According to some notes I rediscovered which I thlnk are from the book "North of Galipoi". The Russian Steamer Merkury or Merkurij had 450 passengers and crew on board when she sank after hiting a mine laid by the UC 15 on 20 June 1916. Of which 178 were saved. The book "The Russian Fleet 1914-1917" lists the sinking date as 15 June 1916. One hops this is of some use.