Recent research suggests iron to be a trace nutrient limiting Banks productivity. Previously, nitrogen was thought to limit Banks' productivity, but tracking dust storms which deposited iron-rich dust into oceans revealed subsequent ocean productivity jumps.
Oceanic bluegreen algae's nitrogen-fixing enzyme uses iron. These algae can fix dissolved atmospheric nitrogen after duststorms supply them with iron, which is otherwise present at only parts per billion concentration in much of the world's deep oceans. Fixed nitrogen becomes available to the blue-green algae and the things that eat and decompose them, and so to the entire ocean ecosystem, unlike atmospheric dissolved nitrogen atoms, which form pairs, and do not react easily with other atoms.
Replenishing amounts of iron previously removed within harvested fish might restore Banks productivity, and thus bring back bountiful fish harvests. But co-ordinating public financing of even research trials may prove impossible or slow. Here trawler self-interest may serve.
Where a small area of the Banks, if limited by iron, receives iron, plankton can photosynthesize more, and rapidly increase - phytoplankton can double in a day. herring, mackerel and other plankton-eating fish that enter this area would stay to feed on plankton, as would cod, haddock and other fish-eating fish with higher market value, to feed on herring, etc.. So trawlers might spread iron, wait, then capture some of the gathered cod, etc., and thus help the Banks while helping themselves.