Diet and trophic position of two mackerel species in the archipelago of Madeira, Portugal

The Atlantic chub mackerel Scomber colias and the Blue jack mackerel Trachurus picturatus are two abundant species in the Macaronesia region which include the archipelago of Madeira, Portugal. Both are key species in the trophic web, being important prey for several local top predators, such as seabirds and marine mammals. However, little is known about their feeding ecology in oceanic environments. In this study, we describe the seasonal variation of the diet of S. colias and T. picturatus in the oceanic region of Madeira throughout a year. Visual inspection of stomach contents revealed that S. colias fed on a broader range of prey groups than T. picturatus, but for both species, zooplankton (particularly calanoid copepods) and fish were the most important food items. The diet of S. colias included a higher proportion of fish, namely Atlantic saury Scomberesox saurus and S. colias, than that of T. picturatus, that included mostly the Longspine snipefish Macroramphosus scolopax. Trachurus picturatus consumed a higher proportion of decapods and other copepods. Seasonal variation was found in the diet of both species, with zooplanktonic species being more important in colder months (February to April) for S. colias and during warm months (May to October) for T. picturatus. Their diet in other seasons were dominated by fish. Despite consuming similar prey, Carbon and Nitrogen stable isotope analysis of muscle of S. colias and T. picturatus showed little overlap in their diets, and T. picturatus showed higher δ15 N and a narrower isotopic niche.