Trophic level, niche width, and site fidelity of Moroccan forage fish in two contrasting upwelling systems

Abstract

Forage fish from two distinct upwelling systems (700 km apart) off the southern Moroccan Atlantic coast were investigated using carbon (δ13C) and nitrogen (δ15N) stable isotopes to estimate their trophic position, niche width and site fidelity. Zooplankton (150 μm - 1 mm), considered as a baseline, showed similar δ15N values but significantly different δ13C values between areas. Sardine (Sardina pilchardus), chub mackerel (Scomber colias), horse mackerel (Trachurus trachurus) and anchovy (Engraulis encrasicolus) showed variable differences in trophic position and niche width within and between areas. Sardine had the lowest trophic position and horse mackerel the highest one. Differences between areas suggested an adaptive trophic behaviour. The δ13C pattern was clearly different between areas for sardine, anchovy and horse mackerel, indicating a high fidelity of these species to their feeding grounds. Chub mackerel, which had a similar trophic position between areas, appeared to be a highly mobile species. In this study, we provide an unprecedented insight into the trophic and migratory behaviour of forage fish in Moroccan waters by comparing four species using stable isotope analysis.