I found two sources that say that Trotsky was detained by British naval authorities in Halifax and spent time in a detention camp in nearby Amherst.
"British naval officials in Halifax, Nova Scotia intercepted his ship and he spent a month detained at Amherst, Nova Scotia."
source: Jewish Virtual Library
"March 27: Trotsky and his family leave the USA by ship, but are detained by the authorities at Halifax, Nova Scotia. They are released a month later at the behest of the Russian government."
source: Glasgow University - Leon Trotsky Timeline
So, yes Trotsky was detained by British authorities in Halifax, NS, in 1917. He and his family were on their way there from New York, NY, and it was not until the Russian government begrudgingly demanded the British release Trotsky that he was released.