Parasites play a key but overlooked role in the functioning of marine systems. Understanding the drivers of parasites infection in fish is thus crucial. Anisakis levels in the Atlantic horse mackerel Trachurus trachurus resulted from both biotic and environmental drivers. Ontogenic variation of the diet was the first driver of Anisakis level. Nonlinear pattern confirmed that the variation of parasitism was related to ontogenetic dietary shifts. The impact of depth on diet also explained the effect of depth on parasites. In deeper areas, fish only access pelagic preys, where Anisakis presence is high, consistently with the pelagic nature of the parasite. Finally, relative proportions of parasitic burden by tissue appeared to be driven by lipid content. In lipid-rich species like T. trachurus, Anisakis can find favorable conditions in the body cavity and in muscle and avoid liver. Parasitism levels in fish are thus the result of patterns at play at different levels and that these factors should be better considered before being able to include parasites in an integrated vision of marine ecosystem functioning.