Composition and spatiotemporal variations in the diet of Atlantic horse mackerel Trachurus trachurus from the Atlantic coast of Southern Morocco
Résumé
A study of the diet of the Atlantic horse mackerel Trachurus trachurus (Linnaeus, 1758) was carried out on the Atlantic coast of Southern Morocco, located from Cap Boujdour to Cap Blanc (26 degrees 30'N-21 degrees N latitude). This area is characterized by a permanent upwelling and is among the highly productive ecosystems of the worldwide ocean. The diet of T. trachurus was studied through qualitative and quantitative analysis of stomach contents. Samples were collected during the warm (June-July-August) and the cold seasons (November-December) in 2007 and 2008. Acoustic surveys were carried out on board of the Moroccan Amir Moulay Abdellah and the Russian Atlantida oceanographic vessels. A total of 456 stomachs were analyzed, exhibiting a diversified food spectrum. According to the results of the index of relative importance (%IRI), which takes into account weight, number and frequency of occurrence, the pelagic crustaceans (%IRI = 80.15%), mainly Copepoda, are considered the preferential prey for T. trachurus. Molluscs (%IRI = 16.76%) are secondary prey and fishes (%IRI = 2.91%) are complementary prey while Siphonophora, Chaetognatha, Appendicularian and Annelids polycheata are accidental prey (%IRI \textless 1%). A Principal Component Analysis combined with a permutation test highlighted a significant difference in diet according to the sampling periods. A MANOVA test applied to the survey periods, the latitudinal position of the stations, the depth of trawling and size of individuals highlighted the determinant factors of this variation. It appears that T. trachurus exhibits significant diet variability with high thresholds for season and space factors.