A round up of answers to the above.
Mackerel usually shoal with others of the same year class, thus if a shoal is netted all the fish are equal in size.
Small mackerel (joeys) are common around our coasts in early summer but quickly grow fat if there is good feeding available, usually juvinile sprats, herrings and such like.
Big Mackerel or 'Channels' as sea anglers call them are caught in deeper water on lines of lures (feathers, tinsle, plastic, etc) run off small boats, these deep water mackerel are highly prized for their eating quality and size.
Your quite right, mackerel cannot be farmed, it would be uneconomical in food costs, but they are used to provide food for captured wild caught Tuna.
For whatever reason mackerel caught early in the season do not freeze well and turn to mush when thawed, this is why sea anglers prefer to freeze only late summer/autumn caught fish.
Ive used strips of mackerel belly on a sea anglers 'clipped down' long distance casting rig and caught Pike up to 22lb and achieved similar results with sandeels.
For heads down baits, yes the swim bladder needs to be deflated, apart from this Ive never stabbed a bait.
Sorry i dont know what 'catching the impossible' was/is (presume a fishing video).
On a +150ha lake I once watched an immense shoal of Roach/Rudd drift past me, all suspended head down about 6' under the surface, also mixed up with the shoal was some very impressive Perch (?), this experience was what encouraged me to use the head down method in the first place.
Pointngo, yes, but not all the time.