Abstract

Key aspects of the trophic ecology of small pelagic fishes are not fully understood along the Humboldt Current. Here we examined the trophic seasonal dynamics, their relationships with oceanographic conditions, and interactions among cohabiting species (anchoveta Engraulis ringens and common sardine Strangomera bentincki) in the central (20 degrees S) and southern (36 degrees S) Humboldt Current System (HCS), using stable isotopes (delta 13C, delta 15N) measured at different levels of the food web (particulate organic matter, copepods, and fish). Anchoveta showed narrow delta 13C seasonal variations in both zones but marked differences between zones. A wider inter-seasonal range in delta 15N values, and a wider trophic amplitude (size of the isotopic niche) were observed in anchovies from the central HCS zone. In the southern HCS, the trophic positions (TPs) of both species showed slight variation during the year (anchoveta: 3.15-3.43, common sardine: 3.08-3.28), with those of common sardine slightly lower than those of anchoveta. However, the isotopic niche size in both species changed seasonally, as did the trophic overlap between them. The lowest trophic overlap between species occurred in winter, suggesting a trophic partitioning mechanism to overcome periods of reduced food availability. Overall, the seasonal variation in stable isotopes in the components of the food web seem closely associated with large-scale atmospheric and oceanographic processes (coastal upwelling) that affect both areas but with different intensity and duration during the year. Local geographic differences between zones also influence the input of organic carbon and nitrogen from certain sources, and interactions between species become significant during specific seasons.