Gillnets are a very selective fishing method in relation to the target species. Fishermen stretch them for kilometres through the Baltic Sea to catch cod, for example. However, gillnets also have a disadvantage: they are practically invisible to porpoises and therefore often become a deadly trap. The reason: like bats, whales orientate themselves with the help of echolocation, they send out signals that are reflected back to them to show them the way. Nylon nets, however, send back practically no echo.
In the project STELLA at the Thünen Institute for Baltic Sea Fisheries, PhD student Isabella Kratzer has therefore been looking for a technical solution to make the set gillnets visible to harbour porpoises and at the same time not restrict their catchability for fish. The results of her research have now been incorporated into her PhD-thesis and will be presented during a public digital colloquium on Thursday, 28 January, at 11 am.
Isabella Kratzer systematically approached the solution to the problem. Table tennis balls played an essential role. They serve science as reference objects for air bubbles. These bubbles reflect sound under water, which is used, for example, in pile driving for wind turbines: Huge bubble veils around the construction sites protect whales and other noise-sensitive marine animals from noise pollution and loss of orientation. Instead of air, very dense objects such as steel balls would also be suitable for reflecting the sound waves. "However, both approaches are useless in the fisheries sector," says Isabella Kratzer. Air makes the nets float, heavy balls make them sink to the bottom. So she looked for an object with a similar density to sea water and the reflectivity of air bubbles.
At the Bundeswehr's Technical Centre 71 in Kiel, she finally varied the material properties, size and density of objects on the computer until she found the optimal shape of an object that would make as much noise as a ping-pong ball by resonating for the porpoises. This optimal object is an eight-millimetre acrylic bead. If a certain number of these beads are incorporated into a set net, it becomes as visible to the whales as a wall is to us humans. The crucial thing is that the nets remain invisible to the target species, the cod, and the fishermen neither lose catches nor have to do any additional work. Acrylic glass beads could thus become one of the most important innovations in fishing technology.