premise: The first salmon farms were in the Atlantic islands but the industry fell into dramatic decline with the value of the farmed fish when the farming method became discredited because of the use of chemicals and concerns over the feed used. Now they are farming cod in the Atlantic isles, only this time they have set rules for fish welfare and are feeding the cod on fish meal produced only from the by-products of fish already caught for human consumption. The enclosure nets in which the fish are grown are cleaned mechanically rather than treated with chemicals to stop them becoming fouled with weed. Cod, unlike salmon, are a gregarious species that naturally shoal and, unlike salmon, they do not need to be treated with insecticide to control fish lice.
hypothesis: The cod farmers are taking measures to avoid the mistakes of the salmon farmers.