Lox is brined salmon, a type of fish. Lox is the homophone of locks. Locks are metal mechanisms used to keep a door, or lid fastened shut. "Secure them with lox" sounds like "secure them with locks" or secure the fish with locks so no one can steal them. The joke is on the word "lox" and its homophone "locks". 
To keep someone from stealing your shipments of smoked salmon , secure them with lox .