Did anyone see that Trawler programme a few years ago when a new £11m purpose built boat was launched in Scotland and on its first commissioning trip, in one haul of its nets it caught 500 tons of Mackerel. Maybe that's where the Mackerel have gone.
And what about the hidden factory that was discovered in, I think Peterhead, where there was a wholesale processing plant just handling "black" fish, hundreds of tons of it.
I'm afraid our own commercial fleet are no saints, it's just that the European fleet is vastly bigger and pull all the same tricks. Greed, greed and more greed.
I've just read an excellent book "Silver Shoals" by Rangely-Wilson which looks at some of the problems. Well worth a read.