Researchers from Wageningen Marine Research annually monitor the migration of glass eel from the sea to Dutch inland waters. To gain knowledge about the possible predation of larger predatory fish on glass eels at water barriers such as locks, researchers use sea bass caught in the Staalhaven of Tata Steel.
During their journey from their birthplace in the Sargasso Sea (thousands of kilometres away) towards the fresh waters of Europe, young eels are hindered by all kinds of barriers; dams, locks and weirs, for example, can slow down their migration. As a result, fish can sometimes accumulate at barriers in large numbers and for long periods of time. These are called bottlenecks in the migration route.
Small fish such as glass eels are particularly vulnerable to such bottlenecks due to, for example, the hunting behaviour of larger predatory fish. They could profit from large numbers of small fish in an unnatural situation. More knowledge is needed to set up good follow-up research so that ultimately, effective measures can be taken.
Wageningen Marine Research has been studying the migration of glass eel (young eel) at the Afsluitdijk since 1938 on behalf of the Ministry of LNV. In the 70s and 90s, ten research locations along the coast were added, including the IJmuiden sluices.
In 2019, a large-scale study was started in which glass eel was also given a small visible colour mark. The objective is to get an even better picture of the annual supply of glass eels along the Dutch coast. The institute is also researching the degree to which glass eel spread, how long they are held up and the effects of possible migration solutions.
Scientists use the eels caught at Tata Steel in behavioural studies. This includes exposing them to marked and unmarked glass eels for a short period of time and observing their behaviour. The researchers want to know whether predators hunt glass eels and whether marked glass eels are at at higher risk. The glass eels are placed in a transparent tube and cannot be eaten. Afterwards, the fish are returned to the wild.
IJmuiden is an important hub for glass eels, as it provides access to the 26-kilometre-long North Sea Canal. From there, the glass eels migrate into the Dutch polders.