Pulse trawling in Lake Hjälmaren (daytime)

En man med skägg, klädd i flytväst står böjd och håller fram en genomskinlig plastlåda. Lådan är fylld med vatten och många mindre små fiskar. Bakom mannen står en blond kvinna med flätor, beige byxor och flytväst. Hon håller också i en plastlåda med fiskar. De står på en båt i en stor sjö. Himlen är täckt av moln och bakom sjön syns en skog.

Lake Hjälmaren is one of Europe’s biggest lakes with eutrophication problems. The lake’s ecosystem is out of balance. One potential solution to reduce the amount of nutrients in the water could involve what is known as reduction fishing. This involves us catching large quantities of white fish such as roach and bream, which are fish that otherwise increase eutrophication. Increased visibility and less white fish in the water benefits large predatory fish such as pike, pike-perch and perch.

This creates a new, more natural balance in the ecosystem.

Lake Hjälmaren is designated as a site of national interest for commercial fishing, and so it is important to find out how reduction fishing would affect fishing conditions. Pulse trawling – a harmless method that involves anaesthetising the fish – makes it possible to take samples, measure and weigh the fish and then calculate which fish are actually in the lake, how many of them there are and how big they are. Knowing this gives us more of an opportunity to reduce eutrophication in Lake Hjälmaren.