The swim bladder is the standard teleost buoyancy organ, and adjusting its gas volume lets a fish hover. Scomber scombrus lacks one entirely. Being denser than seawater, the fish sinks whenever it stops, so it uses lift from its pectoral fins and body while swimming, effectively without pause, throughout its life.
The cost is continuous energy expenditure. The benefit is vertical freedom. A fish with a swim bladder that ascends quickly must vent expanding gas, and rapid depth changes can injure it; a mackerel has no gas volume to manage and can move between the surface and depth as fast as it can swim. That suits a species that follows plankton up and down through the day and forages across the water column.
The absence is not universal in the family. Several other Scomber species, including the chub mackerel, do possess a swim bladder, so this is a trait of Scomber scombrus rather than of mackerels in general. It is often reported as though all mackerel lacked one, which is not correct.