Channel 12 is the primary traffic channel for Halifax harbour; it's monitored by the coast guard VTS (vessel traffic services) who identifies on the radio as "Halifax traffic" - they keep a watch on ships in the harbour and give out harbour traffic information to ships, updates on other ships' locations, etc.
Channel 14 is also used by VTS for the outer approaches to the harbour, but depending on your location you may not hear it as often since the ships that they are communicating with are farther out.