Due to a built-in time delay the Lerøyosen mines were not active when the German warships passed over them. However by the time the supply fleet of the German forces in the Bergen area started coming into Lerøyosen late in the evening the mines were ready. The first ship to hit the mines was the 4,297 ton supply ship "Sao Paulo", which sank with a great loss of both lives and materiel at 2300hrs, 9 April. As the mine barrier had now been discovered the Kriegsmarine needed to sweep the Lerøyosen but had no dedicated minesweepers in the area. In an improvised reaction to the mines the invaders sent out the naval trawlers "Schiff 9" and "Cremon", together with two launches from the depot ship "Karl Peters".