So what's the solution ? Assign a surveillance UAV to every Russian ship parked "without a good reason" over a cable ? It would be expensive, but doable, and create a reserve of vehicles for wartime use.
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So Denmark can start assuming every vessel (or at least more vessels) are in violation. Russia can take that to some international court if they so desire. Inspect every ship. Question the crews. Take plenty of time doing it. Perishable goods on board will perish before reaching St Petersburg and Kaliningrad. Tankers will be refused entry, limiting or delaying export income for Russia.