On 9 April 1940, the day of the German invasion of Norway, the commander of Lerøy guard district, Captain F. Ulstrup, received a message informing him that German warships had forced their way past the small Lerøy Fort on their way towards Bergen. Captain Ulstrup immediately boarded "Tyr" and sailed out to mine the Lerøyosen approaches to the south-western Norwegian city. At 0130hrs the Norwegian minelayer began mining the stretch of water between Lerøy island and Sotra, putting out seven mines right before the German invasion flotilla arrived at Lerøyosen. Making good her escape "Tyr" placed another sixteen mines in the Vatlestraumen approaches north of Bergen.