This study describes the scheme of the interspecific trophic relationships in the pelagic zone of the Argentine continental shelf in the coastal water region along the Province of Buenos Aires, in which the mackerel occupies the trophic levels corresponding to a plankton feeder of copepods of the zooplankton (third level), and to a small predator of the micronecton fish (fourth and fifth levels). The specific food spectrum is wide in the taxonomic composition and it consists of: Copepods, Amphipods, Euphausiids, Decapods (larval and postlarval stages), Chaetognats, small squids and fish, mainly anchovy in all development stages. The size of food species varies from 1 to 140 mm in length and the maximum diameter of the body from 0,8 to 20 mm; the diet is that of a small carnivorous fish tending to the euphagy characteristics of an opportunistic feeder. During the summer months some qualitative, Quantitative and chronological variations in the specific composition of the diet become erudent; these derive from the relative abundance of food species and the monthly sequence of the dominant modal classes in the shoals in this period, the anchovy is always frequent in the stomach contents of youngs and adults of the mackerel, such as larval and postlarval stages, young and adult fish. The capacity of food ingestion varies considerably between 5 and 22% with respect to the body weight of the consumer and the number of ingested specimens with the size of food species. The trophic equivalences calculated between the different food species also show qualitative and quantitative variations, mostly caloric and gravimetric in relation with the anchovy taken as basic food standard; therefore the adults of this species are considered as having the greatest utilitary value for the feeding of the mackerel during the period from October to April. The trophic habitat of the mackerel is included in the spawning and growing area of the anchovy, that at the same time is characterized by a high productivity of calanoid copepods, common food for both species. The food chain of the mackerel belongs to the intermediate type, between chains of simple disposition and food webs, and the main food is represented by two different successive links, one of the zooplankton crustaceans and other of anchovies. Generally, the intake of food is made by two different mechanisms: by filtration of elements of zooplankton (copepods) and by pursuing and capturing small preys (sergestids, chaetognats, squids, anchovies). From this point of view, the mackerel shows a high trophic adaptation characterized by the alternation and transitory change of trophic level and ecological niche, and by the morphological structure of some parts of the digestive system with regard to feedint habits. This last characteristic is also verified by the determination and statistical analysis of certain morphometric relations, mainly the buccal, mandibular intestinal and coelomatic quotiens.
