Employees of the Flemish Environment Agency (VMM) recorded a new daily record for the fall for mitten crabs in the Kleine Nete in Grobbendonk in mid-April. “With around 17,000 animals, the previous record of 15,000 was easily broken,” says Paul Van Loon of the VMM. “The peak seems to be over at the moment.”
The trap for mitten crabs near the water mill in Grobbendonk was placed in 2018. It is a narrow box as wide as the river with a slot at the top. The crabs crawl across the bottom of the Kleine Nete and fall into the tank through the slot. Then they can only crawl along the end and through a tube to a collection container. “The crab migration season started a little earlier this year,” Van Loon explains.
READ ALSO. 1.3 million crabs have already fallen into the trap (and that is only a fraction of the total population)
“Normally the peak occurs at the beginning of May, but this year they were already present en masse in mid-April.” Just as in nature, more things started a little earlier this spring. 17,000 animals is of course an estimate. “We weigh one kilogram of crabs and they are actually counted. If we know the number of crabs in one kilogram and we multiply it by the total weight, we arrive at a very reliable figure.” Mitten crabs have a habit of migrating upstream in the spring. When they are sexually mature after a few years, they move downstream in search of more salt water to mate. Then they die.
After the crabs end up in a narrow slot, they can only crawl along the end and via a tube to a collection container. — © Joren De Weerdt
Currently, the employees of the Flemish Environment Agency empty the collection bins every other day. On average, the catch is around 7,000 to 8,000. “The caught crabs go into the freezer as quickly as possible. A fairly quick and efficient way to kill the animals. Afterwards they are thawed again and used as animal feed in the zoo.”
With around 700,000 crabs caught, 2019 was a peak year since the crab trap was put into use. “We are currently at around 225,000 for this year and we suspect that it will be more than 300,000. That is a lot less than in 2019, but still a large number.”
© Joren De Weerdt
Mitten crabs originally come from Asia, but they also ended up in our region via international shipping. They are wolverines that have no natural enemies and can therefore seriously disrupt biodiversity in the Flemish rivers due to their massive presence. As the year progresses the crabs grow. The little ones are a few centimeters in size, adults are the size of a palm.
Tags: daily record mitten crabs trap Peak earlier year Grobbendonk
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