Small Mackerel are forage food in the sea food web
Quoting from Wikipedia (https://en.wikipedia.org/wiki/Forage_fish):
Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding. The ocean primary producers, mainly contained in plankton, produce food energy from the sun and are the raw fuel for the ocean food webs. Forage fish transfer this energy by eating the plankton and becoming food themselves for the top predators. In this way, forage fish occupy the central positions in ocean and lake food webs.
Quoting from Wikipedia (https://en.wikipedia.org/wiki/Mackerel):
Smaller mackerel are forage fish for larger predators, including larger mackerel and Atlantic cod. Flocks of seabirds, as well as whales, dolphins, sharks and schools of larger fish such as tuna and marlin follow mackerel schools and attack them in sophisticated and cooperative ways. Stop quoting.