Seasonal and size-dependent variability in diet of Scomber colias (Gmelin, 1789) of the Atlantic Coast of the Northwest Africa

Atlantic chub mackerel Scomber colias is one of the major fisheries resources in the North African Atlantic coast that has an important role in energy transfer in the marine ecosystem. We conducted quantitative and qualitative analyses for the diet of Chub mackerel, collected between Cape Barbas (22°N) and Cape Bojador (26°30'N). Sampling was carried out during the warm and cold seasons of 2008. Stomach contents of 475 specimens were collected and analyzed. In terms of Index of Relative Importance (IRI), Crustaceans (IRI=62.3%), especially Copepods was the most important prey. Secondary, Teleosts (IRI=32.88%), especially small pelagic fishes (Engraulidae, Clupeidae, Scombridae, Myctophidae) and Mollusks (IRI=4.79%) mainly composed of gastropods. Other groups: Annelids, Appendicularians, Cheatognaths, Radiolarians and Hydrozoans were a minority, in terms of abundance and biomass and were considered as accidental preys. Scomber colias seemed to shift its prey from Copepods to fish at threshold size ranging from 21 to 24 cm (FL). Results from the visual analyses with geographic information system suggested the spatio-temporal variation of dietary composition. Significant statistical relationships (p<0.05) between diet composition, size classes and seasons were confirmed as well as their horizontal and vertical variations. Spatio-temporal difference of the marine environment and available prey derived the difference in the functional relationship of the Scomber colias depending upon the different gulp sizes as it grows.