is always a meeting with the mussel farms in which we provide them with advice. Sometimes we are also invited to consultations within other management covenants, for example on the regulation of oyster harvesting.” Troost and her colleagues then studied the importance of mixed banks (containing both oysters and mussels) for birds. They recommended allowing harvesting only on banks where at least half consist of oysters.

Furthermore, WMR researchers are regularly asked for advice where supporting research is needed. At the request of the Ministry of LVVN, for example, they investigated the Manila clam, a newcomer to Dutch waters that may be a food source for oystercatchers. They can also give unsolicited advice, if there is reason to do so. Troost: “Between 2018 and 2020, we found that extremely high numbers of cockles had died during heat waves, in some places even more than half the population. We flagged this to LVVN, following which the additional stock counts were introduced in the autumn. In fact, due to climate change, such heatwaves may become more frequent.” 

WMR's shellfish monitoring is used for several purposes, besides determining fishing quotas. Rijkswaterstaat needs the data, for example, when carrying out beach nourishment; the replenishment of sand to make beaches wider and higher. By checking shellfish stocks, Rijkswaterstaat can avoid carrying out replenishment in places where large numbers of shellfish live. 

The government also uses the counts to comply with the European Marine Strategy Framework Directive (MSFD). This directive obliges every European member state to protect its seas, including by monitoring how the marine environment is evolving. “For the same money, we try to serve as many purposes as possible,” says programme leader Sieto Verver. 

Finally, the counts are publicly accessible so that anyone can see how shellfish populations are evolving. Thus, interested parties can see for themselves that the number of mussels in the Wadden Sea in 2025 was much higher than in previous years. Researchers counted as many as 7,000 hectares of mussel beds. This is much more than in previous years, when the acreage hovered around 2,000 hectares. Most of the small mussels Karin Troost saw in 2024 had survived the winter.