I have trouble believing that those giant CO2 spewing cargo ships are so small a factor.
It’s why the “buy local” movement wouldn’t really save much CO₂. Driving the trucks from the harbour to the consumers emits more, AIUI.
The majority of CO2 from moving food is created by you driving to the supermarket for groceries. It’s not hard to see how when you compare a 15 ton truck moving 30 tons of food, compared to a 2 ton car moving 4 kg of food. That truck can move 1000 times further than a personal car for about the same amount of CO2 per kilo of food.