The Productivity Overfishing Index is based on the production of biological material in the form of carbon. By far the most of the carbon produced in the ocean is phytoplankton (plant plankton). As we move up the food chain, the biomass diminishes by a factor of about ten at each level and there are major differences in transfer efficiency between different ecosystems. In the huge upwelling systems off Chile, Peru, California, and Northwest and Southwest Africa phytoplankton production levels are among the highest in the world, but the transfer efficiency is low. Much of the phytoplankton in these systems falls to the seabed and is not exploited by zooplankton or fish.
At the other end of the scale we find arctic-boreal spring bloom systems such as the Norwegian Sea and the Barents Sea, where there is a moderate annual production of phytoplankton, but the transfer efficiency, which leads to production of zooplankton and fish, is high. Our ecosystems are thus capable of supporting some of the largest fish stocks in the world, and therefore some of the world’s largest commercial fisheries, because the food web is “designed” for the very efficient production of fish.
What this means is that it is highly inappropriate to use the Productivity Overfishing Index in the same way for all the oceans of the world, and conclusions drawn on the basis of using it in this way will be wrong.