I think the over fishing of sandeel, capelin, anchovies, sprats and so on will have forced mackerel to seek a new source of food.
After all, salmon and mackerel have co-existed together for hundreds of thousands of years without an issue.
Mans demand for fishmeal (56% goes into feeding aquaculture..mental!) has cost the oceans and the oceanic food chain dearly!
We are currently removing a whole layer of food that is affecting larger fish, sea birds, sea mammals and ultimately, us. Taking 5kg of small fish to create 3kg of fishmeal to deliver 1kg of Salmon on a plate is unsustainable.