Transporting fish or any food whatsoever by plane results in a substantial increase in environmental impact. Container ships are by far the most efficient and CO2-friendly way to transport food. Even when it is frozen. For cod, the difference in CO2 emissions has been calculated, depending on the transport to London.
A transport by ship from China to London (17,500 km) is 190 kg CO2 equivalent / tonne. A transport by truck from Aberdeen to London (862 km) is 250 kg CO2 equivalent / tonne. And a transport by plane from Iceland to London (1678 km) is 3250 kg CO2 equivalent / tonne. Despite these major differences, research shows that in general the determining factor of greenhouse gas emissions is often the actual breeding or fishing activity (60-80%), unless the products are transported by plane.
Regionally caught fish in the North Sea has unfortunately often a much higher carbon footprint than freshwater fish grown in Asia and transported frozen to Rotterdam by container ship. After all, catching a kilogram of sole with the traditional beam trawl requires an average of 4 liters of diesel oil. In short, for fish, environmental impact cannot be expressed in food kilometers.
Concluding: organically or locally produced is often not the most sustainable option with fish. And when it comes to carbon footprint, fresh is often less environmentally friendly than frozen fish. But “fresh” in the supermarket can also be thawed. It is usually not stated on the packaging.