Middle-sized pelagic fish constitute an important part of world fish catches. The biomass of this group principally includes mackerel and herring-like species. They live in all the oceans, especially on the shelves, and feed principally on planktonic organisms. Middle-sized pelagic fish are composed by numerous families with some common characteristics including colouration, the ability to swim well and the habit of living in schools. These species make long migrations for feeding and reproductive reasons. They include herbivores, carnivores and omnivores, and there is great flexibility in the trophic requirements within each species. They also show high behavioural flexibility. These species are intensely fished, and all of them show important fluctuations in abundance, which are examined with special attention in Sardinops caerulea, Engraulis ringens, Trachurus symmetricus, Scomber japonicus, Clupea pallasii, Sardinops melanostictus and Sardinella longiceps.