Weever fish are poisonous and live near the shores in Danish waters. Weever fish stay in calm waters, and are only rarely seen along the west coast of Jutland.
The venom is located in the dorsal fin, which consists of sharp spines, and it can be very painful to step on a weever fish. The venom can cause swelling and fainting, but often it just feels like a bee sting. You can ease the pain and neutralize the venom by placing your foot in hot water.
Be very aware of weever fish in June, July and August, when the fish hide at the bottom in shallow water in order to spawn.
(Source: Naturguide.dk, October 2010)