On the surface the water is fairly fresh, resulting in bathing areas that are free of jelly fish, while deeper the water has a higher salt concentration, with saltwater fish such as cod, pollock, flounder, and mackerel. The tide sets up one of Norway's strongest currents through the strait (outflow can reach speeds of 5 knots and the strait is known Selvikstrømmen (the Svelvik current) in Norwegian. Large ships can only sail in daylight and the largest only when assisted by tugboats.
The surface of the water appears fresh, resulting in safe bathing areas free from jellyfish, whereas deeper down, it has a higher salt concentration, inhabited by fish such as cod, pollock, flounder, and mackerel. The tide generates an exceptionally strong current through the strait (up to 5 knots), designated as Selvikstrømmen (the Svelvik current) in Norwegian. Only large ships can navigate during daylight hours, with the largest vessels requiring assistance from tugboats.