Upvote:8
The book "Gateways of Asia" by Frank Broeze, pg. 266, contains the following:
In spite of its good natural harbor, Vladivostok had several disadvantages....The head of Zolotoi Rog is frozen over for no less than some 100 to 110 days of the year. Freezing usually starts just before Christmas and the ice breaks in early April. Around the shores and at the head of the harbor, the ice is sometimes half a meter thick. The first rather futile ice-breaker arrived in January 1895. Today the port is kept ice-free by the exhaust water from the power plants churning water in the bay.
This leaves open the question of when effective icebreakers were introduced.
According to Wikipedia: Icebreakers, the Russian ship Yermak, launched in 1898, was "the first true modern sea-going icebreaker." Wikipedia:Yermak states that the Yermak could break 2-meter ice, sufficient to open the Vladivostok harbor.
Thus, we can conclude that at the time Russia occupied Port Arthur in 1897, Russia already had an icebreaker in Vladivostok, and was no more than a few years away from being able to effectively break the ice there.