The young herring likes to grow up in the Wadden Sea

Small fish that live in large schools in the water column of the sea, i.e. not on the bottom, have hardly been mapped in our coastal seas. Since these fish are the food of birds such as gulls, terns, grebes and divers, as well as porpoises and seals, they can tell a lot about the occurrence of other animal species, explains researcher Margot Maathuis.

Maathuis: “The presence of suitable food often has the most influence on changes in numbers of a species and their area use. How is it possible that guillemots suddenly die in bunches? Why is it that the number of terns is decreasing? Why is the distribution of grebes changing? ?”

Sound waves technology

This made a research team from Wageningen University, Wageningen Marine Research and NIOZ want to know more about that food, those small fish. Maathuis: “We had questions such as: how big are those schools? How many schools are there? What species are they? How big are the fish?”

Mapping this group of small school-forming fish is not easy according to Maathuis. “They are small – less than 20 centimeters long – and live in large groups in murky water.”

Within the project Waddentools Swimway Maathuis was given the space to experiment with a new technique in the Marsdiep in the Wadden Sea, between Texel and Den Helder. “Since that is the passage for fish that migrate from the North Sea to the Wadden Sea and vice versa.”

The idea was to place acoustic equipment that spreads sound waves on the seabed. Maathuis explains that camera systems do not work in murky water and that the fish are too small to track with a transmitter. “The sound signals are reflected by objects in the water, comparable to the technique bats use to find insects.”

By directing the sound waves upwards, towards the water surface, all the fish swimming there were recorded. Maathuis: “We had no image of these fish, so it is very special that we have data from them every 90 minutes for a year. In this way we have been able to map detailed seasonal patterns.”

Growing up area

For example, the research team discovered that school-forming fish such as herring and sprat peak in late spring and are found at approximately ten meters deep. Maathuis: “They mainly swim with the current, but with outgoing water they appear to do their very best to stay in the Wadden Sea.” This confirms the research group’s idea that the Wadden Sea serves as a nursery area for herring and sprat.

Maathuis explains that the results of this research serve as a starting point for other research. “This new equipment is now also on the seabed near the island of Borkum.”